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Automotive Electrical Troubleshooting

Troubleshooting is the act of taking logical steps to solve a problem. In the case of automotive electronics troubleshooting, this is regarded as black magic. In reality, it’s no more difficult than troubleshooting a problem of any kind. I’m betting if you didn’t have the first six chapters under your belt, then maybe it would be.


This chapter starts out with an easy-to-solve problem and works up to the complex ones. What I hope to show you is the thought process that goes into solving each problem versus the actual steps involved, or even how difficult the problem is to solve. Afterward, you’ll be prepared to dive in with both feet when something goes wrong.

First off, here are the tools you will need:

  • Knowledge of the problem (or symptom) at hand.
  • Some knowledge of the circuit you’re diagnosing.
  • Diagram of the circuit if possible.
  • DMM.
  • Wire probe kit for your DMM.


Understanding Flowcharts

One of the best things I ever learned in school was how to use and make flowcharts. I have many years of experience in troubleshooting and each time I do it, I go through the process as if it were a flowchart in my mind. I guess I should thank my computer science teachers for this ability!

Respective manufacturers can provide flowcharts to professional auto mechanics so the amount of time mechanics spend troubleshooting problems is reduced. As mechanics are paid by book hours, it cost the OEMs less money to develop this tool on the front side than to pay the mechanic the time the job would take if they didn’t provide it. Unfortunately, these are not readily available to vehicle owners. I’ll give one to you for two of the scenarios covered in this chapter. If you’re troubleshooting a really complex problem, then a flow chart may be the only way to solve it in a timely fashion.


Typical Problems

Most of the examples assume that you do not have anything other than a basic understanding of the circuit and your DMM. A few of the examples have been recreated based on problems I’ve had to troubleshoot in real time. I hope you find them enlightening!

  • Circuit inoperable
  • Circuit works, but blows fuses
  • Wiring burned up
  • Battery is drained overnight
  • Intermittent circuit operation


Circuit Inoperable

Example—Reverse light circuit, 1972 Olds Cutlass

Symptom—Driver-side reverse light not working

We have a bulb that doesn’t work—how do we determine the problem? First, a basic understanding of the circuit is necessary. In order for the bulb to come on, the following have to occur:

  • Ignition switch in IGN/RUN position.
  • Shifter in REVERSE.

Before we start troubleshooting, let’s be safe by following the safety outlines I gave you at the beginning of Chapter 5. Now:

  • Move the ignition switch into the IGN/RUN position.
  • Depress the brake pedal and move the gear selector in the REVERSE position.

Now, one reverse light is working and the other is not.



Simple problem, right? Let’s dig in and find out …

Since one of the lights is working, we know that overall the circuit is working properly. This means that there is an open circuit between the wiring and the non-working bulb. Possible causes are:

  • A bad bulb.
  • A bad connection between the bulb and the bulb socket.
  • A bad connection between the connector and the bulb socket.
  • A bad connection somewhere in the wiring harness to this connector.


 Figure 7-1


Here are the correct steps in determining this. I always choose the most obvious problem first, that way I can eliminate extra work. In addition, it’s always best to work from one end of the circuit to the other, don’t start in the middle. In this case, we go to the very end and test the bulb first, as it’s easy to remove. The flowchart above (Figure 7-1) shows the procedures:



Checking the bulb itself.


Meter Settings

  • Selector switch in the Ω Position
  • Red probe in V/Ω
  • Black probe in COM



Step 1:

Remove the bulb from the socket and connect the black probe to the metal bulb sleeve

Step 2:

Connect the red probe to the tip of the bulb (Note: Many automotive bulbs have two filaments and have two electrical contacts—one for each filament. This one does not.)

Step 3:

A reading of close to 0Ω shows continuity and means that the filament is good. No reading means that the filament is burned out, and the bulb needs to be replaced.

The bulb is good, as indicated by the 0.6Ω reading on my DMM, so we need to move on to the next step.




Checking the connection between the bulb and the bulb socket.


Meter Settings

  • Selector switch in the DCV Position
  • Red probe in V/Ω
  • Black probe in COM and connected to vehicle chassis ground



Step 1:

With the bulb out of the socket, the Ignition Switch in the IGN/RUN position, and the shifter in REVERSE, attempt to measure voltage at the brass (+) connection point within the bulb housing.



