1998 F150 speedometer is intermittent. The speedometer sometimes works ok, but more and more frequently, it quits and goes into a "float" mode where it gradually floats either up or down. I can be driving at 50MPH and the speedo will slowly float to over 120MPH. It also sometimes slowly drops to 0 MPH.

My guess is it's the speedo guage itself, but there are other possibilities like, the circuit board that drives the speedo, the wiring harness somewhere, the speed sensor etc.

When the speedo has floated to the max, if I stop the vehicle and shut off the engine, the speedo will stay at the max.

Is this common? What is typically the cause?

 

Sounds like a loose cable. Typically, there is a wire cable that runs from the tranny to the back of the speedo that physically spins (the wire inside the cable, like a shaft). Sounds like you should double check the tranny end first for something loose, or possibly a worn gear. If it's not there, then it sounds like something behind the dash.

TonyPTX