The ever popular TH125


There are a bunch of things about the TH125c transmission that need to be kept in mind when working on it. This is a collection of the issues asked about in the forum. Itís by no means a complete list of things. It does however cover some obscure things needed by customizers.


TH125 Torque Converter Lockup.


TH125c has a locking torque converter system controlled by 4 electrical items. They are listed in the order of their location in the circuit. (Starting at the fuse end)

  1. The Brake Pedal Switch. This switch opens to unlock the TC when you press the brake. It is on the brake pedal hardware.
  2. The Third Gear Switch. This is inside the transmission. It closes only when the transmission is in third gear.
  3. The Converter Lockup Solenoid. The device closes a valve that pressurizes a clutch and locks the converter.
  4. The ECM. The ECM switch closes a connection to ground only when the car is doing a fairly constant speed above 35mph.


There are 2 tests for problems with the Torque Converter lockup system. These tests should BOTH be done before any shift problems are looked at.

The first, and easy, test for TC lock is to press the brake while doing a constant speed over 35mph. (The test WILL NOT work if you are accelerating or slowing.) You only need press the pedal enough to activate a switch. (About 1") If the TC is working right you should see the tachometer jump up about 200RPM then drop again when you let off the brake.

The second TC test is to pull the TC lock's electrical connector. It's on the front of the transmission. Unplug it and see if you still have problems.

The second test is very important if you are stalling when stopping the car or if the car bucks badly when slowing. There are known issues with the Torque Converter Lockup Control Solenoid. This device can hang the Torque Converter Clutch in the locked mode, usually when the transmission is hot.


If the second test stops the problems then the TC control solenoid likely needs to be replaced. This repair can be done with basic tools. Itís not hard just very time consuming. The Solenoid is under the side cover of the transmission. While replacing the solenoid itís a good idea to replace the Third Gear Switch, which is next to it. Then youíll have all new electrical parts in there.


If you donít see the RPM jump while doing the first test you may have a TCC apply problem. This could be electrical, TCC solenoid, TC clutch or hydraulic problems.


The test for the brake switch is in the Manual lockup section at the end of this page. The test for the Third gear switch is a bit different. If you watch ALDL terminal F youíll see 12V when the TG switch closes but then youíll see it drop voltage when the switch in the ECM closes. At this point youíll be able to watch the ECM lock/unlock the TC depending on driving conditions. This only tells you that the electrical system is OK. You could still have a hung TC solenoid or other problems in the transmission.


Once the electrical problems are ruled out then itís either solenoid hydraulic or the torque converter itself. Hydraulic or TC problems are beyond most people but if you do work on this yourself the best source of information on this is a GM document that covers the whole TC lockup system step by step. It was provided to trainees at a GM seminar and is not part of the shop book. It is however on ALLdata disks. You may be able to get this from a friendly shop. It might even be on the Popular Mechanics CDís made by ALLdata that are available a Wall/K mart etc.

Shift Problems.

Do the Torque Converter tests above. Often what feels like a shift in TH125 is really the TC locking or unlocking. If there is a problem with the TC lock then it can feel like you've lost a gear. Does the problem happen with the TC lock plug disconnected?

Does the problem happen at the 1-2 shift, the 2-3 shift or both? If it is only one shift then you very likely need a shop to work on it. You can try the stuff below but donít be surprised if nothing helps.

First of all make sure that the transmission fluid is at the right level. Over or under fill will both cause shift problems. What condition is the fluid in? If itís turned light or even clear then itís oxidized and you might want to try changing the fluid and filter before taking it for repairs.If itís brown to black or smells different from new fluid then the transmission may have fried a clutch. You could try changing fluid but you should probably take it to a shop to be checked out.

The next item to check is the Throttle Valve Cable, which runs from the throttle linkage on the engine to the top of the transmission. This cable IS NOT a ďkick downĒ cable like many people are familiar with. It works with an internal governor to control all transmission shift points. If this cable is binding or out of adjustment you will have shift problems.

Another thing that happens is a passage inside the valve body gets blocked with a flake of dirt or sludge. This can affect one shift or the whole transmission. You can try one of the cleaning products that you add to the fluid. Some people have reported good luck with these. Depends on just whatís blocking the passage if it will work or not.If this fixes your problem then do a fluid and filter change to get the rest of the dirt out.

