I am trying to fix my speedo. I did some searching on here for a fix and I just replaced the buffer box under the dash with a used one from ebay with out fixing it. I was under the impression the cruise control wouldn't be working if the problem was the VSS so I thought the buffer box was bad since my cruise still works. Is this true? Could the VSS still be my problem?.
91 RS 305 TBI.

 

I would say no. It has an input signal from the VSS so it can determine if your speed changes so it can correct it. Here is a link to the diagram for a 92 camaro, should be the same as your 91.

http://www.austinthirdgen.org/mkport...se_control.jpg

 

The sensor on your trans is probably shot. Like a 5 minute fix. Longest part is lifting the car.

 

It could also be your VSS Buffer has no power or has gone bad.

Question on the cruise control diagram. I have an 89 TransAm with an 89 Pontiac Firebird Service manual and my cruise control digram don't match the colors on that one for the multifunction lever and cruise control modual for your post. My original harness matches your digram, could Pontiac/Helm have screwed up when writting the manual or does that mean my harness has been swapped out prior to me buying the car?

Joe

 

Doubt it

" the tbi uses a 2,000 pulse per mile sensor wire to a vehicle speed buffer and from there it sends a 4,000 optical signal
to the speedometer and a 2,000 pulse per mile optical signal to the tbi ecm and cruise control"

So if the CC is working the VSS and buffer ( at least the CC output ) must be working OK

 

I'd say your harness was probably swapped at some point. I doubt Pontiac screwed up their factory shop manual, but stranger things have happened.

 

 

Well this helps. Since the Cruise control is working with both the old buffer box and the one I purchased off ebay. It probably isnt the VSS and both buffer boxes are sending at least the CC signal. I am leaning towards the speedo unit at this point. I dont have any volt meters or audio signal meters. I should buy one soon. Thanks everybody. Ill post up when its fixed.