Cruising down the highwAy today. Lost 3rd and 4th. Might've been low on fluid. Filled it and same thing. Would shift good in low gears.but when going into third and fourth. It does nothing. It just revs as if it was in neutral.

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Time for a rebuild. You have burnt up the 3/4 clutches. VERY common in the 700R4 transmission.

Awww. Man. Ok that's what I was thinkin but wasn't sure. Any good places that sell them cheap online.

Also how hard will it be to put a 4l60e in. Car is 93 z28.

With transmissions they are one of the things that you REALLY get what you pay for. You can get it fixed cheap but the repair won't last long.

You can use a TCI t-com stand alone controller. They work great.

Could It also be the governor

This is a 3-4 clutch problem, not a governor problem.