Another possibility, although rare, is that the filter has failed or come unsealed inside the tranny. The result is that the fluid is picked up higher in the transmission. This can have the same result as low fluid.


Shift Kits: These kits WILL NOT fix a transmission with problems. Worse they could hide some problems. Be sure that your transmission is good before adding the kit.

Source of parts and manuals besides GM dealer: Transmission Exchange Co. they have full rebuild kits as well as shift kits and TC lock delay modules.

Throttle Valve Cable

The adjustment procedure for the TV cable is in the Haynes book and is correct for all years. Note however that the location of the adjuster moved on the DIS (87-88) 4 cylinder setups. It is now on the transmission end of the cable.

Fluid & Filter changes.

There is a way to significantly reduce the mess associated with this job as well as get the maximum amount of old oil out of the transmission. Iíve used it a number of times and it works well.

The transmission will stay oiled well enough to prevent damage but donít take chances. Follow the warnings. This method will get a good amount of the oil normally held in the Torque Converter. The most oil Iíve gotten out of the TH125c using this method is 6.5-7 quarts of the 9 possible.

Many people say you need to flush out the rest of the old oil. My feeling is that you only need to do that if the old fluid is burnt. If the old fluid is burnt then you likely need other major work done. Flushing is extremely wasteful for most situations.

WARNING! The following procedure can be dangerous. It should not be performed on a hot transmission. Use of proper blocking and jack stands is a must. Do not allow transmission oil to run onto hot exhaust parts.

This is not a GM approved method. There is the remote possibility you can damage the transmission. DO NOT shift the car from park during this operation! DO NOT allow the engine to run once oil flow has stopped. DO NOT GET UNDER THE CAR WHILE IT IS RUNNING!

If you are uncomfortable with this then you could suck the oil out thru the dipstick tube. That will empty the pan but wonít get near the same amount of oil out.

1.     With the car raised on jack stands and the front wheels blocked. Disconnect the bottom cooler line from the transmission.

2.     Install a temporary piece of line long enough to reach a catch basin under the car. Make sure the line is aimed so oil canít spray out of the pan. The oil will flow at a small amount of pressure. Anchor the line in the pan so it wonít jump out.

3.     Start the car and let it idle until oil flow stops. Shut off the car as soon as oil flow stops. DO NOT GET UNDER THE CAR WHILE IT IS RUNNING!

4.     Let the car stand 5-10 minutes and then repeat step 3 one time.

5.     Disconnect the top cooler hose and put the bottom line in a jar. Use low pressure, not more than 15-20PSI, compressed air to blow out the lines.

6.     Reconnect the cooler lines to the transmission. (REPLACE the rubber lines if they are at all damaged!)

7.     Continue with the rest of the filter change procedure. Donít forget to clean the magnet in the oil pan. (Note: Insert the filter bushing in the transmission then insert the filter neck.)

8.     Take the car off the jacks.

9.     When filling the transmission pour in 2-3 quarts of fresh Dexron III oil and start the car. Check the dipstick. Pour in more oil until the stick reads just below the full zone. (The oil is cold. You donít want it to be over full when hot.)

10. Start the car and let the transmission warm up a few minutes. Put your foot on the brake and shift thru the gears. Check the fluid again. Drive it a block. Go fast enough for the car to hit third. (Donít race!) Then recheck the fluid level one more time. Add more if needed to get the level to the middle of the full zone.


If the magnet is missing from the oil pan replace it. Any flat magnet(s) 3/16 to 1/4 inch thick will work. (Donít use those thin ďrefrigeratorĒ sheet type magnets. They arenít very strong.) Just make sure itís near the dimple in the pan used to locate the original magnet. That way it will be out of your way when you put the pan back on. The magnet is part of the oil filtration. It catches the iron flakes generated by the clutches.

When installing the filter put the bushing in the transmission opening THEN push the filter into the bushing. If you try it the other way it wonít go.

The filter has changed over the years. Follow the directions included with the filter to alter the support clip on the older transmissions. If you forget this then the filter will obstruct the oil pan. I believe this only applies to 84-85 and maybe some 86 transmission.

