First post on this forum, but anywho the speedo only works 50 plus mph. Throwing a p500 vehicle speed sensor issue I replaced it and still got the same thing. 50 plus is fine but nothing under works. The VSS is the smaller updated screw in kind, and the truck is a 1998 1500 4x4 5.9

 

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You replaced the speed sensor on the top of the rear differential?

If you have, I have also seen where the teeth on the tone ring on the carrier have been broken off. Take off the diff cover (probably time for a gear oil change anyway) and inspect the tone ring on the carrier. You can also check where the sensor comes through the housing to make sure nothing is bocking it.

You can also follow the sensor wire up from the differential and make sue that is has not come unplugged anywhere else. On my D60, there are two other connectors on this harness within 4 feet of the rear axle.

 

I changed the VSS in the trans. I could take a look at the sensor in the diff but I thought that was only used for abs.

 

your 98 uses therear speed sensor in the diff to determine speed as well as for abs i belive only the 94's used the VSS in the trannyfor speed

 

So I am guessing that they corrilate with each other thus speedo working fine over 50 mph. I would think that abs should be on a seperate system... Why would you need two sensors pretty much inline with each other? Hmm I'll give it a shot.

 

The screw in sensor on the trans is only there for transmission operation. It has nothing to do with speedometer operation.