Mike and I purchased a 1998, F150, 4.6L, 4X4, 3.55 LS for his daughter. I am in the process of going through the truck and changing all fluids, belts, hoses, etc. While changing the rear differential fluid I noticed a toothed ring mounted to the ring gear. Near the top of the differential is a sensor, which appears to a Hall Effect sensor. Is this used for speedometer correction? Given a stock baseline, if the pulse did not match a correction factor would be applied (??)

 

That ring is used for rear abs. The speedo gear is in either the trans or the transfer case depending on if you have 4wd.

 

Thanks for the information. This also explains why I could not find the ABS sensors on each rear wheel.

 

It is the Vehicle Speed Sensor (VSS) sensor, its output is used by the
odometer, speedometer, rear Anti-lock brakes, and the powertrain computer.

I have no sensors on the tranny for speed, just the VSS sensor on the
rear-end on my 1995 f-150.

 

The speed sensor in the diff. is only used for ABS. Your VSS in a 98 is on the rear of the tranny, on the drivers side. The VSS has a gear connected to the outputshaft of the tranny. This is where vehicle speed is measured.

 

Thanks!!