Beck2013

I have a 2000 Trans Am A4 that just today stopped shifting into fourth gear. I am 2 hours away from home and I am wondering what it could be and also is it safe for me to drive it? It just revs up higher and higher, It isn't beaten on and has had a Transmission flush in the last 9 months. Thanks, Sid.

mrvedit

If both 2nd and 3rd gear feel strong without slipping, then a possible (most likely?) cause is a leak in your 4th gear servo. I'm no trans expert, but I think you are OK to drive it, just leave the selector in D3. If you feel slippage in 2nd or 3rd, then you have other problems.

Assuming it is the 4th gear servo, this can be replaced without even removing the trans. (Its under the round cover on the passenger side.) I understand it can be a bitch to get out in your model car, but a trans shop should have no trouble with it on a hoist.

Beck2013

Well this morning I drove it and it went through all the gears fine and felt strong as ever when it went into 4th so as of now I am not having any problems anymore. Interesting..

z28camaro82z

cracked 4th accumulator piston could be a valid guess. Most of the factory pistons were made of plastic in attempt to save money and they tend to crack or break completely and cause a leak.

mrvedit

That is certainly plausible. (It is often called the 3-4 accumulator piston.) You have to remove the pan, valve body and separator plate to get to it. A Sonnax Pinless piston is a good choice for a replacement. ($12)

Or the 4th gear servo piston is cracked, or more likely, has a badly worn seal which works some days and not others.