97 Tahoe crusing 200-300 rpm fluxuations
 

I made a post the other day that I thought my tranny kept shifting between 3rd and 4th randomly at freeway speed (50-70mph). I'm not all too familiar with transmission, but it dawned on my that this fluxuation that randomly accurs isnt so random after all. I realised that this 200-300 avg. rpm drop/rise happens on parts with a gradual incline and sometimes flat. I have limited knowledge on transmissions but this lead me to believe that it is the torque converter locking and unlocking. As little as a second intervals and has done it for miles off and on. I also realized that if it were shifting between 3rd and overdrive then the rpm jump/drop would be around 700 rpms at 60 mph, a much larger change then whats accually accuring.

So, Im wondering if my conclusion is accurate, if people have had this happen, and what can be done to fix this/price. The transmission shifts strong and smooth so the torque conveter would be the only thing I'd want to fix. Im thinking that the locking mechanism itself or possibly a computer is messed up. Please correct me If I'm wrong about my conclusion.

-Chris

97 2 dr Tahoe LT
105k miles

RE: 97 Tahoe crusing 200-300 rpm fluxuations
 

That certainly makes sense to me. My 97 was doing a similar thing today while driving 65mph. It almost feels like it is occasionally mis-firing or something, but the engine runs flawlessly under 55mph. I'm glad you posted this. Now I have someplace to look to fix this. I will let you know if I find out any information about this.

RE: 97 Tahoe crusing 200-300 rpm fluxuations
 

Sounds like the same thing, basically it doing it under 55 is rare because from my understanding the torque converter only locks at high crusing speeds. Hopefully someone else who has come across this problem has a solution and will post.

RE: 97 Tahoe crusing 200-300 rpm fluxuations
 

A torque converter "locks" based on engine RPM, not vehicle speed. The lockup is just the fluid pressure building to the point that it spins the rest of the transmission and produces power.

The simple version of how it works is this. Your engine spins an impeller (a mechanical, physical connection). That impeller swirls your transmission fluid, which turns the torque converter (which looks like a donut-shaped turbine with curved blades inside it). The turbine spins the input shaft that connects to the gear train. (There is more to it -- there is a fluid return system to increase torque, for example, but that explains the basics.)

I know how the transmissions work, but I'm utterly useless for diagnosing them. However, one thing about your description jumps out at me.

GM uses a solenoid to control something called the Transmission Converter Clutch (TCC) and basically this device reduces power loss at cruising speed by directly coupling the engine output shaft to the drivetrain input shaft -- in effect it takes the torque converter out of the picture. Normally when this fails, your vehicle stalls, so I could be barking up the wrong tree, but the TCC engages at around 50-55 MPH, and disengages at around 45 MPH, and it also disengages upon acceleration. It could be a problem with the solenoid, the check ball that the solenoid moves, the fluid pressure sensor, or even the ground to the TCC. If you can identify the TCC solenoid on the transmission, it's easy enough (and harmless) to disconnect it and see if the problem goes away...

That's just a guess, and it's easy to imagine a whole bunch of other explanations, but the TCC is at least easy to check. Actually I originally posted this just to explain how the torque converter works, but the TCC occurred to me, since I had a TCC mis-diagnosed as the source of my troubles on an old Suburban I owned years ago.

RE: 97 Tahoe crusing 200-300 rpm fluxuations
 

Chris,
This last post was some very good information and it sounds as though your transmission may be working normally or near normal.

With 105K miles on a 10 year old Tahoe, the first thing that I would do is make certain that you had fresh fluid and a new filter before attempting to diagnose any problem.

If you are not getting any DTC lights on the IP then the computers thinks the torque converter clutch and transmission are operating correctly.


There were a couple of software updates for the ECM and TCCM that you may want to have a dealer check to see if your Tahoe has installed, the updates were written toaddress symptoms similar to what you describe.

Also make sure that all tire pressures are correct, I have seen low tire pressure cause excess drag which has lead to TCC business.

RE: 97 Tahoe crusing 200-300 rpm fluxuations
 

i also have this same problem, but my truck onlt has 75k miles on it, i travel from campus to an off campus site, and as i go up hills the truck will down shift, out of overdrive, but not really into third, cause i can drop it into third and it revs up more, cause it is actully in third, but i also notice this on the highways heading home and back to college, im glad i found this forum, it has helped me out alot

RE: 97 Tahoe crusing 200-300 rpm fluxuations
 

That is just the torque converter clutch unlocking, it is normal not a problem.

I have a 1997 tahoe with 95,000 on it and it is doing the same thing. Did anyone ever come to a conclusion on this issue. Thanks

Anyone out there?

ZX1100F1 already has posted the answer to the problem