Step 2:

If voltage is present at the brass (+) connection point, we have a poor connection between the bulb and the bulb socket. In some cases, it may be necessary for me to use the tip of the DMM to get a good bite into this brass (+) connection point by scraping it back and forth across the brass contact.

That is the case here, but it isn’t actually the brass (+) connection point within the bulb socket that is the problem. Rather, the bulb socket itself isn’t getting a good bite into the ground lugs of the bulb as it is spent. In all fairness, it is 36 years old! Although I can get the bulb to work as pictured, it is only be a temporary fix. The long-term fix is to replace the socket entirely.



If you determined that the brass (+) connection point was the source of the problem, then you’d need to clean it and the contact on the bulb itself to ensure a good connection. But, before cleaning either, put the shifter back in PARK and turn the ignition switch to the OFF position. Now you can clean the contact points, without danger of shorting the one in the socket, and re-install the bulb. (The tip of a small flat blade screwdriver works nicely for the socket contact and a file of any type or some sandpaper can be used to burnish the bulb contact.)

Since you narrowed the problem down to the socket itself, it is not necessary to check the integrity of the connection between the wiring and the bulb socket. This proved to be a good example as I thought the problem would be a little simpler than it actually was. Either way, it’s the thought process that’s important. Note that we didn’t have to run to the auto parts store to get a new bulb that wouldn’t have solved the problem anyway. Now, let’s move on to a more challenging problem.


Circuit Works, but Blows Fuses: Scenario 1

Example—Aftermarket electric fans in the same vehicle.

Symptom—The circuit works fine most of the time, but the driver-side side fan stops working occasionally when the vehicle is being driven. When this happens, the fuse that powers the relay for that fan is blown. I can replace it and restore operation to the circuit for a period of time before the fuse blows again. Before we begin, Figure 7-2 is a diagram of this circuit.


 Figure 7-2


This problem is a little tougher to solve, but let’s start with what we know:

  • Spal 16-inch electric puller fans being used, and according to the manufacturer, each requires 22 amps at 12 VDC.
  • 30 amp Tyco Relays for the fans are mounted under the dash and approximately 12 feet away.
  • Relays are approximately 4 feet away from the fuse panel.
  • Relays are fused with 30-amp fuses each.
  • Ground wiring is about 3 feet long.
  • 10-gauge wiring is used throughout.
  • 10-gauge wiring has .00102 Ω of resistance per foot.

From Chapter 1, let’s first calculate how much voltage is lost in the wiring:

E = I x R

E = 22 amps x (19 x .00102 Ω)

 E = 22 amps x .01938 Ω

E = .43 volts of loss in the wiring

That’s certainly acceptable. Now that we’ve quickly ruled out that the wire gauge is insufficient for the task, let’s get on with the troubleshooting.





Since there are two identical circuits, we focus only on the one that is blowing the fuse. Possible causes of this problem are:

  • A faulty fan.
  • Intermittent short in the wiring between the fan and the relay.
  • Intermittent short at the relay itself.
  • Intermittent short in the wiring between the fuse block and the relay.

Notice how I basically just dissected this circuit into four different parts. Some of the wiring is within split loom tubing so it is naturally out of sight—again, out of sight out of mind… This can certainly be an obstacle to some, but if you follow my lead it won’t be an obstacle for you.


  Figure 7-3

Here are the correct steps in determining where the problem lies within the individual parts of the circuit. Again, I like to go the end of the circuit and work my way back. The flowchart (Figure 7-3) outlines the procedures:

Checking the Fan Itself—The Process of Substitution: The easiest way to determine if the fan on the driver’s side is bad is to swap the wiring from this into the other fan and vice versa. This allows me to rule out a bad fan by the process of substitution. (Incidentally, if you were troubleshooting a circuit with only one fan, assume the fan is not the problem and proceed to the next step. Remember, troubleshooting is not the act of replacing parts until you find the one that was at fault.) I don’t need tools of any kind to do this, just a little common sense and one extension harness that I can easily fab up on the bench so that I can connect the driver-side fan to the passenger-side fan harness. (The driver-side harness is obviously long enough after I cut loose a few of the cable ties holding it in place.)


This is the fan harness as originally installed. The radiator in my Olds is a Ron Davis Racing unit, an optional fan wiring kit that has mating plugs to the ones on the fans is also available. As I elected to make my own wiring harness, I chose not to purchase the kit.


Swapping the fans from one side to the other is simple with a short extension harness. Note that I was careful to tie this up properly before taking the vehicle for a test drive.


