Hi - my speedometer has begun to bounce between zero to whatever my actual speed is. The problem is sporadic, but happening more frequently as time goes on. It also spends more time at zero as time goes on.

The odometer hasn't worked since I got the truck, but I track mileage by gas consumption to keep my oil changes on schedule.

I've checked and determined my truck has an electric speed sensor rather than a cable driven speedometer unit.

A new replacement part is costly, so I'd like to ask anyone with a history of similar problems:

Did a simple replacement of the sensor unit on the transfer case solve the problem? Or did you have to go into the gauge cluster to fix it?

Is there any good way to check if a junkyard sensor unit is still good short of buying it and installing it?

Anything else you'd suggest I try before substituting in another sensor unit? Any other info your experience might offer would be appreciated too.

Keep on truckin' and thanks in advance for responses.