I have a '98 GC and yesterday the transmission starting shifting in and out of 2nd. Now, while this is happening the speedometer is jumping up to 120 mph. I have taken it to a garage and hooked it on the computer and nothing is coming up. So i talked to a few friends of friends and they have all suggested it could be the transmission solenoids.

I guess what I'm asking is it typical for these to go out, or burn up? And is this maybe whats wrong?

 

I had to change two of the tranny solenoids in my daughter's '01. It did throw several transmission codes pointing at what had failed, though.

So, to clarify the symptoms:

The truck is moving at a basically steady speed, feels like it jumps out of gear and the speedometer spins up to 120?

If that is the case, whatever keeps the output shaft engaged to the rest of the transmission is going bad.

If it were just popping into neutral, the speedo wouldn't spin up.