About eight weeks ago I bought a 1998 Plymouth Voyager 4cy, 3sd. After driving it a week I noticed oil all over the back door. Returned to dealership where they fixed a rear seal leak. When I got about a mile away the "check engine light" comes on and the speedometer stopped working. Since they kept the vehicle for almost two weeks for the first repair I have been afraid to return. While on the freeway the engine "bogs" and the speedometer jumps to 120mph. It continues to do this until the speedometer kicks back in and everything is fine. The "check engine light" from AutoZone said it was the speedometer sensor. A friend checked it and said the dealership didn't plug it in all the way and when I left everything was fine....until I turned the car off. So I broke down and ordered a speed sensor. Here's the freaky part...while sitting at the park with my dog I had the key in the "accessory" position, engine off mind you, while sitting there off the speedometer kept jumping up to 120mph AND was clicking on mileage!!! It clicked in four miles while I was just sitting there, parked and engine off!!! Now before I go pick up my 60.00 dollar speed sensor can someone explain what the heck is going on???

Shorted Sensor?

Have you seen the Excorsist? Maybe that's the route to go.

Only thing that comes to mind is the Body Control Module, better known as the BCM, is in the circuit for the speedo and odometer. It's powered no matter where the key is positioned. It may be sensing the bad sensor and letting that pass to the speedo. Try pulling the IOD fuse out for a few minutes. It's in the fuse box next to the battery. That will reset the BCM.