Tach wiring question

67Dart273 said

Electronics DOES die once in awhile. What exactly is the type of your ignition? Tachs will not work on CDI/ MSD ignition without special adapters or some built in trigger wire, and on CD they don't go to the neg side of the coil, which is grounded on a CD

Frankly I'm surprised the shift light works (it's integral?) and not the tach

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This is all stuff that has worked for many years...nothing new about it.

Ignition is a 4-pin LX-101 box. The tach works off the negative coil terminal as with any 'normal' ignition of the period (it's a '73 car). I ran an MSD box for 20 years and, yes, the tach signal was separately derived INside the MSD box.

That the shift light (yes, part of the tach) works means to me the tach is not completely blowed up...it just quit working.

IIRC, this Mallory tach uses a 'derived' tach signal, taking the input and running it through some voltage protection circuitry to keep 'too hot' stuff out of the tach. Any arcing/sparking of such an input will not bode well. IF a 'normal' input signal is, for the sake of argument, 500v, an arc will be well OVER that. Another IIRC, but I think the tach signal needs to come from something OVER a particular voltage..too low and it won't get past the protection circuit.

It just occurred to me that MAYBE the tach has worked only BECAUSE of the crappy connection. Maybe fixing that made the tach signal TOO hot. Hhhmmm..... I'll try a resistor in series to the tach....