GM installed tachometer signal filters starting the 1970's. The ignition system produces voltages upward of 30,000 volts to produce a spark. That high voltage causes electrics noise, and voltage spikes and surges, in the signal wire going to the tachometer circuit board. The original signal filter deteriorate with age. This can an cause erratic signals, or block the signal going to the tachometer circuit board.
This tachometer signal filter -
Compatible with - Most 1975-1990 GM vehicles that have "circuit board" driven Tachometer. Not compatible with Tachometers that are simply a coil (these are usually pre-1975 models).
Easy to install - Three color-coded wires allow fast and easy fool-proof installation. Provides protection and superior response on all Classic Tachometers.
See our Blog post: How a signal filter may help your Tachometer read properly