The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - Does the Tach in a 1980 require a filter?

ray_mcavoy · 04-09-2011, 07:55 PM

I believe the tach filters were used simply to prevent radio interference. So you won't damage the tach by not using a filter.

By the way, the tach filter's internal circuit is shown in some of the factory wiring diagrams and a functional equivalent could easily be built using parts from RadioShack (or other electronics stores).

The circuit consists of two resistors in series, and a couple capacitors to ground. The "filter input" wire coming from the negative side of the coil (TACH terminal on HEI) goes to a 5.6K resistor, followed by a 10K resistor (in series), which then goes to the "filter output" wire that plugs into the tach harness. A .15MFD capacitor connects from ground to the junction point between the two resistors. And a 100pF capacitor connects from ground to the junction point between the 10K resistor and the "filter output" wire.

04-09-2011, 07:55 PM	#6
ray_mcavoy Registered User Join Date: Nov 2009 Location: Sherman, ME Posts: 2,373	Re: Does the Tach in a 1980 require a filter? I believe the tach filters were used simply to prevent radio interference. So you won't damage the tach by not using a filter. By the way, the tach filter's internal circuit is shown in some of the factory wiring diagrams and a functional equivalent could easily be built using parts from RadioShack (or other electronics stores). The circuit consists of two resistors in series, and a couple capacitors to ground. The "filter input" wire coming from the negative side of the coil (TACH terminal on HEI) goes to a 5.6K resistor, followed by a 10K resistor (in series), which then goes to the "filter output" wire that plugs into the tach harness. A .15MFD capacitor connects from ground to the junction point between the two resistors. And a 100pF capacitor connects from ground to the junction point between the 10K resistor and the "filter output" wire.