View Full Version : Troubleshooting Vehicle Speed Sensor / PCM


Gary U
05-07-2012, 11:04 PM
I have an issue that has me a little stumped and looking for insight from anyone. Have a 6.0L LY6 in a 68 Chevelle with a GMPP harness and controller (PCM). Transmission is a Tremec Magnum 6-speed with a mechanical speedo drive for the original speedometer. This transmission also has a Ford style VSS (like a 2004 SVT Cobra for example).

I completed the swap over two years ago and the VSS originally worked fine – HP tuners scanner always showed vehicle speed when moving. Last fall I noticed that vehicle speed was no longer registering in the scanner. Around the same time I also noticed that the engine would hold on to RPMs between shifts more than it originally did. I don’t know for sure these are related.

Briefly, what I have done to investigate is:
-checked the continuity of both wires from the PCM connector to the VSS connector.
-checked the resistance across the VSS as well as through the harness and VSS together. Both were the same.
-replaced the VSS (it was cheap)
-verified that the VSS is outputting an AC voltage that increases with vehicle speed.

This all tells me that the VSS seems to be working and the signal should be getting to the PCM. I also tried to measure the voltage of the PCM speedometer output (not currently used), but not sure what I am looking for here. The result was a constant AC voltage of 7.5V AC while the DC voltage was fluctuating – range of fluctuation was lower as speed increased.

I need to figure out what the root cause is here. Anyone else ever have a problem like this? Any ideas on how to confirm if the signal of the speedo output from the PCM is good?

Sorry for the length – thanks for reading.

Gary U
05-09-2012, 09:59 PM
Does anyone have info on how to confirm/validate the PCM output signal for the speedometer?

Raul_Chile
12-25-2012, 04:53 PM
Does anyone have info on how to confirm/validate the PCM output signal for the speedometer?

Hi Gary

I have similar problem in my LY6/T56, but I don´t have installed some VSS and HPTuners display 255km/hr ¡¡¡ and also I can´t make to work external tachometer(Serial crank, 15/15 resolution).

Do you have some idea?

Best Regards
Raul