what are the most common causes of the speedo not working at all on a 140k mile 91' RS 305 (L03)? thanks

 

Does the cruise control work (if equipped)? If not, I'd start with checking the VSS & its drive & driven gears at the trans. The VSS connects to the VSS buffer box, which is near the ECM. The VSS buffer sends a 4000 pulse per mile signal to the ECM & the cluster, and a 2000 pulse per mile signal to the cruise control module. Both VSS buffer outputs should toggle between 0 and 5 volts while the car is moving.

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Tim

1992 Z28, 305 TPI 5 speed, 12.1 sec @ 114.7 mph

1984 Berlinetta 305 TPI Auto, 13.8 sec @ 99.7 mph