Tachometer '81 Z trouble

Discussion in 'Electrical Systems' started by kwicker, Dec 2, 2017.

  1. kwicker

    kwicker New Member

    3
    0
    Oct 9, 2017
    I have a '81 Z with a troubled tach. When I turn the ignition switch on (without cranking) the tach goes to 500 rpm. As I watch it, the needle creeps up to about 1200 rpm and seems to stop there. When I crank the engine, the tach needle pops clockwise to the 6 o'clock position. I checked the continuity of every circuit on the printed circuit board. I check the continuity of the brown wire from the circuit board harness to the distributor. I disassembled the distributor to make sure everything looked ok inside. I put a voltmeter on the brown wire pen of the wiring harness and read the voltage from there to the chassis ground when I turned the switch on without cranking and got none. I checked the voltage when I cranked the engine and got about 14.7 DC volts. When I rev the engine, the voltage doesn't seem to move notably. Please help. Thanks.
     
  2. Rdobbs1977

    Rdobbs1977 Huge Guns N Roses Fan

    272
    8
    Dec 1, 2014
    Nashville
    Your voltage is not going to move beyond what you've stated with the motor running. i take it you must have a factory tach as you've mentioned a brown wire and circuit board?

    Do you have an aftermarket tach that you can quickly wire up to see what happens? Were it me, that's exactly what i'd do to eliminate the tach as a possible culprit.
     
  3. kwicker

    kwicker New Member

    3
    0
    Oct 9, 2017
    Thanks Rdobbs. Yes, a factory tach and no, I don't have an aftermarket to wire in. Maybe I can find someone local who has one I could borrow. As for the voltage, I thought the tach was like a voltmeter calibrated for engine RPM vs. voltage. That's why I expected the voltage to vary with speed. Bad assumption?
     
  4. Rdobbs1977

    Rdobbs1977 Huge Guns N Roses Fan

    272
    8
    Dec 1, 2014
    Nashville
    no problem kwicker. No your assumption rings true, after all one wire on your tach goes to the coil where voltage takes place. IE there is a correlation there without getting scientific on things (which i'm not that guy ha ha). however, imo if a guy had a voltage problem, the tach would be the least of your worries, as in you'd prob have bigger problems going on than just your tach such as misfires, hard starts, no start, etc. i keep a cheap $15 tach (can buy at a local parts store) on hand as just another test tool when its needed. I recently had an autometer monster tach act up on my drag car. took it out and put the cheapo tach in to see if it was perhaps my msd box, wiring, or the auto tach giving me problems. turns it it was the auto tach as the cheapo tach worked like a tach should. even put the auto tach on another motor i had, and it would not work right. IE, the tach was my culprit.
     
  5. kwicker

    kwicker New Member

    3
    0
    Oct 9, 2017
    Very good information, indeed, Sir. Thanks.
     

Share This Page