Questions and Answers

GMC Jimmy Won't Stay In Over Drive Q. I have a 1996 GMC Jimmy that has one small problem. It has a 4.3 liter V-6, automatic transmission, 136,000 miles, fuel injected, ABS brakes, A/C and cruise control. The problem! It will inadvertently shift out of over drive into drive constantly. It does this down hill, with or without the use of cruise control, or up hill.

The speed it occurs is around the 65 mph to 75 mph with the engine at around 1,700 rpm in overdrive. You can release the accelerator pedal but it still takes a couple of seconds for it to shift back into over drive however, it will go right back into drive if you do not accelerate to a higher speed or slow down to a slower speed.

The problem is not severe at all on good flat highways however, if there is a slope of any kind it will occur like clock work. I had the transmission rebuilt last year at around 110,000 miles and it has done it since that time.

The transmission mechanic had no answer to the problem. He didn't seem to be very knowledgeable about the electronic controls that may exist on this transmission. I know some vehicles have a throttle position sensor that relays information to various locations but I don't know about this make and model.

Please Help...

A. This car has all kinds of sensors on it. The transmission is electronic too! My guess is that since someone was in the transmission and it didn't do it before, that there is a problem inside it.

A GM scanner hooked up to it can help in diagnosing it. Watch the speed sensor, TCC solenoid control circuit. Maybe even a mis-adjusted brake pedal switch. This switch tells the transmission when you are braking and releases the TCC. If it's giving a wrong signal, know what I mean? Lot's of possibilities here.