I have intermittent issues with my speedo. I speedo jumps while in park. This shows up on the speedo as well as the computer datalogs. I have replaced the cluster, vss buffer and vss. I've also checked for power on the pink wire at the buffer which shows the correct battery voltage with key in RUN. Ive also checked the voltage on the green wire off the buffer. Thats showed approx 2 volts below battery voltage. I then checked that green wire for open/short to ground, it wasn't open and didnt have continuity to ground. I find it odd that the speedo and datalogs are showing the MPH moving while the car is in park. I was wondering if I could be getting some inductance from my MSD wiring running close to the VSS wires?? I found this while do a search sounds a lot like my issue. https://www.thirdgen.org/forums/elec...-ignition.html

 

http://www.msdignition.com/product.a...659&terms=8830
any thoughts?? Haven't noticed any noise on the radio but its not on much. I'm running the MSD 6AL see they say to use the 8830 with MSD 7 & 8 series

 

Bump

 

The VSS buffer is a fairly simple circuit, and is reasonably immune to extraneous noise if grounds are reliable. If you suspect interference from ignition switching noise (MSD) or inductance from the coil or plug wires, you may be able to see if with an oscilloscope by watching the VSS input pulse lines. A slight spike there may be enough to fire the optoisolator and trigger an output pulse.

If there is no measurable noise in the VSS raw inputs, the noise may be making its way directly to the ECM and affecting its reading of the buffer output. You should verify good grounds at the ECM, VSS buffer, and ignition unit.

If there is higher frequency noise in the input lines and a normal square wave output from the buffer module, you could try to attenuate it by adding some capacitance across the VSS+ and VSS- connections (red caps):

 

Did you ever fix this emi issue ?

 

I moved the MSD wiring away from the cars wiring harness with no luck. So just as a reminder I had replaced the VSS, VSS buffer, cluster, and tested the circuit for opens/shorts.
I reinstalled the original VSS buffer which I had replaced with a used Ebay buffer and it seems better now. I think I saw a MPM spike once on the datalog but I'm not seeing it anymore. I'm thinking the cluster was the original issue... I then got a bad VSS buffer from ebay and went with a new cluster fixing the original issue but giving me another problen a bad buffer! So when I put the old buffer in with the new cluster things seem to be much better! I'm just hoping I figured it out and it don't come back to haunt me.