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If you have a GM front-wheel drive car, with a 125C
(3 speed automatic transaxle) that stays in lockup, when hot, as you approach a stop,
and may kill the engine when coming to a complete stop, then you
may have a defective lockup solenoid. It is a very common occurrence
for a 125C to develop this problem, and judging by the letters
I have received, is common enough to warrant writing an article
about the subject.
Replacing the lockup solenoid on a 125C is relatively easy for a transmission technician, and within the realm of an experienced, and thorough "backyard mechanic". The lockup solenoid is located on the valvebody/pump assembly, within the side cover of the transmission, located on the driver's side of the car, and is accessed mainly through the wheel well. On most applications, it is easy to remove the cover, requiring removal of the wheel, the plastic shields covering the side cover, and, in some cases, removing the motor mount assembly on the transmission (driver's side) of the vehicle. In some cases the subframe must be loosened (not removed; just loosened) to gain more room to remove the side cover. Consult your manual for more exact instructions applicable to your car.
Once you have removed the side cover, and gained access to the solenoid assembly, you will need a torx-bit (number 30, I think, but please do not quote me…I am doing this from memory.). Remove the two 10mm-head bolts that retain the TV pressure bracket assembly, and move the bracket out of the way (you can usually leave it hooked to the cable; just move it out of your way). You will need to unhook the wiring loom from the connector on the inside of the aluminum transmission cover. There is usually a pressure switch (sometimes more than one) that grounds the lockup circuit; while you are in there, spend a few extra dollars and replace the switch, unless you have the means to check the switch, in which case you may want to check and replace if it is bad. Reassemble in reverse order. Torque the 10mm-head bolts to 75inch pounds.
The solenoid, as well as the switch is available at most transmission shops and at the GM dealership. There are some aftermarket solenoids on the market, but I prefer the use of Genuine Delco parts, as I feel that they may be of greater quality than the after market parts I have seen. Make sure to match the parts exactly; There are a couple of solenoid configurations for all of the GM front-wheel drives, and you can easily get the wrong one. Another one of Murphy's Laws.....
When you are ready to install the side cover, tap the bolt holes back with a small ball-peen hammer (where the holes warp from tightening the bolts at the factory). Install the new gasket (obtainable from any transmission shop) with no sealer. I prefer the black "fairpak" gaskets, but cork will do, as long as there is absolutely no oil, silicone glue, or weatherstrip adhesive, etc. on the gasket or gasket surface. Make sure that the gasket surface of the side cover, as well as the case, are clean and dry (I know it is hard, because fluid is dripping from the transmission, but I jack up the driver's side of the car a little more, just before installing the side cover; this usually will stop the transmission from dripping). Moderately tighten the bolts, installing all of the bolts before tightening any; these bolts are only 8mm shank bolts, and the side cover is only sheet metal, so do not get too carried away tightening the bolts.
There are a few different configurations of lockup solenoids, as well as switches. The switch, that you will be replacing, is, most likely, a normally open, single-pole-to-ground switch. This means that the switch has only one wire connection, and that connection will not connect to ground, until there is pressure at the port that the switch is screwed into. 12volts is fed from the brake switch (it should have power when the key is on, and the brake is not depressed) down to the "hot" side of the solenoid. The "ground" side of the solenoid will then hook to the switch. When the switch port is pressurized, the switch will then ground the solenoid, which will then allow the converter to lock-up. How does the clutch lock up the torque converter?
When you are through, confirm lockup operation (How?), check for leaks and, if all's well, you are done......well almost......; You must then tell all of your friends about my page, and then take me to dinner... :-) .....If all is not well, don't tell your friends about my page; chances are that I did not explain the operation well enough!!!