Help! Speedo eratic/inop!!

[billscac]

96 CE LT4 Convert. 8000 Miles, manual trans and RTD. At actual 45MPH, speedo indication will jump up to 135 then down to 7 then to approximate road speed of 45 and hold there. When it does jump up to higher indicated speed the SCV Radio volume will get louder (based on the speed signal it gets from the PCM) so a signal is getting to the radio head but it is incorrect. Sometimes the indication will drop to 0 and stay there regardless of actual speed. When there is no speed reading the engine will not Idle when I come to a stop. I've replaced the VSS with a new ACDelco unit as it was the easiest (and cheapest) way to start diagnosis. New VSS... same symptoms. Took it to a local Corvette shop and After scanning with a Tech1 which showed DTC 500. The tech suggested that I unplug the RTD module to eliminate it as a possible cause. Tried that and no change in speedo function
I'm leaning towards the PCM as It decodes the VSS signal and sends it to the various other components (CCM, RTD, Cruise Control Module, and Radio head). The Radio and speedo display are both obviously getting the same signal, albeit an inaccurate one. Those signals come from the PCM which gets and decodes the signals from the VSS. I doubt that two different VSS units would both be malfunctioning the exact same way which is why I'm leaning toward the PCM as the culprit.
Any thoughts will be most appreciated.
Bill

 

[vetteoz]

I doubt that two different VSS units would both be malfunctioning the exact same way which is why I'm leaning toward the PCM as the culprit.

Or a wiring / connection fault from the VSS

Could try pulling the fault codes to see if anything present

The "paperclip method" - it does more than you think, even on '96 cars! - Corvette Forum


C4 Diagnostic Trouble Codes | CC Tech - Corvette Blog, Restoration and Repair

 

[billscac]

Or a wiring / connection fault from the VSS

Could try pulling the fault codes to see if anything present

The "paperclip method" - it does more than you think, even on '96 cars! - Corvette Forum


C4 Diagnostic Trouble Codes | CC Tech - Corvette Blog, Restoration and Repair

Thanks OZ I'm going to go thru all the ground connections first and then on to the paperclip!

 

[Ken Anderson]

Common problem, cluster needs repair.

 

[billscac]

Problem Sloved!

Or a wiring / connection fault from the VSS

Could try pulling the fault codes to see if anything present

The "paperclip method" - it does more than you think, even on '96 cars! - Corvette Forum


C4 Diagnostic Trouble Codes | CC Tech - Corvette Blog, Restoration and Repair

Common problem, cluster needs repair.

Thanks OZ and Ken...
Sometimes it's the simple things. After removing the battery and battery tray I located the two ground connections on top of the frame rail just below the master cyl (FSM calls them G101 & G106). One is the skinny wire from the negative battery terminal (part of the neg cable) and the other is a braided strap from the bellhousing. I removed them, hit every contact surface with my dremel tool wire brush , and reassembled with new bolts. Problem solved. Hopefully this procedure might benefit someone who is chasing an intermitent electrical gremlin as these two ground points effect almost very electrical system in the vehicle and WAY cheaper than throwing ECMs or other components at it. BTW OZ the paper clip deal was great, thanks for the link, it showed an unrelated C12 for the CCM which I promptly cleared.. and no other faults.