1993 GMC

[jeff1953]

I have a 1993 GMC 3500 454 auto. when i get out on the highway and set the cruse control and then turn on the a/c the speedometer jumps up almost 10 mile per hour is there a short in the a/c control panel, also sometimes the transmission wont shift into 4th gear and you might stop and shut it off and then start it and go again it might shift or not.

 

[Erik the Awful]

The speedometer jumps up, but your speed doesn't? How does it read compared to your actual speed with the A/C off?

 

[someotherguy]

Start with all your engine compartment grounds, be sure they're attached, tight, and in good condition. Negative battery cable has a second smaller wire to the inside fender wall, passenger cylinder head rear has a braided strap down to the firewall near the heater hoses and a second strap goes from that same spot to the frame rail, there's several ground wires attached at and near your thermostat housing, and also check under your dash where the diag connector is attached - there's a ground wire there, too (for the cluster!)

Richard

 

[jeff1953]

Thank's for the info. I will get it checked out. also with the a/c on it reads about 10 miles an hour fast but you are still going what you set it at before you turned the a/c on.