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Interesting Starting Problem, Not What I Expected To Learn

Discussion in 'Jeepster Commando and Commando Tech' started by Robert Wight, Oct 20, 2021.

  1. Oct 20, 2021
    Robert Wight

    Robert Wight New Member

    Houston, Tx
    Jan 5, 2021
    Was troubleshooting the aftermarket tachometer which stopped reading and discovered that if the signal wire from the HEI distributor to the Tach is shorted to ground, the engine WON'T start. BUT, if the short is downstream of a signal filter, the short will not interfere with the starting of the engine. I guess the signal filter is acting as a surge suppressor.

    I had pinched the wire between the dash and the metal backing in the dash pad, and this pinch wore through the insulation.

    I hadn't thought about it until I was troubleshooting the Tach, but I assume a tachometer is nothing more than a frequency meter, it measures the pulses from the HEI, and calculates the RPM based upon the Hz of the signal. If the signal is shorted, there aren't enough amps left to fire the spark plug.

    Had it not been for the signal filter, I would have driven myself mad trying to find the reason why my engine wouldn't start.... It never would have occurred to me that the signal to the tachometer would interfere with the starting of the engine.
    :idea: Perhaps I should install a fuse in the signal wire... :idea:

    Part of my troubleshooting involved bypassing the signal filter to localize the problem, the pretty spark was an obvious clue that something was amiss with the signal wire.
    dnb5853 and Fireball like this.
  2. Oct 20, 2021
    Howard Eisenhauer

    Howard Eisenhauer Super Moderator Staff Member

    Tantallon, Nova...
    Nov 22, 2003
    Yes the tach output is really just a connection to the switched side of the coil, no isolation at all.
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