After a quick test drive, the problem still exists, but now the fan on the driver’s side works fine. Since the problem changed sides, I know it isn’t the fan itself. We’ve now eliminated one of the four parts of the circuit as the source of the problem. I can now put the wiring back to normal and proceed to the other three parts.

Checking the Wiring between the Fan and the Relay: Continuing to work our way down the circuit, this is the next step. Also, this step does not require any special tools other than your keen sense of observation. As the wiring is covered in split loom to protect it, the chances of it being damaged are pretty slim. Inspect it along its routing, paying particular attention for any screws that may have pierced the wiring or possibly chafing as it passes through the firewall. Yes, this means that you have to cut the ties that anchor it to the inner fender and open the tubing so that you can see the wiring itself. As the wiring passes through a rubber grommet in the firewall and is properly anchored, we did not find any damage to the wiring or its insulation in the run from the fans to the firewall. Now, we need to get under the dash and follow the run from the firewall to the relay. This run is clean.


This run is for the electric fans. It contains the power wires from the fan relays to the fan motors as well as the trigger lead for the thermostatically controlled switch. Since the Olds has no innter fenders, I’ve anchored the harness (up high) to the bottom of the fender.


When inspecting a run of wiring like this one, it’s best to cut all the ties and open it up as best you can. This allows you to inspect the insulation of the wiring for signs of damage.


As you can see, the wiring passes through the firewall through a snap bushing. The wire insulation is protected from potential damage by the drilled hole’s sharp edges in the firewall.


Follow the run through the firewall and to the relays to look for evidence of any place that the insulation of the wire could be damaged by.


In the event that this is an OEM-installed circuit, the wiring to the fan is not so easy to access. In fact, it may travel through any number of lengths of tubing or conduit along-side any number of other wires. This can make this step more challenging, but you need to spend the time to be sure that the wiring hasn’t been damaged in the run. Pay special attention to areas where accessories (OEM or aftermarket) have been mounted that are in close proximity to the wiring harness—like horns, sirens, cruise control modules, etc. Over the years, I’ve seen a number of cases where a loomed harness was pinched by a mounting bracket, bolt, or screw and it just took time for the insulation of a wire within the harness to wear through. If the wiring passes through the firewall, you also need to pay close attention to its proximity to moving parts under the dash, etc. The more thorough your investigation, the quicker you’ll solve the problem.

Checking the Wiring of the Relay Itself: This is a typical S.P.D.T. relay. As pictured, connections to it have been made with female push-on terminals and they have been insulated with Super 33+ tape.

Even though the relay’s electrical terminals have been insulated, let’s remove the relay from its mounting position and inspect it more closely. When removing the relay, close inspection reveals that there is a puncture through the tape on the rear right next to terminal 30, which is the output to the fan itself. This could not be seen without removing the relay.


This brings us to the relays themselves. All looks good from the front. But you need to remove the relay specific to the driver-side fan and inspect it closely in hopes of finding and resolving this problem.


Well what do you know, the tape has been torn at the rear of the first relay removed. And terminal 30 is no longer fully insulated.


Look closely, see the screw coming through the firewall from the engine compartment? This screw came in contact with terminal 30 of the relay, causing our problem. This problem was intermittent because the screw was just far enough away from the terminal of the relay that the chassis had to flex a little bit for it to touch, thereby blowing the fuse.


Further inspection reveals that a screw run through the firewall from the other side caused the short.

This screw was just close enough to terminal 30 of the relay to cause it to become shorted only when the chassis flexed a little bit when driving. A shorter screw easily solves this problem. This is an excellent example of a simple, yet elusive, problem that was solved with simple and logical troubleshooting steps.(OK, rest assured that I’m more careful than that. I chose to replicate this based on how often I’ve seen a similar scenario occur!)





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Circuit Works, but Blows Fuses: Scenario 2

Another more obvious case where a circuit works but blows the fuse is where some kind of aftermarket electronics has been added to an existing circuit and its current requirements, combined with the current requirements of the accessory(s) already connected to said circuit, exceeded the circuit’s capability. This is where folks are tempted to replace said fuse with a larger value fuse. In the example I gave, the gauge of wire, relay, and fuse were all selected based on the fans’ current requirements, so this circuit is in harmony. This is exactly what the OEM does for every circuit in every vehicle they build. In such a case, the solution is to connect the aftermarket accessory to a different source of power, such as shown in Chapter 6 in the tach installation.

