Hello: I have a 1994 GMC 6.5 turbo diesel 4 by 4 that I have owned since 2002. A good solid truck for the most part, replaced the water pump 2005, Vac pump 2011, re-routed some wires that were burned by being too close to exhaust manifold from factory 2015. Struggled for a year with faulty Hi-res sensor in injector pump, till a guy in Finland posted how to change it out. New sensor in, truck engine starts correctly now and runs just fine. Just before I installed the new injector pump internal Hi-res cam sensor, I noticed the speedometer was getting a little bit "jumpy". The truck drove, all shift points were just fine and cruise control was solid. I looked on internet for advise on this problem. For a while the speedometer would read correctly to about 40 mph, then run wild to 100 plus mph and back at random. This became slowly worst until the speedometer was peg to peg while engine running. Same thing while driving. I have checked the two wires to the VSS at the end of the transfer case for true wire continuity, opens and shorts to ground. The wires tested to be good. To be extremely sure I was testing the correct wires, I put on long wires attached at the VSS location, then looped back to test at the connector at the Vehicle Speed Sensor Buffer. Yes, I was testing the correct wires. I cut the 2 wires to the VSS to take the VSS "out" of the signal input to the buffer. When I turn on the power key to "run", the speedometer reads well below zero mph and does not move. As soon as I start the engine, the wild zero to 100 plus (peg to peg) jumping begins. I bought a used buffer that matches the code on the old buffer (used or rebuilt is all that can be had now). On install, the "new" buffer is doing the exact same behavior as the "old" buffer. If any of you out here have info on this issue, I would like to hear your suggestions. Thanks for your help.