When I get up to 55 to 70mph my speedo moves up and down. Is it the speedo or maybe the VSS and also I went to advanced and autozone to get price on VSS and they show me a sensor that looks like it goes in the tranny or transfer case, the abs speed sensor is what they showed me that went in the rear. What should I do??

 

The speed sensor in the rear is usually the culprit of speedo bounce.

Cowboy Steve

 

If you go to the parts store, tell them you need the ABS sensor and not a VSS sensor or they'll give you the wrong part. I just went through this with my truck, turned out to be the PSOM (programmable speedometer/odometer module) that sits behind the speedo. Ford wants $350 or so for one!!! You can junkyard one, but this unit also stores the mileage on your rig so unless you get lucky like I did and find one with close to the same mileage your odometer will be off. Used instrument clusters are pretty easy to come by though if that is the problem. Good luck!

 

OK we'll give the abs sensor a try. $23 ain't bad

 

When you are back there,, look at the wiring to it, & look for rust build up on the mounting surface.....