What if you removed a stock piece of electronics and replaced it with a better-performing aftermarket piece? For example, let’s say that you upgraded your stock headlights to aftermarket units that consumed more current. If you simply connected them to the stock headlight harnesses, they might cause this exact same problem. This is where folks get into trouble—real trouble. Since it’s easier to replace the fuse with one that’s “only” 5 amps larger, this is commonly the next step. Next step is another 5 amps larger, right? Wrong! At this point, the circuit is no longer in harmony because the factory wiring isn’t capable of the additional current. This may also be true for the contacts in the headlight switch or even the bright light switch. This creates problems of all kind, maybe even burning up some of the wiring in the process. (Chapter 8 shows how to do this the right way.)


Wiring is Burned Up

This is hopefully not something you’ve experienced recently. I’ve seen it happen many times over the years and it is typically caused by one of three things:

  • Gauge of wiring insufficient for current demands of the circuit—as I just discussed.
  • Wiring not fused/fused improperly, and pinched or shorted—this causes a short circuit, which causes an extremely high amount of current to flow through the wire, which results in the insulation being burned right off of it.
  • A high resistance connection with high current flowing through it, such as a connector termination.



Burned cables and wiring can cause vehicle fires. Here are three examples of what can happen as a result of rushing things or poor planning.


Since I just covered the first bullet point above, let’s look at the cause of each of the remaining two scenarios.


Wiring not Fused and Pinched or Shorted

CAUTION: Note that this is extremely dangerous and can cause a vehicle to burn to the ground! In the event that this occurs in a vehicle that is being driven down the road, the problem can escalate so quickly it can be nearly impossible to pull to the side of the road, find, and fix the problem before serious damage occurs.

Example—Simple short circuit Symptom—Insulation burned off of the wiring Pictured is a length of wire that came out from under the dash of the Olds. This piece of 10 AWG wiring wasn’t fused between the positive connection and the accessory it powered. Somewhere along the way, it got pinched and the resulting short circuit burned the insulation off of the wire. Interestingly enough, the prior owner fixed the short, but chose to keep the burned length of wire in place just as you see it here. In addition, he chose to leave it unfused.


I forget exactly what this was connected to under the dash of my Olds, but it’s a keeper. Notice that the ring terminal is crimped incorrectly and the insulation of the wiring is burned badly, even totally gone in spots. Nice!


More than likely, this damage took fractions of a second. Fortunately for the prior owner, the damage was limited to this cable. Imagine what damage this could have caused had it been laying across the plastic feed line for an oil-pressure gauge. Or even if it had been routed near or under carpet, plastics, or anything it was routed along side or anchored to.

If you’re troubleshooting such a circuit, you have to assume that it wasn’t protected or protected improperly (yeah, a 150-amp circuit breaker is too big for 10-gauge wiring.) and start at the very front of the circuit. If you happen to experience such a scenario first hand, the best thing that you can do is cut either of the battery cables as close to the battery as possible.

Obviously, you understand the value of protecting a circuit now that you’ve read Chapter 4, but you just never know when you might see this occur at a buddy’s house, the local gathering, in the pits at the drags, etc. Thinking on your feet might save your (or somebody else’s) vehicle.

Repairing such a problem typically requires removing the entire run of burned wiring, replacing it, and fusing it properly. Don’t even think of trying to re-use it. In some cases, this is an extremely ambitious and time-consuming repair, especially if this wiring is routed inside loom with a factory harness or tied to one. Over the years, I’ve seen numerous cases in which large sections of the wiring harness required replacing because of damage created from such a simple oversight. In other cases, I’ve seen a burn in the carpet from the firewall to the rear of the vehicle, requiring its replacement. These guys got off easy and were luck they didn’t have a fire!


High Resistance Connection

Example—Same fan circuit in the 1972 Olds

Symptom—Relay housing and wiring burned up

Interestingly enough, this really did happen to me. Remember when I told you earlier that I had a preference when it comes to relays? It just so happens that I didn’t have any Bosch relays on hand when I built this circuit, but I had a few that came free with something I’d purchased over the years. I figured what the heck, and used them. Two weeks later, I noticed that the fan on the driver’s side wasn’t pulling as much air as the one on the passenger’s side.

When I dug into the problem, using the same procedures I outlined above, I noticed that the body of one of the fan relays was melted slightly and so was the wiring and female push-on terminal connected to terminal 87.


