I have a OBD1 1992 P30 Chevrolet chassis with the 454/4L80e combo.

Unfortunately, I can't remember exactly how/when it started, but it has a problem that happens when it kicks down from 4th to 3rd (passing gear).



Everything works perfectly on the transmission UNTIL the computer kicks it down under acceleration. If I manually move the shift lever from 4 to 3, all is good. But as soon as the computer kicks it down, OD is lost. It stays in 3rd gear and If I let off the throttle, it freewheels down to near idle.

Pull over, shut it off, wait 10 sec (about) and start it up... All good again.



I am completely stumped by this. Because it is very repeatable, my thinking is that it is electronic. I am thinking about buying an aftermarket TCU, but before I shell out the cash for that, I really want to make sure that it is indeed computer related.

Ideas? Thoughts?