Mechanical or electric?? What are common failures. I'm not getting a reading until a few miles down the road then it starts working. Bad vss if electric or speedo gear if mechanical????

 

If you haven't gone as far as to look under your truck yourself, I suggest you start there.

The speedo output is on the output shaft off the back of the T-case.
If there's a wire plugged into it, it's electric. If there's a cable coming out of it, it's mechanical.

 

As advised, look under the truck first. The cable and VSS1 plug into the same place in the back of the Transfer case, and you won't mistake one for the other.

But your symptom sounds like a bad (slowly degrading) VSS1. DO check that the odometer stops working when the speedometer drops. (Otherwise the problem is in your cluster.)

 

will do

Definitely electric. Will check to see if odo is turning when speedo is not working. Hoping the problem is not in the cluster. Do the teeth on the gear attached to the end of the vss have a tendancy to wear/break over time?

 

I don't know about "tendency," but my gear was pristine. And that's the easiest part to check. Just remove one bolt, pull out the unit, and look at it. On mine it was white Delrin (nylon).

Checking the VSS1, though, is not easy. It is a 3-wire Hall-Effect device, with +12v, gnd, and signal. One person on this list thinks he can check it with an ohmmeter, but he's ... wrong. Even though it's an expensive part (+300, but definitely shop around), at some point you have to make a leap of faith.

[I was able to check mine by using a signal generator and putting signal into the wire to the cluster. By adjusting the frequency, I could set any speed I wanted, so I knew the cluster was good. And the VSS1 was bad. But you're not too likely to have access to a signal generator.]

Good luck.

 

I am having the same problem on a '92 22re 4x4. Speedo waits a mile or two while the odo/trip works no spikes.