lost overdrive

[lostoverdrive]

hey everyone, I am new to this. I am having problems with the transmission in my GMC2500. It is a hydramatic 400 and was rebuilt about 5 months ago at 218,000 kms. It seemed to work fine but just recently on a long trip, I felt the tranny shutter in overdrive, then few minutes later i was kicked out. I never regained overdrive again for the rest of the trip. She screamed at 110kph the whole way home in drive. The next day, i had overdrive again, but the next time I drove it for more than 1 hour, same thing. I took it to the tranny guy, he thinks it's an electrical problem, he got a code saying overdrive gear ratio problem, and sent me to another mechanic. That mechanic told me it was most likely a torque converter problem,maybe a tcc soenoid, a tear in the seel or maybe just a bad torque converter. I don't know much about transmissions but isn't the torque converter supposed to be replaced durring a tranny job? shouldn't the tranny guy do this under waranty? if i do have to replace the tcc solenoid, could some one give me an idea on how much it costs, where it is located, and how big of a job it will be to replace it? My truck has been from shop to shop now for about a month, this is getting very costly and frustrating, please help!!!

[gmctd]

That should be a 4L80E, as the TH400 never had TCC or o\d - TCM senses fluid temperature and will restrict several functions if overheating - sounds like your symptoms.

Also, has two speed sensors, used to measure TCC and gear range ratio slip - should have some DTC's for those symptoms - prolly should get it scanned to see what's happening.

If you have a TH400 with external add-on o\d, you will need to check with the vendor.

[damork]

I've been down the same path as you are, and believe me there are some things to consider before agreeing to pay big $$ when it might just be an electrical problem. I lost overdrive (lockup to be exact) and was screaming along to and from work for two weeks the first time it happened.

Someone here responded and advised looking at all grounds, especially to look over my rear trailer wiring connector. I did so and what a mess, the wires had been soaked in the salty winter slop and eventually the connector became a rusty corroded mess. I was getting feedback of some kind that eventually refused to let my lockup work correctly. A new connector and rewiring job fixed that then I proceeded to clean up all the grounds I could find and it fixed the problem right away.

The brake light switch up under the dash is another potential culprit, which I have standing by but have not needed it.

The speed sensors used with the 4L80E (front/rear) should be clean as well. If you have strange off the line shift characteristics such as delays or unexpected upshifts, mark a line on the switch so you reinstall it at the same torque then take it out and really clean it up. Made a huge difference with how my truck shifts.