so my car has been running way rich lately so i hooked up the ol laptop and found that when im going like 75mph on the freeway the VSS is telling the computer im only going like 8mph WTF?? also im getting a hidden code 72.. meaning that the SES light doesnt come on because of it! the speedo is working just fine.. i removed the VSS and theres really nothing on there to see if its broke or not soo wtf? does neone know what this could be from? im pretty sure this is the cause of my running rich cause the computer is staying in the wrong cells due to it thinking that the car is going slower than it really is

its a 96SS M6

thanks

 

I swapped my rear end a few weeks back, and in the process snapped the wiring harness that goes to the VSS in the "pumpkin" of the rear diff.

The Speedometer gets its reading from your transmission (I believe) so it still worked - but the PCM was not seeing the correct speed. I had to drive it like this for 2 days while I waited for the dealer to order in the new wiring harness.

After asking the local f-bod owners, checking junkyards etc, I believe I paid $75.00 for the 2 foot section of wire at the dealer I only wish that $75.00 included a jar of vaseline - woulda made the process alot easier

Edit: check your wiring that runs to the sensor, also make sure the connectors arent corroded etc. Hopefully this helps.

 

yea cause the speedo is working fine... its the computer that isnt getting a good signal

so the speedo uses the transmission signal and the computer uses the rear diff for a signal i thought that the sensor on the rear diff was used for the abs???

 

I believe the Rear diff sensor feeds the ABS and the PCM a signal.

When I had the wire off - the ABS INOP light was on, as well as the speed readout in the PCM being wrong.

Plugged in the new harness and she was good to go

 

 

I really don't think the computer determines speed from the rear end, or transmission for that matter. If it did, your speedometer would jump up when you spin the tires, and it doesn't. I imagine the speed sensor would be connected with the front wheels.

Good Luck.

 

uhhh i think it does actually! i think i remember seeing my speedo jump while doing a burnout before but i could be wrong!

so wtf then! where does each signal come from!

where does the speedo signal come from

and where does the ecm speed signal come from

 

The speedometer is controlled by the transmission. I can't seem to find where it is plugged into the T56 but the 4L60E is towards the rear near the output.

Found a neat little website that has the entire T56 schematics to include the location of the gear drive for the speedometer but not where the connection comes out of the tranny. http://csce.uark.edu/~jgbertr/rb/4th_gen_tech2.html#t56

Hit me up on Messenger later on and I'll let you know if I've found anything else.

 

The VSS signal comes from the sensor in the tailshaft of the transmission. This tells the ECM how fast you are going and the ECM feeds the speedo signal. The sensor in the rearend is just for ABS. Maybe the speed sensor in your tranny is messing up.

 

There is only one VSS sensor, and its on the tailshaft of the tranny. It does not connect directly to the speedo, just to the PCM. The PCM needs to process the data to provide the correct signal to the speedo based on the programmed constants for gear ratio and tire diameter.

Doesn't answer your question, as to why the PCM and speedo are showing different numbers, but it should clarify some of the confusion in the responses.

 

dammit bish quit post whoring! you coulda called and told me this

 

cool thanks that definately explains that part of it but its just weird! WHY is my computer not reading correctly but my speedo is you would think that if the computer was off than the speedo would be off as well

 

well i REPLACED the VSS and im still getting code 72 (OUTPUT SPEED LOSS) wtf?? this mother ****** i swear these cars do the gayest sh*t sometimes

anyone have NE ideas

 

You must be running an OBD1 PCM in your '96 SS

What I can tell you is that, from everything I've seen, the OBD1 M6 cars do not even have this DTC enabled from the factory.

 

oh it does trust me im looking at the code from the scanner

heres what else ive discovered...

ok in THIS PICTURE HERE it shows on PINS 31 PURPLE (VSS GROUND) 32 YELLOW (VSS SIGNAL) these wires come DIRECTLY from the VSS sensor itself that bolts into the tranny... now i checked these wires and they are both getting signals FROM AND TO the computer from the VSS SENSOR itself!!

NOW... if you look at THIS PICTURE HERE you will see that on PIN 8 DK GRN/WHT (VSS OUTPUT) this is the wire that i think is somehow causing the problems! cause the code im getting is code 72 Speed Output Loss! so WTF?? where does this wire go and WHY is my speedo getting a signal and NOT THE COMPUTER it just doesnt make sense!

heres another thing! i disconnected the VSS connector from the sensor and started it back up and drove it and the speedo and the computer dont work! so its definately sending a signal to the speedo but how the FU*K is the computer not getting it