Flaky overdrive on automatic transmission

1996 Chevy S-10 pickup, 2.2L I4, 59K miles

I bought this truck a couple months ago in order to save a bit of gas (my other vehicle is a 94 Chevy K1500 Blazer, 5.7L V8). The S-10 runs beautifully with the exception of one thing.

After it’s fully warmed up and been run around for ~15 minutes, overdrive gets very temperamental. What I mean is that, when accelerating while moving at highway speeds, it will bounce back-and-forth between overdrive and drive, and the engine RPM will also bounce back and forth. I’m not sure if OD is slipping, or the transmission just doesn’t want to (or can’t) leave OD and downshift. You really have to mash the accelerator pedal hard to make it downshift and keep it downshifted. This is at its worst between 50-60 mph; I might only be trying to keep up with traffic or give it a bit extra to make it up a small hill, and it will start revving up-down, up-down, up-down.

The transmission fluid does seem to need changing (dark red instead of just red), but could this be a sign of a larger problem? If Transman or someone equally knowledgeable about auto trannys can post an opinion, I’d much appreciate it.

Step one, have the transmission serviced. Step two, lock out the overdrive until you are clear of traffic and can maintain 50-55 mph. This will prevent the overdrive from “hunting” when traveling right at its shift point…

Bump for Transman618. Does this have a Throttle Valve cable? If it is fully electronic, have a look at the Throttle Position Sensor. Make sure that no codes are set. Likewise, you should probably have freeze frame data viewed when this happens. Maybe something will be revealed.

Hope this helps. Will keep an eye on this post for additional replies.

Scan it and check for a P-1870 code. Its most likely the TCC chattering. This trans is electronic, no TV cable. If you get the code, you’ll have to drop the vb, ream and replace the regulator and isolator valves.


Okay, thanks for the reply TM. Any thoughts on what this sort of job will cost?

And am I correct in assuming that this code is a trans-only code that needs a special scanner, and hooking up a regular OBD-II scanner to the ECM data junction won’t show this?

Yes, a regular OBDII code reader should pick up this code.


Update: I just had the truck in the transmission shop today for a fluid/filter change, and I asked them to check out the overdrive condition.

The guy doing the work said that no codes were stored, and he couldn’t find anything wrong with overdrive or anything else. He had it out on thei highway and did duplicate the problem, but he said it was a possible engine weakness problem and suggested a tune-up (which it’s already had).

As it happens, I think the fluid change was all that it needed, because I gave the thing many chances to repeat the problem after the work was done, and it doesn’t seem to be doing this anymore. It’s now shifting pretty solidly into and out of overdrive. The handwritten notes on the work order say:

  • OK now
  • OK hot
  • In/out of lockup 2 times
  • Engine perf?

In any case, the problem is resolved. Thanks for your input transman, fortunately it didn’t need what you had in mind.

What can a trans service do? Now if you want to drop the pan cut the filter up and look for metal this could be considered a acceptable diagnostic step. Trans service never fixed anything


Trans service never fixed anything [/i]

I wouldn’t go that far. Certainly it can’t “fix” a real mechanical problem. But that doesn’t mean that it can’t alleviate some problems.

I can’t speak specifically to a '96 S-10 transmission, but ATs in general are full of all sorts of things that require the free flow of fluid to function well including pressure switched solenoids. Getting things cleaned up a bit can, in fact, make a big difference.

I “fixed” my own transmission problems this way. They were minor problems to be sure - some hard shifting and a recurring TCC solenoid code. Fluid and filter made a HUGE difference in addition to eliminating the code.

A tranny service is not that expensive and a good idea no matter what, so do it. It may or may not make a difference.

Not that expensive? Here in Tucson you will pay $99.00 to $150.00. You have to justify that kind of money

For what it’s worth, it ran me $125. The work was done at a VERY good local independent transmission shop that I’ve gone to for years now, and they always do top quality work. The trans service wound up taking care of the problem I started this thread over anyways, so it’s money well spent.

You get what you pay for.

What was the operational concern? A odd overdrive shift pattern? Just trying to be clear. Any metal or fiber material found in the pan or in the filter?

Just like I stated in the original post, it would seemingly bounce in-and-out of overdrive at highway speeds (the RPMs would go up-down-up-down-up-down almost rhythmically). The tech who worked on my car found nothing wrong, the pan and filter had no debris. I know it needed the fluid/filter service because the fluid was a very dirty dark red, and I’ve only had the truck for three months.

My guess is that the old fluid wasn’t lubricating all that well, and the engine was straining to make it up those hills in OD. Whether it was actually having trouble staying in OD or just the engine’s way of saying it wants to downshift, I’m not sure.

“Not that expensive? Here in Tucson you will pay $99.00 to $150.00. You have to justify that kind of money”

Well, if the only other options are to scrap the vehicle or rebuild the tranny, then $100-150 is pretty darned cheap - esp since it does seem to help a lot of things. I easily justify trying that before anything else.

It is also normally pretty easy to DIY - so it doesn’t have to cost even close to $100+

Have you really tried to do a tranny fluid replacement at home? Lots of fluid spills everywhere,homeowners association rules forbid it for me,spills on your nice clean conncrete (unless you want to work in the dirt,then you have to get rid of the fluid hoping it doesnt spill on the way and they dont fix a thing

I have done 4 on my car (97 Escort) and 2 on my van (95 Caravan) right in the driveway. Its really not that hard and I didn’t make a mess.

And apparently they do fix things (read the rest of the post) - which is why you should just relax about it already.

Its the placebo effect you have already spent the money. Technicaly why would it work. Read the other blogs there asking for freeze frame data,check throttleposition sensor,Transman618TCC chattering. Techs sugesting tune-up concerns “It doesnt seem to do it now” not a clear cut statement Its fixed You should read some of the other posts.

Get a grip, man. I know how my truck drives, and when I say that it is not having this symptom anymore, it’s not having this symptom anymore. Just because you don’t think the job actually does anything doesn’t mean that everyone needs to think like you do.

Have you ever heard the saying “sometimes you need to fix the customer”?