Transmission falls out of drive at stoplights


#1

My car is a 1993 Buick Century, 3.3l fwd, with 137k miles. Yesterday I had the transmission fluid and filter replaced. Afterwards, my mechanic said that the old filter was very dirty and was probably the original but that the pan was fairly clean. It drove home no problem but today when I come to a stoplight, wait, then hit the gas it just revs. If I then shift it into N then back into D, it drives fine again. It also shifts fine while at any speed - its only when I stop that it falls out of gear. The fluid level is right on the full line, but I tried adding a pint (then siphoning it back out after a short drive) and the symptoms did not change. My mechanic is not a transmission expert so I need to go someplace else but know of no trustworthy place to take it, so am hoping to get some input here first in order to be educated. Thanks for any help.


#2

Sounds like the fresh fluid cleaned out the varnish that was maintaining pump pressure. Now, at idle, the pressure drops to the point the servos relax and it drops into a false neutral. A tranny shop can check the pump pressure to confirm this. Usually, when this happens, the tranny will jerk into gear when you rev the engine a little and oil pressure increases… Good Luck…


#3

That makes sense - thanks! Any idea what sort of job/cost I would need to get it fixed?


#4

Something else to consider. It is possible that filter seal to the pump inlet is broken; was not replaced; or not sealing. If air is getting into the suction side of the pump, line pressure will drop allowing the forward clutch to release. As soon as you go from D to N to D, the air lock is cleared; pressure rises; and function returns. You might have a tranmission technician look at the things done in the original service.

Hope that helps