The melted relay housing is a sure sign of a problem, but it will take some detective work to figure out what caused this.


Look closely at terminal 87 (front and center). Notice that the whole terminal is pushed up about 1/8 inch into the body of the relay.


Notice the round contact point peeping up over the switch from behind (on the far left). This doesn’t leave much of the contact left to mate with the switch when it is thrown. This resulted in a very high-resistance connection, and the associated heat melted the body of the relay as well as the push-on connector and wiring from terminal 87.


This is a jewel. Not only was the cable routed way too close to a source of high heat (and not protected in high-temperature sheathing), the ring terminal was improperly terminated on the cable.


Further investigation revealed that the relay was poorly manufactured and when I pushed the female connector on to terminal 87, the terminal itself pushed up into the body of the relay slightly. This was enough to break two-thirds of the electrical connection between the contacts within the relay itself when it was powered up. This caused high resistance, high heat, and subsequently damage to the relay and wiring.

The solution was to swap the free relays for some quality units (Tyco in this case, as the company I work for sells them and I thought I’d try them out) and this solved the problem long term. Although the problem I experienced was a problem within the relay itself, the same kind of thing could be experienced between a wire and connector or between a connector and accessory. This problem is magnified as current flowing through the circuit increases, which is why it is so important to make quality connections.

Example—Starter Circuit in same 1972 Olds

Symptom—Wiring burned up

Here is another example of a high-resistance connection. This length of 4 AWG cable went from the starter motor, between the header primaries, to the frame, and then finally to the battery in the rear of my Olds.

As you can see, there are two problems:

  • Close proximity to the headers, as evidenced by the solenoid trigger lead melted into both chunks of wire, as well as the burned insulation.
  • Poor termination of the ring terminal to the cable, as evidenced by the hole burnt through the ferrule of the ring terminal itself.

Finally, when I removed this cable to replace it, I noticed that the electrical connection between the solenoid and armature itself was loose, further contributing to the heat generated at this connection point. Believe it or not, the motor actually started with the cable in this condition. However, the copper in the solenoid trigger lead and the copper in the 4 AWG starter cable have maybe 1/16 inch of plastic insulation left between them. Finally, the copper in the 4 AWG cable was nearly in contact with the frame due to the lack of insulation protecting it, which was totally melted by the heat from the headers.

Best case scenario would have been the starter getting stuck in “start” mode when the solenoid trigger lead finally fused to the starter cable. (You’d have a hard time fixing that if that occurred when you were backing into your parking spot at the cruise!) Worse case would have been the length of cable from the frame to the battery burning, and it was tied to the fuel line its entire length. It gets worse.


Fixing Burned-Up Cable

This is a classic example of what happens when you’re in a hurry! I had the negative cable off of the battery within about 3 seconds of sizing this up. To fix this problem the correct way, here’s what to do:

Step 1:

Remove the starter (which requires removing the passenger side header) so that you can clean and tighten the connection between the solenoid and armature. Use a lock washer to prevent this from occurring again.

Step 2:

Replace the entire run of 4 AWG cable from the batteries to the starter with new 1/0 AWG cable (to ensure the starter would never bog down).

Step 3:

Loom said cable to protect it the length of the run and anchored it along the frame rail so that it was secure.


The cables and fuel line are properly anchored to the frame rail. Even though they are in close proximity to the exhaust and rear tires, a minimum of 2 inches is between them. The 1/0 AWG supply cable and the 4 AWG charge lead from the alternator to the batteries directly, as this vehicle has a bumper-mounted main power switch on the supply cable.


Step 4:

Install snap bushings in the tin surrounding the fuel cell to protect the insulation of the cable itself (before, there were none) as it passes through the tin in three different spots.


As the charge lead passes through the tin that surrounds the fuel cell, it must pass through a snap bushing to protect its insulation. The 1/0 AWG supply cable is directly above this and out of the photo. It also passes through the tin-in the same fashion.


Step 5:

Install a distribution block on the firewall so that you can terminate the 1/0 AWG and have numerous 4 AWG and 8 AWG take-offs for anything you need—one such 4 AWG output goes directly to the starter.


The 1/0 AWG supply lead feeds this firewall-mounted distribution block. The 4 AWG red wire (top right) is the supply to the starter motor. The 4 AWG red wire and the two 8 AWG blue wires (bottom) supply power to the interior fuse panels.


