1996 Suburban Transmission Issue

Volt · January 4, 2006

Hi, I have a 1996 Chevy Suburban 2500 2wd 7.4L with a 4L80E trans.

After driving for 20-30 minutes at steady throttle, 50-60 mph, the transmission downshifts from OD to D, and will not upshift back. After I come to a stop, the shifts from 1 to 2 and 2 to 3 are hard/abrupt, and it does not shift to OD. I can stop, shut off the ignition for a couple of minutes and when I restart, everything is as it should be, shifts smooth and OD is working fine.

The transmission was replaced a year ago under warantee with a GM unit, for a different problem (stuck in 2nd gear and the dealer could not figure out the problem so they replaced the trans). However, I had this problem before the trans was changed, and still have it now so I think it may be electrical or computer related. I have about 5,000 easy miles on the new trans.

The event does not log any codes.

I am willing to replace any parts that are suspect, or do any additional troubleshooting.

Thank you!!

Dennis

GM-Tech · January 4, 2006

This sounds like a classic case of a torque converter that's not staying engaged or slipping. Are you absolutely, positively sure it's dropping out of OD (4th) and going into D (3rd)? Next time this happens, manually pull the shifter down into D and I think you'll find it downshifts into D, meaning it didn't go from 4 to 3, the TCC's dropping out.

And I think you need to recheck the no codes thing. Here's what happens. When the computer sees tranny slippage, such as when the TCC should be engaged but isn't, it sets a code in the computer. This in turns causes the tranny to default to high line pressure, which you don't notice until you come to a stop and then pull away and get the neck snapping shifts. Turning the ignition off for a moment, then back on, will clear this condition (but not the code) and return the shifts to normal until the next occurance.

It could be something other than a mechanical problem in the trans causing this. Check for codes again.

Volt · January 4, 2006

Thank you very much for the reply.

I have tried manually moving the shifter to "D", and it does not change RPMs or shift. So I am certain that it is dropping to drive rather than just unlocking the TC in OD.

I will check for codes again and report back.

Thanks!

Volt · January 4, 2006

I just went to AutoZone and connected the scan tool. It did not have any codes stored.

Are there more advanced scan tools that the dealer uses? If so, I will take it there for diagnosis.

Is the fix for a converter slip to replace the converter, or is more likely related to line pressure from the pump, valve body, etc?

Thanks again!

Volt · January 5, 2006

Thanks again for your help. Please let me know if the dealer is my next stop or if there is anything else I can do.

Does the fact that it is actually dropping to Drive, rather than unlocking the TC change your view of the issue?

Dennis

GM-Tech · January 5, 2006

Does the fact that it is actually dropping to Drive, rather than unlocking the TC change your view of the issue?

Not really. No offense, but I'd want to drive this myself with a scanner hooked up to see what's going on.

Don't feel bad. In many cases, I don't even take a tech's word for something in our own service department. I've learned that if I don't see it myself, I consider that what they're telling me is going on may not be 100% accurate.

I've been bit more than a few times by, "Yeah, I checked that. It's okay!"

Volt · January 5, 2006

Hi GM-Tech,

No offense taken - definitely!

Although this is where I was afraid I would end up. I know the right way to diagnose it is to replicate the problem on the road with the scanner hooked up, and to see what gear the computer thinks the truck is in versus what it is actually in. Unfortunately, it takes a long steady drive to get it into the failure mode, and here in Southern CA, it is not always possible to get out and drive continuously. Hence it may be difficult for a tech to replicate unless they live with the vehicle like I do.

Thanks for your help and for the ideas. I guess I will drop it off at the dealer and see if they are willing to do something with it.

Dennis

GM-Tech · January 5, 2006

it takes a long steady drive to get it into the failure mode

I know you seem pretty sure it's dropping out of OD, but boy, this really sounds like a TCC apply failure. Those years were known for it.

Volt · January 5, 2006

If TCC apply is indeed the problem, what is the common fix?

If it's internal to the trans, good, I have a full warrantee on it. If it's external / electrical is there anything I can test or try?

GM-Tech · January 5, 2006

If TCC apply is indeed the problem, what is the common fix?

Valve body.

neptune1230 · January 8, 2006

If TCC apply is indeed the problem, what is the common fix?

Valve body.

<{POST_SNAPBACK}>

I have a '99 2dr tahoe with the same problem. Is a new valve body something that can be done by myself or is this a job for a pro? I am an above average shade tree mechanic that would rather do it myself. Also what is the approx. cost of a new valve body? thanks

GM-Tech · January 8, 2006

If you're above average, you should have no trouble replacing a valve body. Just make sure you know where all the little balls go when they go rolling around the garage floor.

Can't help ya on price.