Dexron III

GM introduced Dexron III in 1995 but many stores only recently started to carry it. Per GM literature Dexron III oil replaces Dexron II (and IIE) and is compatible with all GM passenger and light truck automatic transmissions built since 1949. (GM TSB 57-02-91 03-1995)

It provides improvements to a number of characteristics and GM claims it requires no service. On cars that came with it, or freshly rebuilt transmissions, I could believe that but on older cars it should at least extend the service interval, especially in situations where it is likely to have some Dexron II mixed into it.

Yes, this means it is OK to add Dexron III to top off transmissions filled with Dexron II.

Manual TC lockup.


Warning! Manual TC locking presents a number of problems and can damage the transmission. It is potentially dangerous to drive with a manual lock active. Manual locking should ONLY be considered when the ECM has been removed such as when a carbureted engine has been installed.


Notice! This is an EXPERT level article and it assumes the ECM was unplugged and that the wiring is intact. If the harness has been cut it is up to you to return it to operating condition. This topic is only covered as an aid to installation of non-stock engines.


Manual locking of the TC WILL NOT help performance. Manual locking is used only help gas mileage on cars without an ECM. Even then it is preferred to find an automatic solution to the problem. On cars with an operational ECM, delaying the locking of the TC can help performance. This company can tell you more about that-> Transmission Exchange Co.


Before you can install a manual lock. You need to test the brake and third gear switches. BOTH switches Must work. Without both switches you cannot wire the manual switch without damaging the transmisson.

There are 2 wires on the TC lockup connector. The first wire, according to GM schematics, comes from a switch on the brake pedal. The other wire coming from the transmission connector is ground. The ECM used to ground that wire to activate the TCC. Your new switch will replace the ECM.


The brake switch. Disconnect the transmission plug and hook a test light to the wire that is always hot with the key on and hit the brake pedal. The line should go dead. If it doesn't you can't wire the switch manually. The Brake Switch MUST work or you'll have nothing but problems. Dangerous problems.


The third gear switch. This switch must also work or the TCC may try to activate as soon as you take your foot off the brake when stopped. This is not a good thing. (Same effect as dumping a clutch every time you take your foot off the brake. You WILL brake the tranny like that.)


To test the third gear switch connect a voltmeter between the TC ground wire and body ground. TCC ground is Terminal F on the ALDL. Put the meter where it can be seen while driving and have a passenger watch it. When the car goes into third the meter should swing up. Warning! You MUST use a voltmeter for this test! Test lights may pass enough current to allow TC locking. This is not a good thing just now. This wire should only be hot when the transmission is in third gear. If you see a voltage on the line when stopped then the third gear switch is bad.


The manual lock switch is connected between the transmission ground wire and the vehicle ground. When the switch is closed the Torque Converter will lock anytime the transmission is in third gear.


Manual locking presents several problems!


You want to avoid using the manual lock except for highway driving. You don't want the tranny locking as soon as it hits third gear, which is exactly what it will do with the switch on. If the TC locks at the same time you hit third you'll likely tear up the tranny. (You also lose some performance that way.)


If you get into heavy traffic you'll want to leave it unlocked, even if the tranny stays in third.


The most obvious method to do this is a simple toggle switch. Problem is a toggle switch doesnít allow for the driver forgetting itís on. If the toggle is on then the transmission will lock as soon as it hits third gear, which isnít what we want for either reliability or performance.


The preferred way to restore TC locking is to make a speed sensor for the circuit. That way the TC will never lock below a specific speed. The ECM cutoff was no lock below 35. In most cases setting the cutoff for no lock below 35-45, and add Transmission Exchange Co.'s TCC delay unit, would return auto locking in the safest way possible. Ideally the speed sensor should only lock at relatively constant speed. It should unlock when speed changes rapidly.


A second option is to make a control with a push button. When the button is pressed the TC locks but when the brake is pressed or the transmission goes out of third gear the circuit resets and leaves the TC unlocked. This gives the same result as a simple toggle switch on the highway without some of the safety and drivability problems. (This circuit is Very easy to do and really cheap.)


Either control circuit would be fairly easy to do. There's a 2000ppm feed back to the ECM connector from the dash. There's any number of ways to make that feed close a switch at a particular speed or above.


All documents in this area are Copyright 1999,2000 D.W.Lane All rights reserved. Not for reproduction in any media without written permission.