By Ralph Kalal

Replacement of transmission fluid ordinarily does not include draining the torque converter. The procedure specified in many factory shop manuals for changing transmission fluid is merely to drain and clean the sump, and then install enough new fluid to replace what was drained.  

But, what if you want to replace the old fluid in the torque converter, too?

On some vehicles, particularly domestic makes, it is impossible to drain the old ATF from the torque converter without operating the engine and turning the converter, that pumps out the contents. However, as this is occurring, new ATF must simultaneously be added to replace what is being pumped out. On other cars—usually European brands, but occasionally on domestic makes—there is a drain plug in the torque converter itself, which allows draining it with the engine off.  

On these vehicles, there will be an access panel in the torque converter “bell housing” that can be removed to expose the drain bolt, which is normally recessed into the converter. It will be necessary, however, to rotate the crankshaft to bring the bolt to the bottom of the converter’s rotation. This is done by putting a big breaker bar and socket or very large box end wrench on the pulley nut located at the front end of the crankshaft. Then remove the drain plug and let the converter drain into a drain pan. When it’s empty, retighten to the torque figure specified in the factory shop manual.

On most cars, though, it isn’t that simple because there is no drain plug in the torque converter. The only way to fully drain these torque converters is to have the converter pump the old fluid out of itself. Here’s how that’s done:

This requires at least one assistant and preferably two. Also, be sure that you have an ample supply of the required ATF on hand.   You’ll need more than the complete capacity of the transmission because some will be poured into the sump, some will go into the torque converter, and some will be pumped out with the old fluid.

After changing the fluid in the sump pan as described in this chapter, disconnect the return line from running from the transmission cooler part of the radiator to the transmission. There are two lines, one to the cooler and one return. Check the factory manual to identify which is the return line. If it is a flexible line, disconnect it at the transmission. If it is not a flexible line, disconnect it at the cooler. It is important to get the return line, because that empties the transmission cooler, as well. These lines are frequently connected with quick-connect fittings and a special tool may be necessary to remove them (see Chapter 1).  

Attach a length of hose to either the flexible line or to the cooler fitting. Route the hose to a container large enough to hold the contents of the torque converter plus a quart or two. You will need to be able to see the fluid pouring into the container.

One person then starts the engine, as another person watches the fluid being drained to see when fresh fluid begins to flow out, and the third person adds fresh fluid as fast as the old fluid is pumped out of the torque converter and drained from the cooler. On some vehicles, fluid is not circulated in the torque converter unless the transmission is in a “drive” gear, so this process requires having the vehicle in gear, with the parking brake engaged and someone’s foot on the brake pedal. But, on most cars, the fluid will circulate in the torque converter with the transmission in “park.”

As soon as fresh fluid starts draining, turn off the engine. Reattach the cooler line(s) and top off the transmission to the full mark with fresh fluid.  

That’s how the theory reads. But on many cars, it’s just not practical. When auto manufacturers claim automatic transmission fluid lasts for the vehicle’s “lifetime,” they have no reason to worry about making the transmission fluid cooler lines easily accessible, and quite often they are not.

But, if you still want to get that old fluid out of the torque converter and can’t access cooler lines, there are two alternatives. First, you could just change the fluid a couple of more times. Sure, that’s messy. But, if you’ve got a reusable sump gasket, it won’t cost more than the price of the extra transmission fluid. You won’t be able to get all of the old fluid out, but you’ll be able to come pretty close. Second, after changing the fluid, you could use a hand pump to suction out fluid from time to time, replacing as much as you remove. That saves you the trouble of dropping the pan and, eventually, you end up with the same result.