Troubleshooting Vehicle Speed Sensor / PCM
Troubleshooting Vehicle Speed Sensor / PCM 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.
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.
Teching In
Joined: Oct 2007
Posts: 23
Likes: 1
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
Registered User
Joined: Sep 2020
Posts: 4
Likes: 2
Thanks Brumby !! Worked perfectly, thanks mate !!
For reference for future readers :
I have 2006 4L80e with TCI ez-tcu. The ez-tcu is outputting 4000 ppm square wave (oc). I used the Dakota Digital to feed the yellow and purple/white wire. Yellow is the "speed hi" and the purple/white is "speed low".
This is a moonie cluster on a 1988 Chevy Silverado C3500
The moonie cluster was looking for 4000 ppm sine wave or ac signal. This little $100 box worked awesome.
Hope this helps someone in the future !!
For reference for future readers :
I have 2006 4L80e with TCI ez-tcu. The ez-tcu is outputting 4000 ppm square wave (oc). I used the Dakota Digital to feed the yellow and purple/white wire. Yellow is the "speed hi" and the purple/white is "speed low".
This is a moonie cluster on a 1988 Chevy Silverado C3500
The moonie cluster was looking for 4000 ppm sine wave or ac signal. This little $100 box worked awesome.
Hope this helps someone in the future !!
Teching In
Joined: May 2022
Posts: 15
Likes: 1
Hello guys, Building my 79 Camaro with LS2 engine from 05 Vette. I'm using a 411 PCM from PSI with their stand alone wiring harness. The PCM/harness is configured to work with cruise control including "Cruise engaged" and Cruise Inhibit" signals. I'm using a T-56 6-speed Tremec with 17 tooth VSS reluctor wheel. PCM is tuned for 17 ppr VSS input. I have a cruise control module from a 2000 Yukon. My instruments will be Dakota Digital but that package is not due in until early January. Will I need a DRAC/buffer module for this system? I'm assuming the VSS out from the PCM will need to feed the speedo first. Not sure if Dakota Digital provides an output to the cruise. If it does, it seems like the DRAC would be unnecessary. Please let me know what I'm up against. Thanks!
