Speedometer failure needing help

[Corbin]

I'm having issues with the speedometer wiring I think..? I have put 2 other known good gauge clusters in it and both do this

When you start the truck the speedometer goes inbetween 30-50mph at idle in park, I have tried two different vssb modules and it does it even when you unplug the vss, it will peg the speedo if you rev it,

Anybody have any idea on what's happening with this?

Truck shifts perfect

 

[Ruff Idol]

The ECM could be acting up. To my knowledge the last leg of the loop (after the VSS on the trans tailshaft and VSSB behind the glove box) is where the final, modified signal leaves the ECM and goes right into the speedo via inst. cluster wiring harness. This fellow has a good writeup on how to test the signals coming out of the ECM and VSSB with a scope to ensure they are the right waveform/amplitude/etc. Link posted below.

Btw, this is in the case of my '92 C1500. I assume you are in a similar year range because you mentioned you have a standalone VSSB unit and it is NOT built into the instrument cluster (like on the '91 and earlier models).

https://atracom.blob.core.windows.net/gears/2005/2005-04/2005_04_04.pdf