I've been researching for days and have not been able to find the solution to my issue..

I have a 98 chevy blazer 4x4 auto. The other day I replaced my stock stereo for an aftermarket stereo, and afterwards my speedometer would not work. I've checked all fuses=OK. I replaced the gauge cluster today because it had other issues so I figured it needed to be done anyways.. resolved other gauge problems, but still no speedo.. Checked Vehicle speed sensor, and it has resistance, which, from what I've researched, means its still functioning..

My Tach works fine and the truck still shifts normally, it just doesn't have any speedometer signal so it also doesn't record miles on the odo..

Is there a vehicle speed sensor sending unit or is that part of the VSS? are there any relays or fuses I should check specifically? And what is the wiring route so I can trace the wires for any damage?

 

I think your radio is hooked into the ecm so that when you slow down your radio volume will decrease. If you don't hook up the wire that goes from the stock radio to the ECM I belive it can effect your speedo.
I switch out my stock radio from an 04 pickup for another stock radio and I had to pay the GM dealer $100. to reflash the radio so it would work. They said it is an anti theft feature..

 

If it had that feature it didn't work, become the stock stereo did not adjust its volume at slower speeds..

When I installed the aftermarket stereo there are two wires I left undone; The gray illumination wire and the violet/white dimmer switch wire.

Initially, I had the dimmer wire hooked up to the dimmer wire on the AM stereo and the gray wire left unhooked... no speedo..

Then, I unhooked both, no speedo..

Then I ran both directly to a ground so as to complete their circuits... no speedo and no dash back lighting..

So now I've unhooked both wires again..

I tried unhooking the stereo entirely, no speedo still..

Also, I drove the truck for a while without any stereo hooked up but still with the stock wire harness, and the speedo worked fine at that point, so I don't think it's an unhooked wire causing the problem, because when there's no stereo, they're all open circuits...

I suppose my next step is I'll switch the harness back over to the original and reinstall the stock HU, and see if the speedo works again.. if it does I'll undo one wire at a time until I lose my speedo again so as to isolate the problem wire..