Step 6:

Route the cable down the firewall and over the top of the starter to the solenoid, to get it as far away from the headers as possible.

Step 7:

Sheathe about 3 feet of said cable within the high-temperature sheathing, just to be safe.


The wiring to the starter motor is protected in high-temperature sheathing. In addition, the ring terminal is soldered to the end of the cable, ensuring a good solid connection to the starter.


Step 8:

Add a pair of 200-amp circuit breakers wired in parallel. These breakers run between the battery and the ON/OFF switch mounted on the bumper. They protect the run of the cable from an unlikely short. (Admittedly, these breakers are large, but the pair of 140-amp units I installed originally would open on occasion during a cold start.)


A pair of paralleled Die Hard Golds ensure that the Olds starts right up, even on a cold morning. When locating your batteries in the trunk, it’s a good idea to protect the run of wiring between them and the front of the vehicle with a circuit breaker, or a pair wired in parallel (shown here).


Remember when I said it gets worse? Well, imagine my surprise when I noticed that both batteries were wired in parallel with 4 AWG and grounded with a 5/16-18 bolt and nut to the tin surrounding the fuel cell. In addition, the tin was painted black on both sides, and tack welded to the fuel cell frame. Also, the bolt and nut were loose enough that the ring terminal would easily move! As I replaced the tired, mismatched batteries, aftermarket battery clamps made the connection of 1/0 AWG wiring between them a snap. I also took this opportunity to tap an existing hole in the frame rail and ground the batteries to the frame directly.


With trunk-mounted batteries, the return path to the starter can get quite long. As shown, I tapped the frame rail (obviously, this car has been back halved) and used a 5/16–18 bolt to firmly secure the 1/0 AWG ground wire to the frame. In the front of the vehicle, a short run of 4 AWG connects the bellhousing bolt nearest the starter to the frame rail on the same side, completing the return path.


Shortly after this, I knew it was time to rip out all of the wiring in the car and start over from scratch. Unfortunately, at that time, I didn’t know I’d be writing this book so I didn’t take before pictures. Personally, I’m still surprised the starter even worked at all!





Battery is Drained Overnight

In Chapter 2 you learned how to use both a test light and a DMM to measure current draw. Now we put the DMM to use to solve a real world problem.

Example—2003 Ford Mustang GT

Symptom—Battery is dead after sitting for longer than 96 hours

I have a suspicion about what the problem is, but let’s go through the motions. This vehicle has a bunch of aftermarket electronics, so it’s a great candidate to separate the men from the boys. Currently, it has the following aftermarket electronics:

Audio Equipment

  • AM/FM/CD Player
  • (2) Audio Processors
  • (2) Amplifiers, totaling 3,000 watts RMS in power
  • (2) 18-inch Subwoofers
  • (2) 61⁄2-inch Component Speakers
  • Neon Accent Lighting (above the woofers)

Security System

  • CPU
  • Shock Sensor
  • Tilt/Motion Sensor
  • Power Window Up/Down Convenience Module
  • Auxiliary Power Source
  • Siren

Radar Detector

  • Front/Rear Hide-Away Detectors
  • Instrument Panel located LEDs and Piezo Beepers

Go-Fast Goodies

  • Water/Methanol Injection System
  • Line Lock
  • Boost Gauge
  • Speedometer Calibration Box

OK, so that’s a bunch of stuff in a Mustang huh? If you ever embark on such a project, it’s helpful to make a legend to keep it all straight.

As I’ve done many installations like this over the years, I learned the value of adding centrally located fuse panels for the accessories. As the legend calls out, this vehicle has four of them and their locations are given. In addition, I provided the details of what’s on each fuse and its value. (The specifics of the security system and its auxiliary source of power are kept a secret!)

In addition, the charging system of this vehicle has been extensively modified to keep up with the current demands of the audio system. It consists of:

  • 200-amp Ohio Generator Alternator
  • Optima Red Top Battery
  • 100 Farad Rockford Fosgate Capacitor (used to soften the blow of the current requirements of the amplifiers, which is in excess of 300 amps at full power)
  • 1/0 AWG Wiring Throughout
  • Dash-Mounted Digital Volt Meter

Before taking measurements, it’s important to separate the troubleshooting into two distinct sections—aftermarket electronics and OEM electronics, because it is possible that we could have a draw from one, the other, or both. In addition, as most late model vehicles are similarly equipped, this vehicle has been designed so that it automatically disables circuits that are unnecessary after the ignition switch has been turned off, and the vehicles doors and trunk are closed for a period of time. This is commonly referred to as “sleep mode” or “hibernation” and minimizes the current the vehicle requires of the battery when it is left parked for an extended amount of time. Finally, it’d be kind of nice to know how much current Ford specifies the vehicle should draw when parked. I placed a quick phone call to a tech support specialist at my local Ford dealer and according to him, “Ford allows up to 50mA for a production vehicle.” Good enough, let’s dig into it.






