[Dynodon4]

Hello: I am trying to repair the speedometer on my Son's 89 Trans Am. When we bought the car a while back, the VSS was disconnected. Plugging it in, the speedometer will jump over 120 MPH when the vehicle is only doing about 5 MPH and shut the engine off. I replaced the VSS and same problem. The car came with another instrument cluster so I tried swapping that out and again no luck. I connected a scanner and it reads the same MPH as the speedo so I figured the Buffer box was faulty. Changed that out and again nope. I thought that maybe the wiring from the VSS to the buffer was damaged so I made up a temporary harness and again nope. I found the test procedure in the GM manual for the VSS and buffer and all checked out except the ac volts from the VSS seemed high. I got just over 2 Volts turning the rear wheels by hand. If I recall 4 or 5 volts is max and would expect 2 volts around 50-60MPH. I've inspected the VSS reluctor ring for filings in case that was causing too many pulses but it was clean. I have also unplugged the red cruise control wire from the buffer in case it was feeding back and causing a false signal somehow. I'm running out of ideas. Any input would be appreciated. Thanks Don

 

[Dynodon4]

Just an update to what I've found. Talking with a GM tech, he suggested and loaned me a frequency generator that I could power the speedo with to mimic the vehicle speed sensor. At 60HZ I should get 54mph. Plugged in at the buffer I got 56MPH and steady. Connected to the VSS pigtail I got 58 MPH and steady. obviously the VSS is reading wrong. I tried another used VSS( the third one now) and still it wants to read 120MPH at about 5mph speed. My next guess is maybe a worn drive shaft yoke bushing that changes the VSS to reluctor gap or some how the wrong reluctor. I'll keep you guys posted.

 

[Dynodon4]

Well I found that the output bushing, just by the governor, was horribly worn out. I would guess around .050-.060 wear. So bad that after looking very closely I could see where the reluctor wheel had been touching the speed sensor. The transmission has been doing some odd shifts so I decided to find another. Took the reluctor wheel off the old trans and installed on the new. Got her all back together and the speedometer still was reading way too high. I located an 88 Firebird in a wrecking yard that had the electronic speedometer and discovered that it had a different VSS. This one is driven by a gear on the output shaft instead of reluctor wheel. Bingo!!! All is working as it should. Looks like in the past the transmission was replaced with one that had the wrong style VSS and the speedometer was left not working. Anyways I thought I should update this in case someone else runs into something similar.