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Torque Converter Lock up

When a lock up converter is used, it locks at whatever speed is programmed into the chip.  This is usually in the range of 40-50 mph.  It should unlock whenever one lets off the gas, or touches the brake.  It also normally unlocks when one accelerates sharply, or, at wide open throttle.  Some chips offer a programming option to input a specific mph at which the converter will lock even at wide open throttle.  This usually picks up a little speed, but it is hard on the converter.

In normal operation, the torque converter is locked up when the tan/black wire from the solenoid is grounded by the ecm.  By inserting a switch between Terminal F of the ALDL connector and ground, one can manually lock up the torque converter by flipping the switch and completing the circuit to ground. 

If the converter refuses to lock up under normal operation, or when forced by a manual switch, one has to determine if the problem is in the circuitry, or in the solenoid, itself.

There are two problems that occur fairly commonly with the TCC circuit.

One is that the solenoid in the transmission stops working and it leaves the converter locked all the time.  This is easy to notice as the car will not want to accelerate easily from low speed and you will notice a lot of "drag" when you let off the gas similar to driving a manual transmission car.  Also, when it downshifts from third to second to first, you will usually feel a "thud" as it connects with the next lower gear.  Finally, the car may idle roughly and not feel right as there is little slip in the converter and the car will want to move more than normally when sitting with your foot on the brake.

The fix for this problem is to remove the pan from the tranny and replace the torque converter lock up solenoid.  Some builders suggest using a different solenoid which reportedly is more rugged.  Ask your favorite transmission builder for his suggestion.

The second problem that frequently occurs is that the converter does not lock when it is supposed to.  Most often this is a problem with the electrical circuit that supplies power to the solenoid in the tranny.

Look at this circuit diagram:  If you understand circuit diagrams, you may cut to the chase and check it out without going thru my suggested routine below.

The lock up circuit is on the left side of the above circuit diagram.  Power comes from the ECM-SOL fuse in the fuse block and goes to one of the brake switches mounted against the front edge of the brake pedal arm.  It comes through the switch and goes to the connector on the side of the transmission under the car.  Inside the tranny it goes to the tcc solenoid and then returns back thru the connector to the ECM which controls the lock up and unlock by grounding the circuit.

Troubleshooting-I do it this way if the converter is not locking.

The first thing I normally do is to ground terminal F on the aldl connector while driving the car.  If this does not cause the converter to lock, I lift up on the bottom of the brake pedal to see if the converter then locks at about a 50 mph steady cruise.  If it does not, I proceed as follows.

Turn the key on to Run.  Pull the connector off the side of the tranny and see if the purple wire in the connector which comes from the brake switch has battery voltage on it.....use a test light or a volt meter to do this.  If it does, then you know that the fuse is good, and that the brake switch is working as it should.

This means that the problem is most likely in the TCC solenoid itself as it did not lock when I tried grounding the aldl terminal F.  Be sure to look at the tan/black wire to make sure it is not broken.  If you ground terminal F, you should be able to verify a ground in the tan/black wire from under the car with your meter.  Then go to A7 on the ecm connector and see if that end shows a ground while terminal F is grounded.  If there is no problem with the wiring, then I would change the TCC solenoid.

Now, if the purple wire at the tranny plug did not have power when you checked it, then the problem is some place between the fuse block and the plug.  Go under the dash to the brake switch (the one with a pink/black and purple wire in the connector).  See if the pink/black is hot with the key in the Run position.  If it is not, then check the fuse in the fuse block (Ecm-Sol).  If you do find power in the pink/black wire going to the connector, then check the purple wire.  Is it hot?  If not then be sure the switch is adjusted so that the top of the brake pedal lever is full depressing the plunger on the switch back into the switch. 

If it is, then it would appear the switch is bad.  Sometimes the switch can be fixed by taking it apart and polishing the brass contact inside if it is burned and by rotating the plunger.  Check it with your meter before reinstalling to make sure it now has continuity.  Reinstall, adjust so that the plunger is fully depressed.

You should now have power at the connector on the tranny on the purple wire.  Hopefully the converter will now lock, or you will have to proceed to recheck the rest of the circuit.