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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.


Automotive Wiring and Electrical Systems

 

This tech tip is from the full book, AUTOMOTIVE WIRING AND ELECTRICAL SYSTEMS. For a comprehensive guide on this entire subject you can visit this link:

LEARN MORE ABOUT THIS BOOK HERE

SHARE THIS ARTICLE: Please feel free to share this post on Facebook Groups or Forums/Blogs you read. You can use the social sharing buttons to the left, or copy and paste the website link: https://www.cartechbooks.com/blogs/techtips/automotive-electrical-troubleshooting


 

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:

     

    Procedure

    Checking the bulb itself.

     

    Meter Settings

    - Selector switch in the Ω Position

    - Red probe in V/Ω

    - Black probe in COM

     

    Step-by-Step 

    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.

     

     

    Procedures

    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-by-Step

    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.

     

     


    Automotive Wiring and Electrical Systems

     

    This tech tip is from the full book, AUTOMOTIVE WIRING AND ELECTRICAL SYSTEMS. For a comprehensive guide on this entire subject you can visit this link:

    LEARN MORE ABOUT THIS BOOK HERE

    SHARE THIS ARTICLE: Please feel free to share this post on Facebook Groups or Forums/Blogs you read. You can use the social sharing buttons to the left, or copy and paste the website link: https://www.cartechbooks.com/blogs/techtips/automotive-electrical-troubleshooting


     

    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.

     

    Both reverse lights are working now, but this is only a short-term fix. The long-term solution is to replace the bulb socket. 

    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.

       

    Procedures

    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.)

     


    Automotive Wiring and Electrical Systems

     

    This tech tip is from the full book, AUTOMOTIVE WIRING AND ELECTRICAL SYSTEMS. For a comprehensive guide on this entire subject you can visit this link:

    LEARN MORE ABOUT THIS BOOK HERE

    SHARE THIS ARTICLE: Please feel free to share this post on Facebook Groups or Forums/Blogs you read. You can use the social sharing buttons to the left, or copy and paste the website link: https://www.cartechbooks.com/blogs/techtips/automotive-electrical-troubleshooting


     

    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!)

     


    Automotive Wiring and Electrical Systems

     

    This tech tip is from the full book, AUTOMOTIVE WIRING AND ELECTRICAL SYSTEMS. For a comprehensive guide on this entire subject you can visit this link:

    LEARN MORE ABOUT THIS BOOK HERE

    SHARE THIS ARTICLE: Please feel free to share this post on Facebook Groups or Forums/Blogs you read. You can use the social sharing buttons to the left, or copy and paste the website link: https://www.cartechbooks.com/blogs/techtips/automotive-electrical-troubleshooting


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