1994 GMC Suburban TCC Brake Switch

There's no '94 Suburbans in the list of unanswered questions so either it didn't get posted or someone answered it but you didn't get the automated e-mail with the link back to that page. You can search by vehicle brand and model to see if your post is listed, then click on it to see if there's a response.

29 - Quad driver module (computer) 4th gear circuit

As you can see, diagnostic fault codes can be not very descriptive. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. There are entire books that cover just the codes and the test procedures to solve them. GM references "quad drivers" a lot. I only know them as the circuits in the computers that run things. The circuits in other brands of computers work the same way, but they use considerably different terminology. To me, this code implies there's a problem in the computer itself, but I would never suggest you replace it as a test unless you have another one already. The proper procedure is to perform a few electrical tests on that circuit to rule out things like a wire being shorted to ground or cut open. Most of the time there are safeguards built into the computers to protect the driver transistors when a wire or solenoid becomes shorted, so logic would dictate a grounded wire or shorted solenoid would not damage the computer, but you still don't know if the fault code refers to a defect detected inside the computer or if it's referring to an external defect affecting that driver circuit.

You didn't say what kind of problem you're having with the torque converter clutch so I can only tell you how the system is supposed to work. I have to defer to my Chrysler experience but almost all of them work the same way. The engine needs to be almost fully warmed up, and the transmission has to be in the highest gear, and for some 4-speed models, it will operate in third gear. Typically the clutch will disengage near wide-open-throttle because a torque converter automatically allows the engine to develop double the torque as with a manual transmission, but that advantage is defeated when it's locked up, in favor of better fuel mileage. You want that higher torque when pulling a trailer up a steep hill while trying to pass a freight train! It will also unlock when you fully release the accelerator, in preparation for coming to a stop, and presumably to allow the vehicle to coast further with less engine drag. The clutch will unlock if engine temperature gets too high so engine speed will pick up and the water pump, and possibly the radiator fan runs faster. The last thing to cause unlocking is hitting the brakes. The assumption is you'll be coming to a stop.

Two common causes of intermittent unlocking are a glitch in the throttle position sensor's reading, and a misadjusted brake light switch. Intermittent problems with the TPS seem to be more common with diesel engines, but that might be a misconception on my part. There's usually two, and often three separate switches built into the brake light switch so you can't judge its operation by observing the brake lights flickering on and off while driving over bumpy roads. Intermittent cutout of the cruise control is another result of a misadjusted brake light switch.

To identify a misadjusted brake light switch, hold the brake pedal up with your foot. If the intermittent problem stops acting up immediately, suspect the switch adjustment. Next, press the pedal down about 1/4" to 1/2" while driving. If the cruise control kicks out or the torque converter unlocks with that little pedal movement, again, suspect the adjustment.

A different common complaint is engine speed periodically jumps up 200 rpm, then goes back down a couple of seconds later. To verify that is due to the torque converter clutch unlocking, especially when you can only hear it and you don't have a tach. On the dash, hold the accelerator and vehicle speed steady at highway speed, then tap the brake pedal with your left foot. The increase in engine speed should feel the same.

Chrysler developed the first torque converter clutch for their big block engines in 1977 and all of them in 1978. They were strictly hydraulically-controlled through internal valves in the valve body. Today they all work the same way mechanically inside the torque converter, but computer controls switch a solenoid on and off to control fluid flow to the clutch. That provides more precise control over a wide range of variables. I read a lot about people having fault codes related to the solenoids, but for most of those posts that I follow, replacing the solenoid doesn't solve anything, and most of the people don't notice any symptoms. Often they aren't even aware the code is in memory until a different problem sets a code that turns the Check Engine light on. Looking into the new problem reveals the TCC code that has been there a while.

Chrysler had a lot of trouble with their first batch of lockup torque converters. I learned about that because I had one that acted up at 40,000 miles. After those first couple of years it has been extremely rare to have a failure inside the torque converter on any brand of vehicle.

I just looked and your two posts are the first two listed for Suburbans, and you were in a conversation that came to an end for some reason. You listed a different fault code number too. Here's the link in case you lost it:

https://www.2carpros.com/questions/1994-gmc-suburban-tcc-brake-switch

38 - Brake input circuit fault

You'll find variations of code descriptions depending on where you look. Your version said the switch was "stuck off". That can be misleading because a switch can be on or it can be off when the brake pedal is released, and a different part of it could be on or it could be off when the pedal is pressed. On the scanners, Chrysler lists switches are "pressed" or "released". That's easier to understand when you see one incorrectly listed as "released" when you're pushing the pedal

In the case of almost all cruise control circuits, its switch within the brake light switch is turned on to make contact when the pedal is released. The switch turns off to break the circuit when the pedal is pressed. They do it that way so if the contacts become arced or pitted, the cruise won't set at all. If they did it the other way and required the switch to turn on to cancel the cruise, an arced contact would prevent it from working, the cruise would not kick out, and they'd find you whimpering in the next county before the truck stopped!

There's a different possibility that has come into play on newer trucks, and possibly cars, and by "newer", I mean much newer than what I drive, like a '94 model. If you hold the brakes applied when starting to move, as in when driving off a drive-on alignment hoist, you can set a fault code related to the brake light switch. The Engine Computer doesn't know you have the pedal pressed. It just knows the brake light switch is activated when it shouldn't be. The code is set because the computer knows it can't rely on it to give it correct information, so it's not going to allow the cruise control to operate when it can't get a turn-off signal.

This could also be caused by a switch that's out-of-adjustment or one that has arced contacts. A brake light switch doesn't get out-of-adjustment on its own or need periodic readjustment, so if nothing was done to it recently, intermittent problems are generally going to be electrical in nature, like arced contacts, not mechanical in nature, like adjustments. The notable exception to this wise observation is on import vehicles that have small rubber caps on the plungers on their switches. Those often fall off, then the switch turns on too easily or the brake lights stay on all the time.

Congratulations on wanting to go into this field, (I guess)! I used to teach Automotive Electrical, Brakes, Suspension and Alignment, and Engine Repair and Rebuilding. The first three were my specialty areas. Holler if you need help or have questions related to them.