By following the procedures I outlined in Chapter 2, let’s take an overall current draw reading with the ignition switch in turned off.


Meter Specifics

  • Selector switch in the A position.
  • Red probe in A and connected to positive battery post.
  • Black probe in COM and connected to positive battery terminal.

When the battery in this vehicle is disconnected and then reconnected, this may cause the vehicle to exit its sleep mode, so we want to leave our meter connected for a few minutes. This allows the vehicle to go back into sleep mode so that we can get an accurate reading. In the case of this vehicle, the wait time was in excess of 10 minutes.


As Ford specifies 50mA, and I have almost four times that much, I’ve got some work to do here. It’s easy to keep adding electronics to your vehicle and pay no attention to this. Then, one day you’ve got a dead battery and you’re in troubleshooting mode!


As I anticipated, the current draw has settled down. This is an accurate measurement of the current draw of everything in the vehicle with the vehicle parked. As you can see, our static current draw is 174 mA, and this is 124 mA higher than Ford specifies. Although this doesn’t sound like much, it is. This is the cause of the battery being drained when it sits for an extended period of time. Now, let’s get to the root of the problem.



Believe it or not, even though this vehicle is stuffed full of electronics, this is actually a simple process. We just need to remove fuses individually until we find the circuit (or circuits) that is the source of the current draw. Then, we need to determine the accessory on this circuit that is responsible. When troubleshooting such a vehicle, start with the aftermarket electronics first, then to the OEM electronics.

Step 1:

Since there is an underhood aftermarket fuse panel right near the battery, let’s begin by disconnecting each fuse one at a time. If there is no change in the overall current draw after removing a fuse, then re-insert it. (Note that according to the legend, fuse number 1 supplies power to the interior accessory fuse panel. Since I removed it and there were no changes to the current draw, this rules out all accessories tied to this interior panel.)







Step 2:

Since there was no change to the current draw, we can now proceed to the next step, which is removing the large ANL fuse from the fuse holder. This supplies power to the audio system as well as a second trunk-mounted fuse panel. Note that our current draw was reduced to 49 mA, a change of 125 mA.





Step 3:

Now that we’ve found a major source of current draw, we need to determine the accessory to blame. This necessitates that I reinstall this fuse to proceed.

Step 4:

Around the rear of the vehicle, access the accessory fuse panel on the passenger side of the vehicle and begin removing fuses one at a time as before. In this case, none of them resulted in any change in the current draw, so we can rule out the accessories connected to them as culprits.







Step 5:

This leaves either the audio amplifiers or the capacitor as the culprit for this portion of our current draw problem. To determine this, remove the fuses for both amplifiers from the ANL fuse block to eliminate them. As the current draw didn’t change, the storage capacitor is to blame as it is the only thing left on this circuit.








Unfortunately, this capacitor has a small parasitic drain as a byproduct of its design. I am left with two choices: remove it entirely, or just live with it. As my audio system consumes more current than the charging system can provide alone, and this capacitor really helps to overcome this problem, I elect to leave it in and drive the car more regularly. At the very least, I put a trickle charger on the battery if I do have to leave it for an extended period of time.

Because disconnecting the capacitor dropped the static current draw to 49 mA, below Ford’s spec, I don’t need to look any further. But what if I did, then what? Simple—I would need to continue the above procedures for the interior fuse panel, then the underhood fuse panel until the source of the problem is found. Start with the interior panel because the fuses in the underhood panel typically power multiple circuits. Remember, this vehicle can’t enter the sleep mode with one of the doors open, which is a necessity when accessing the under dash fuse panel. No problem, I can close the switch in the door itself with a screwdriver. This allows the door to remain open, which allows easy access the interior fuse panel.





This begins the countdown to sleep mode. Once the vehicle has entered it, I can resume my troubleshooting. (Just don’t forget to pull up on the door handle before attempting to close the driver’s door. This releases the switch, which in this case also happens to be the latch that secures the door around the striker to keep it shut.)

