A few things to be aware of;
The sensors can be had for $20 on eBay
The sensor may not be bad
There may be corrosion at any one of a number of harness connections. (from sensor to main harness, from harness to computer, from computer back to main harness, from main harness to front harness, and from front harness to cluster assembly)
Corrosion can also occur inside the cluster and on one of the PCB boards and or the rubber connecting strip.
I had an erratic speedo, eventually having it read 0mph all the time. I purchased a new circuit board, installed a new sensor, cleaned the old board, and wiggled wires everywhere before finding corrosion at the front harness connector at the pink/green wire. I'm still not 100% certain it is fixed, but the corrosion was cleaned off and it is now working on the stands.
<edit> And yeah, if you have big hands, the sensor is a PITFA to get to and you will need socket extensions and a swivel to make life easy.
Last edited by MikeinCtown; 05-20-2015 at 03:20 PM.