I was able to fix this problem so I am posting the problem and a solution in case someone else can take advantage of the alternate fix that avoids removing the transaxle. The problem was on a 2001 Seville SLS with a 4T80-E automatic transaxle. The malfunction indicator light was on and P0741 and P1860 codes were indicated on the drivers’ information screen in the dashboard. (Press and hold two of the driver info buttons at the same time to view the codes on the Seville.)

The trouble codes were:
P0741 indicating a problem with the torque converter clutch solenoid valve.
P1860 indicating a problem with the TCC circuit.

Last summer the Cadillac dealer proposed a new transmission so I took it to a trusted transmission shop instead. The transmission shop verified the transmission was operating properly and suggested the problem was electrical. They said to ignore the light and keep driving. The decision tree in the factory service manual lead to the PCM, so in another attempt to fix the pesky light I had another repair shop replace and program a new PCM which did not fix the problem. They insisted that the problem was with the mechanical components of the transmission. Considering the difference in opinion by the repair shops and after spending the money on a replacement PCM, I got used to the light ... until the car failed emissions due to the P0741 code.

I thought about the benefit of having the MIL light available when I really need it so I decided to look into diagnosing the problem myself. After a thorough reading of the service manual and testing the wiring for shorts or opens, I checked the resistance across the terminals of the torque converter clutch pulse width modulated (TCC PWM) solenoid valve. There should be 10-15 ohms, but was no resistance which indicates a bad solenoid valve. (If the decision tree in the service manual told us to check this, it would have saved the cost of a PCM.) The quote from the transmission shop for replacing the valve came in around $1500 because they said the transmission had to come out to replace it. The transmission shop suggested obtaining a repair waiver because the cost of repair outweighed the cost of obtaining a waiver and the problem does not harm the transmission. The service manual confirmed that the transmission should be removed to replace the valve. The repair shop disagreed with the bad solenoid diagnosis and proposed that they run a diagnostic check on their computer.

Inspired by another post on this site who said the transaxle can be removed without taking out the motor, wanting the MIL light fixed, and considering the savings in gas mileage that I would get when the TCC PWM did its thing, I decided to replace the valve myself. But I decided first to try to replace the solenoid without removing the transaxle and it worked. So when someone else has this problem here is the procedure. I hope someone is able to make use of it.

PROBLEM:

MIL light on with both codes P0741 and P1860.

TROUBLESHOOT:

Step One: Test the TCC PWM solenoid valve on the transmission side of the AT inline 20-way connector with an ohmmeter. There should be 10-15 ohm of resistance. The two terminals are U and T, which are the two pins to the right on the upper row when viewing the connector with the alignment tab to the right. If there is zero or infinite ohms resistance the solenoid is most likely bad. The wiring between the connector and the solenoid could also be shorted or open. (In my case there was zero ohms.)

Step Two: If the solenoid and wiring on the transmission side are good, look at the wiring on the engine side of the connector. When viewing the connector with the alignment tab to the left the connectors are the two left holes on the upper row. There should be no voltage with the ignition off and voltage with the ignition on and the car not running. Disconnect the C2 connector at the PCM and check for shorts from the upper left or T terminal and other wires or to ground to verify the wiring is OK.

Step Three: If the wiring and solenoid are good, then look into the PCM.

FIX:

To replace the TCC PWM solenoid with the transmission in the car. Verify that there is enough room to get your fingers between the transaxle side cover and the car body, because after moving the side cover, you will need to get at least one finger in there to press in the hairpin.

Remove the parts required to get at the side cover and everything between the side cover and car body. You will need good access to the upper rear corner of the side cover. Remove the upper engine mount in the left front wheel well. The lower bracket can stay in place if the frame studs are removed from inside the engine compartment. Loosen the side cover and catch just over a quart of transmission oil that comes out. Move the side cover open as far as possible. It cannot be removed. The TCC PWM solenoid is located at the upper rear corner and is attached with a hairpin that is just behind it. You should be able to feel the end of the hairpin with one finger. It will move up and down easily. You can see the end of the hairpin with the aid of a mirror on a stick.

Pull out the hairpin with a pick or bent needle nose plier. If you use a pick it will fall to the bottom of the side cover where you must retrieve it. Replace the solenoid.

Now for the fun part. Wrap a piece of tie wire once around the end of the hairpin. The hairpin should slide out of the loop easily. Then run the end of the tie wire thru the hole between the end of the hairpin and the tie wire so that the hairpin is held in place. Pulling the end of the tie wire out of the hole will allow the tie wire to release from the hairpin later. With the hairpin dangling from the tie wire you can get the end of the hairpin started in the hairpin slot and get a finger on the end of the hairpin, then pull the end of the tie wire out of the hairpin and the tie wire will come loose. Use a gentle push on the end of the hairpin to install it and confirm the installation with a mirror.

Reassemble the car and add just over a quart of new transmission fluid.

 

There's more than one way to skin a cat, eh?
Great job there. I'll bet the solenoid was melted, or at least had evidence of overheating.
The real problem, if the solenoid is melted, is NOT the solenoid itself. It is the seals for the input/turbine shaft of the trans. What happens is they get hard over time, and no longer give a good seal for the fluid pressure to engage the Torque converter clutch(TCC). When this happens, the PCM tried to modulate the solenoid to increase the fluid flow going to the TCC to keep it from slipping. Eventually, the PCM cannot control the slippage, so the TCC solenoid winds up being "ON" 100% of the time the TCC is supposed to be engaged. This creates major heat in the solenoid, which will cause it to pretty much melt. I've had some that required harness replacement due to the plastic connector for the solenoid melting to the solenoid connector.
I hope the new solenoid corrects the problem.

 

Thanks for your suggestion. It was a good thing that the TCC PWM solenoid wasn't melted or I would have had to pull the transaxle. There was a hairline crack in the solenoid plastic because it was old and brittle. It was probably due to expansion and contraction due to heat. I verified the operation of the TCC clutch with a drive test so the turbine shaft seals must be working.

I case someone in the future needs the TCC test: Operate the car in drive (D) at a constant speed of 50 mph. Leaving your foot on the gas, observe the tach while tapping the brake lightly with your other foot. The tach should go up briefly and then down. The rise is is a result of the TCC disengagment which allows transmission slip. The drop is because your car slowed down when you taped the brakes. You can also see the tach drop when the TCC engages after the test.