I have a 1988 chev. with a 700R4 that I replaced with a GM factory rebuilt unit 18 months ago. I put about 6000 miles on it and started to reduild the truck (frame & cab) I used all the orginal eletrical system, sensors, wiring harness, motor, trans, etc. Since I have it all back together & was able to drive it I noticed that in 3rd gear the torque convertor tries to lock and unlock it feels like a constant bumping as soon as it lock it unlock. If you layyour foot on the brake it will quite.In 4th gear it does not ever try to lockup. I have traced the problem to the tan wire with black tracer(in the 3 wire conn. D term.that plugs into the tranny) as soon as it recieves a ground it looses it which is cosistant with the bumping. I do not have any codes and the truck runs great. All the tranny shops tell me to cut the wires and dont worry about it but I want the truck right. Can anyone tell me where it gets its ground from and what sensors that it looks for to apply ground to this wire

 

Sorry I have no idea on this one, but best bet is a chiltons manual.

 

check chiltons because i had the same problem and i have haynes and it dont say...i never checked the chiltons