I am looking for help/suggestions troubleshooting a non-working speedometer in a 1989 Formula 350 car.

As stated, the speedometer does not move at all. I have tried:

• swapping out gauge cluster
• swapping out VSS buffer box
• taking sensor out of side of trans (gears are good) and spinning it with a drill. It gives the correct pulsed voltage.

Not sure what else to check. Computer? Is there a way to check the signal coming to the VSS buffer? (green wire I think) while car is moving to see if there is voltage? Ground issue?

Check engine light is on with a generic code pointing to VSS issue.

Thanks all.

 

Is the gear on the output shaft of the tranny in position to turn the sensor gear?

 

What equipment do you have available to test the VSS output signal? An oscilloscope would be ideal, but a multimeter with a duty-cycle and/or frequency counting scale could also be used.

If the vehicle has a cruise control, disconnect that for testing, just in case the cruise module is shunting the signal to ground making it impossible for the speedo head to rear it.

 

Quick update. Got a used buffer box from Hawks and put it in. Now I have some life to the speedometer but it only works above 25 mph and is bouncy. Kind of like when you had an old car with a cable driven one and it was really worn and the needle would bounce.

Check engine light no longer on for VSS circuit mismatch. I think I am on the right track, but not sure which way to proceed.