When you remove a fuse from either of the OEM fuse panels and you see a huge drop in current draw, refer to the owner’s manual to determine which fuse supplies each accessory. If a fuse supplies power to multiple accessories, re-install the fuse and then disconnect each of the accessories one at a time so that you can determine which one is the problem. Typically, you can unplug them, thereby removing them from the circuit. Sometimes interfaces between a piece of aftermarket electronics and the stock electronics results in current draw issues. In these instances, isolate the aftermarket electronics to solve the problem. As long as you have the right tools and a little know-how, it doesn’t take that long to figure out.


Intermittent Circuit Operation

These kinds of problems can be the most frustrating of all. Sometimes the accessory works, sometimes it doesn’t, with no rhyme or reason. Worst of all, Murphy’s Law ensures that it always works when you take it in for someone to look at it.

Example—Power door lock circuit in the 2003 Ford Mustang GT

Symptom—Switch in passenger door works intermittently

This kind of problem is easy to ignore—after all, the switch in the driver’s door works just fine and that’s where I sit, problem solved, right? Not so much. To help with the troubleshooting, I got my hands on a diagram of the circuit (Figure 7-4), courtesy of Mitchell.

At the end of the day, this circuit is fairly simple. Specifically, the circuit has the following components:

  • Two lock/unlock switches.
  • A GEM module.
  • Two door lock actuators.             

Although the diagram doesn’t illustrate this, the following components also exist:

  • Length of wire from each component to the GEM module.
  • Plugs of some type in the kick panel area that facilitates removal of either one of the doors.

Furthermore, according to the diagram, the switches used are of the S.P.D.T. variety and rest open. At rest, they have no electrical connection. (These are simply “center off” switches.) In addition, the diagram shows that the switches switch +12 VDC only and since the actuators require both +12 VDC and ground to operate, we have to assume the GEM module contains some relays to make that happen.

Since we know the passenger switch is the only part of the circuit that is giving us trouble, let’s concentrate our efforts there. I said above that it works intermittently. Specifically, neither the lock or unlock functions work all the time from this switch. (You already know too much; you’re beginning to diagnose the problem already, aren’t you?)



Checking the switch itself.


Meter Settings

  • Selector switch in the DCV Position
  • Red probe in V/Ω
  • Black probe in COM and connected to vehicle chassis ground



Note that, currently, the switch works fine.

Step 1:

Remove the passenger-side door lock switch so that its terminals can be accessed.






Step 2:

Connect the red probe of the DMM to the white/violet wire. This gives us a reading of +11.85 VDC.







Step 3:

Closely inspect this terminal and the connector itself for signs of a poor connection, corrosion, etc. We suspect that this wire is the culprit somewhere between the GEM module and the switch based on our symptoms, but it looks fine. Wiggle the switch/connector assembly while operating the switch in hopes that we can replicate the symptom—no go.

Step 4:

The next area to inspect is the plug above the passenger kick panel area that connects the door harness to the main wiring harness. (I just followed the harness as it comes through the jamb and then goes up into the darkness. Then, I removed the glove box to get better access to this remote area.) As pictured, there are two plugs high up in this area, but only one that contains the door lock wiring.







Step 5:

Unplug the plug and inspect both halves for poor connections or loosely fitting pins.







Step 6:

What do you know? The white/violet wire pulls right out of the plug on the wiring harness side—problem found!







As this pin is actually in a plug with a lock retainer, this is quite unusual. As the pin was still in the plug, it did make connection some of the time to the mating pin in the other plug. And from time to time, it didn’t. This explains why the circuit worked intermittently. To solve this problem, we just need to remove the lock from the front of the plug with a pick tool, snap the pin securely back into the plug, and reinstall the lock.

A problem such as this is likely a manufacturing-related issue and is relatively uncommon, but there you have it. Problem solved!

Now that you’re versed in the basics of troubleshooting, there should be few problems you can’t solve. Again, I hope you have an understanding of the thought process involved in each of the previous examples. Using simple, logical steps is always the best approach and almost always leads to the solution. Knowing what you know now, can you imagine stabbing in the dark with a test light?

Now you know the difference between diagnosing the problem and making the appropriate repairs and replacing parts until you find the one that solves the problem. As I said earlier, there really is no substitute for experience when it comes to troubleshooting automotive electronics.




Written by Tony Candela and Posted with Permission of CarTechBooks




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