Sounds like the shift, and torque convertor lockup solonoid (s). this plugin feeds the P.C.M (main computer for the car), reciprocating information (back and forth communication) allowing the transmission to shift into and out of each specific gears. It also should have connections for a neutral safety switch.AnswerThere are two eletrical plugs on the top of the 4T60-E transmission. One is near the back, under the shift linkage. It is for the neutral safety switch, back up lights, gear selection, etc... The other plug faces forward on the front of the transmission and it connects to the PCM (powertrain control module)which allows it to vary the transmission shifting pattern according to the needs of the car. According to my rebuild manual, the transmission will manual shift if there is an electrical failure, although not as well. The fact that yours will not shift when it is connected but does when disconnected would indicate to me that it is shifting manually, and one or more of the internal shift solenoids has failed and requires replacement... Chuck AnswerI had a 88 Pontiac 6000 4 cylinder that would stall when I stopped at lights and sometimes not start again. Turned out to be a plug that was connected to the tranny, not sure what it was call, but I unplugged it and didn't have a problem again.