What is wrong with a 1989 Chevrolet that cranks fine but dies when put in gear and the speedometer moves up past 100 when put in forward gear?

Answer 1

had an 88 that somebody put in a tail light bulb wrong, and was back feeding & would stall the engine every time

Yes, when I saw this issue, I immediately thought bad grounding. Check that the ground strap between the engine and the frame of the car is secure and has good contact. Had a Fiat one time that did the speedo thing when this condition was present. Did lots of wierd things until the strap went back on

Could be your TPS (throttle posistion sensor) sensor

I just need to say that it's very unlikey that the taillight builb in the 88 was the actual problem, an 88 didn't have enough elecftrical systems to have problems with electrical feedback.....and it most likely is the TPS (throttle position sensor) or the IAC (idle air controller), the ground wire that is connected on the bottom left (from standing in front of the truck) is actually an extra ground but there are some systems that use that as the main ground. Check that ground anyway because it is a common problem for those gm's (Chevy and gmc, both gm vehicles) but test your IAC and TPS first