Join Date: Mar 2002
Location: Austin, TX, USA
Re: Tach filter revisited
Tachs are a mystery component. These archives are full of tachometer stuff. It's got to be a top 10. Noise is usually temporary and the filter will reverse the effect. But there is another possibility that would leave the tach permanently damaged.
A tach is a frequency to voltage convertor. At the heart of it is a diode on the input to rectify the signal from the primary ignition circuit. Diodes are susceptable to breakdown at higher voltages. I suspect the input diode is failing. Semiconductor technolgoy wasn't all that great back in the 60's anyway. I'd hope the replacements are a bit more robust.
The tach filter will remove some of the noise as Jim said, and reduce the amplitude of the signal from the coil which protects the diode. It's good insurance.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
Last edited by dnult; Mar 24th, 08 at 12:54 PM.