Speedometer reads double actual speed

sounds like internal dashboard issue

 

Either your sender unit at the gearbox or the speedometer unit itself....my guess is the speedometer head itself. The internal 'k factor' is adjusted and controlled by setting the DIL switches at the rear of the unit. I have seen several of these units go faulty over the years and do weird things like you outline.

 

Because the problem seems to change by keying on/off I would also say this is dashboard issue. If you have a good friend or someone you know with the same year you can try to swap out dash clusters for a couple of days and see if the problem follows the cluster. I am not sure on Petes, but the speed may be fed to the cluster from the engine ECM, so it may be having issues.

 

98 Pete's should have a signal going to the speedo head and engine at the same time unless you have the two plug speed sensor on the back of the transmission. If it is a single the same signal will feed ECM and speedo head. Either way it sounds like one of the possibilities is a dash ground issue or you can get a shadow reading from the VSS on transmission if the sensor is adjusted too close to the gear. Those are easier to check then look at speedo head. I used to get trucks that would shopw vehicle speed just sitting still. CAT did come out with a in-line filter for the VSS that helped with this problem.

 

Maybe you're just tired and are seeing double?

 

The sensors are the same, try swapping if the connectors are the same and see if it still does it. Second is check the adjustment, I normally gave them an additional 1/4 turn out to stop the speedo issue..