Brake Switch

HDdave · December 16, 2007

Well my truck has had the problem as long as I can remember where the tcc will not engane at highway speeds or even on a country 2 lane highway at around 60. I can get it to engage on back roads at around 45 or 50 mph though.. When I had it looked at, the guy said it was definetly an electronic issue but he couldnt find it, because he could engage it with his scanner. He chacked my brake switch and said it is working properly reading on and off. Where my skepticism comes in is on a vehichle that is working properly you can tap your brakes to unlock the tcc when it is engaged. When I can get mine to lock on back roads, tapping the brakes does not effect it at all.. Also, it will engage on back roads even in tow haul, and turning on tow haul mode when it is engaged doesnt effect it either. I really want to figure this problem out because it will help my milage a lot. I was also thinking that I could have ECM malfunctions or just the program that is in the ECM is bad. I will be getting a diablosport predator soon, and I am thinking that could possibly fix the problem also by changing the program.. Are there any other reasons you guys could think of that would cause this? I am thinking about getting a new brake switch, but I dont know if it will just be a waste of money...

Genuinefine409 · December 16, 2007

I would think it would either be the brake switch or the TCC solenoid. If the brake switch shows it's opening and closing on the scanner maybe you can try changing the solenoid.

http://www.powertraincontrolsolutions.com/...80E_harness.pdf

There is a wiring diagram that breaks down the wiring and you can see on the 13 pin connecter that goes to the transmission, this is for an aftermarket TCU so the rest of the wiring will be different but the transmission side should be the same. The brake switch may be working and the solenoid could be not engaging correctly. I don't have an ATSG manual for the 4L80E but in the 4L60E you can check resistance on some things to see if they are bad, I'm not 100% sure if you can on the TCC solenoid or not.

http://www.powertraincontrolsolutions.com/ftopicp-399.html

Here is another link to a TCC no locking up on a 4L80E. I am going ot my dad's shop later and I can check in the GM service manuals he has to see what it should test at; if it can be tested. What year is your truck? I think 93+ 4L80E's are the same but not certian about that. I hope you can get this figured out, I had a 4L60E with no TCC lockup after a rebuild and the guy that did it had me running down all these roads and here it was something he did, but I know how you feel with trying to figure this stuff out, its no fun.

HDdave · December 18, 2007

Ok, I will look into that too. Thanks for the help, this is really frustrating me. As best has I know, the truck has been doing it since I bought it from my grandpa in October of last year. Now with the gas prices going up so much I really want to get it fixed. I can still push out around 12.5 mpg's in the warmer weather without it locking, so I think that getting it to work correctly will help a lot. I thinkg the guy I took it to was a moron, he didnt even mention the selenoid. Would the selenoid show up on a scanner if it was bad though? I really appriciate any help you can give, anything is a step closer then where I'm at now..