Hey, I just purchased a 1999 Tahoe 5.7L 4x4 about a week ago with 85,000 miles. Everything on the truck seems pretty solid except for at highway speeds (65-75mph) there is a rpm fluctuation of a couple hundred rpm almost as if it is shifting in and out of overdrive. This occurs about every 30 seconds so it get rather annoying. I'm new to the forums but I did run a search and found an old thread that opened my eyes a little bit to the problem and I found that other have had a similar problem.

https://chevroletforum.com/forum/sho...nverter+clutch

That is the link to the thread I found. According to this thread that what I am experiencing of the torque converter clutch locking and unlocking and that is normal but I just can't see how this could be a normal thing that happens that often.

 

That thread really did sum it up well and I would be inclined to also ask if the tranny has been serviced since you have 85k. 85K isn't bad but 10 years of the same fluid and filter is...

 

Ok when I get up to Highway speeds the rpm's will jump back and forth and I can feel a very slight jerk. I can't really tell if the transmission is shifting out of overdrive and into third then back into overdrive really quick or what. If I give it more gas it doesn't happen, the car will accelerate fine until I come to a steady speed then It will start to happen again. The car actually has 84,000 miles on it so the first thing I'll probably have done is the trans fluid and filter changed out as you recommended. According to the older thread I found some people have had similar problems but no one has seemed to find the culprit. Oh yeah I forgot to mention this doesn't seem to happen until the engine has warmed up. Do you think a simple transmission service would fix something like this?

 

alot of cars can sense the tranny / engine temp and run shift points at different speeds when cold...

it does sound like the TC is locking / unlocking

I'm not a transmission expert; so I wouldnt know how to fix it on your model... but I agree with the symptoms...

 

If you haven't already done it, see if the shop has a flush machine. Get the fluid flushed and change the filter. Dropping the pan and changing the filter only gets about 30% of fluid. Flushing it gets around 95% of the fluid

 

when they flush, do they does that process get the debris out that the filter cough?

 

the filter will be replaced so any debris caught goes with it, many tranny pans have a magnet in the pan to also trap metal particles

 

My problem is when I sit in traffic, my Tacho needle goes up to and over 6K rpm and gets stuck there. After it cools down, I turn the key on and off (without cranking the engine) till the needle goes back down to zero. Any ideas before I take it to the dealer is greatly appreciated.

 

I think there was a class action lawsuit related to the instrument cluster. Might try googling on it.

 

Well I took the Hoe to the dealer and all they could do is to change the whole cluster. How hard can it be to replaced the cluster? I could probly get one cheaper than what they're asking, you think?