What causes the transmission in a 2002 GMC SIERRA 2500HD not to shift into Drive "sometimes"?

Question

2002 GMC 2500 HD 6.6 liter Diesel w/ Allison Transmission sometimes will only engage in 2nd or 3rd gear (can remember which it said) but most of the time its normal. service engine soon light is on. It shifts through the gears great like there is no problems when it goes into Drive. it also shifts normal when it is in 2nd or 3rd when it doesn't engage in the Drive position. Could this be a "speed sensor" problem?

Answer 1

The service engine soon light is a service reminder that indicates something is do for a service. If your truck has 80k or higher miles, it is most likely do for a transmission service. Dirty trans. fluid could be the cause of your shifting problems and is usually noticeable when the engine temperature increases. Check your owners manual for the service that is due for the vehicles mileage.

I don't think the speed sensor has anything to do with your shifting problem.

Also, drive/overdrive is for highway speeds over 50mph and 3rd is intended for lower speeds. Most lock up torque converter/overdrive systems can engage around 35-40mph, but this adds extra wear on the internal clutch packs and increased trans. temps.

Hope this helps. Christopher

Answer 2

if your service engine soon light is on have it scanned because the Allison will all so turn it on and set codes. most of the time its a shift cyl in the tranny or the shifter switch on the side of the tranny.

It may just be dirty fluid as well. But I would start with scanning the codes first. Because if it is a shift cyl the fluid has to be drained off anyway. If you drive the truck off road alot you should all so have a good look at the wiring harness at the tranny I have see it damaged lots of times too...

Answer 3

2002 Allison Transmission

Answer 4

Your truck may still be in park. Try shifting the handle into the drive "D" position, from the park "P" position.