TCC will not stay locked up

2004 Silverado 2500hd 6.0 4l80e 4x4

Let’s see how good you guys are. I have stumped a lot of people with this.

After I bought my truck, I noticed driving at speeds 65mph or higher the converter would cycle in and out of lock-up for no apparent reason. This eventually got worse - now cycles above 55.

I did the easy stuff - changed trans fluid and filter, flushed the cooler. Filled with dexron VI. No change.

Replaced throttle pedal position sensor. No change.

No DTCs, no slippage.

Took truck to a good local mechanic. He said all pressures are within spec, all sensors within spec. He replaced vehicle speed sensor and pcm. No change.

I later installed sonnax 34994-01k tcc regulator valve/seal kit, replaced valve body gaskets and seperator plate. No change.

Here is the kicker.

When I pull my 5th wheel camper the trans works like a dream. The weight seems to make the computer happy. When I pull my car trailer, the trans only works correctly when I have the weight of a car on it. When I pull it empty, it still comes out of lock-up.

Thanks for your help everyone.


I am not by any means a transmission tech - but for reasons I’d rather not discuss I’m familiar with some of the more common problems with GM electronic transmissions.

I’m wondering if the kit you needed was the 34994-18K ( rather than the 34994-01k.

That one corrects for valve body wear.

The kit I installed does correct for wear, just minus the steel sleeve. The replacement valve has an extra groove cut into it with a teflon seal to ride on the bore. I installed this just by chance it would help. It made absolutely no difference.

The 18K kit is for when the teflon seal won’t cut it. At least that’s the way sonnax tells it. Just putting it out there as a possibility - since it is quite peculiar given that none of the normal things are involved.

Of course, it still wouldn’t explain why it doesn’t act up while hauling the camper.

My mechanic and I both think it is electronic. The last time he had my truck, his scan tool was showing the computer opening the circuit for the solenoid with no definite reason why. I put that valve in “just in case.” I didn’t think it would fix it, but I wanted to be sure. If it was leaking past, it should continuously cycle. right?
The truck will come out of lock up every time I drive it, but it’s not entirely predictable when it will.

Simple thought though it may be did you happen check out the brake switch? I still can’t think of why a problem there would make any sense with the info about the trailer pulling. But the stop light switch is also a TCC switch. I believe it breaks a circuit contact (though you’d need a wiring diagram to find out exactly how it goes). What if it is worn and creating intermittent interruption of contact to create an intermittent unlock?

At the very least, if I keep bumping this thread long enough the real trans tech who visits these boards might come through.

When the mechanic checked line pressures did he also check “Line rise”?? This is the line pressure readings when the EPC (PCS) raises it for load changes. Those pressures have to be good too in order for lockup to be right. I’m thinking its a pressure issue because with the weight of your 5th wheel on the truck you will be pressing the accelerator down farther increasing line pressure, and that could account for the lockup working while towing.


Thanks for the replies. I don’t believe it’s the brake switch because I don’t have a problem with my cruise control. That would affect both the tcc and cancel the cruise, right?
Yes, he did check for “line rise”. We discussed the same thing. He has around 25-30 years automotive experience including transmissions. However, I’m not saying he knows everything. Keep the ideas coming!

Are you sure it is the converter, not the engine shifting in and out of overdrive? If it is shifting in and out of overdrive some thing like terrain, a sticking caliper etc. could be a cause. As a test put it in 3 and see if it happens, that will prevent overdrive from kicking in.

I did go an look at a wiring diagram and yes, your cruise & TCC are hooked to the same signal from the brake switch. (I have a GM but a whole different vehicle where the TCC is on its own switch signal). So if the cruise isn’t bothered I’d imagine you can forget that.

The only thing I’ve got left is to still wonder whether you need that sonnax with the metal sleeve rather than just the teflon seal. But I’m pretty sure that pressure loss there should set a TCC code.

Perhaps transman will come back through and have much better ideas/suggestions.

Yes, it’s definitely the converter. My mechanic could manually control the tcc with his scan tool. When he locked up the converter, it would not disengage until he commanded it to.
No, I have not found any other factor like you have mentioned to be the cause. Definitely not a sticking caliper, and not any type of terrain. It will come out of lock up on flat ground or even going downhill.

One more thought - has anyone scoped out the MAF sensor behavior? Or cleaned the MAF?

Good thought, but was cleaned two weeks ago. Made no difference. Sorry, forgot to add that to my main post. I have been talking to another mechanic in town, he’s researching into problems related with the throttle body and actuator. Maybe that will lead to something.

If you do ever get it sorted out I’d be happy to know about it. I’m driving with a 4T65E - and by symptom I’m currently having the same kind of TCC lock up issue (no firm diagnostic info yet, long story). Its not the 4L80 but these GM electronic transmissions share a lot of characteristics and apparently some of the same issues.

Hopefully I will know something early next week. My new mechanic has a buddy in the parts room at a dealership that’s going to loan out a throttle body to try. This same mechanic said he had fixed an identical problem in a buick terraza with replacing the throttle body.