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Thread: Sudden loss of tach and no power

  1. #1
    absotively posilutely bytes311's Avatar
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    Sudden loss of tach and no power

    This issue used to occur every once in a blue moon after I'd go out and start the car if it's been sitting in the sun for a few hours. The issue would resolve itself just by simply restarting the car, however, today it happened in the middle of a drive.

    While driving, I noticed my tachometer jumping around and becoming "sticky" before quitting all together (within a span of a couple seconds) as if it was no longer receiving a signal. Immediately, the car had no power during acceleration and would stumble when given gas. I shut the car down and attempted to start it up again. Normally the tach would reset itself with the key in the "on" position, but this time it would not wake up, even after several on and off attempts and bumping the binnacle. I also observed that I wasn't getting an oil or battery light in the "on" position. My alt gauge was also bizarrely low. I had no issue restarting the car. It would run fine at idle, but it would not take any acceleration,

    Another strange observation: When I turned my headlights on, there'd be a loud and repetitive "clunk" from the engine bay. I was not able to pin-point it exact, but it sounded like it was coming from the ignition distributor or thereabout.

    After several attempts of key "on and off" and messing with the ignition ECU connector, and touching other various connectors, the tach went back to normal. The "clunking" noise with the headlights on in the engine bay also disappeared. Now I have no idea what that was all about and I can't reproduce it.

    Everything ignition related on my car is stock, except for the alternator.

    Any ideas?

    Thanks!

  2. #2
    Senior Member Bitsyncmaster's Avatar
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    Tach acting up could be ignition (ignition coil primary) was failing. Lack of power would also confirm that.

    Now headlights causing a clunk makes no sense unless it caused a misfire which may also be related to the the ignition problem.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #3
    absotively posilutely bytes311's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    Tach acting up could be ignition (ignition coil primary) was failing. Lack of power would also confirm that.

    Now headlights causing a clunk makes no sense unless it caused a misfire which may also be related to the the ignition problem.
    Thanks Dave. When you say ignition coil primary, do you mean something along the positive circuit is failing, or is the coil failing?

  4. #4
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by bytes311 View Post
    Thanks Dave. When you say ignition coil primary, do you mean something along the positive circuit is failing, or is the coil failing?
    It could be the coil, the ignition ECU, wiring or even the pickup coil or it's wiring.

    Make sure the two pin connector on the pickup wire is clean. Mine was corroded so bad I removed it.

    The ignition coil does tend to fail with heat so replacing that next is probably the least effort and cost.

    Then you may be able to find a used ignition ECU to replace yours or borrow one if your problem shows up often.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  5. #5
    Senior Member Rich's Avatar
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    Check out Posts #1-4 in this thread, too, plus the others there.

    Same symptoms you report - jumpy tach and engine stumbling. 3 possible issues listed; pointing to the ECU or its wiring as Dave M. also mentions here.

    The thread ends without the OP reporting a positive resolution so it's only a guess for now.
    March '81, 5-speed, black interior

  6. #6
    Senior Member
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    Because you disturbed the problem it is hard to find again because it is intermittent. Makes it very hard to replicate and therefore difficult to know what it is and if you fixed it. In all likelihood it is a wiring/connection problem, not a part problem. Unless you can find a way to replicate it the "fix" is to go over every connection in that circuit.
    David Teitelbaum

  7. #7
    Administrator Ron's Avatar
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    Try checking that Auxiliary Relay 2 is working.
    It feeds fuses 1,3,4,5 and the ignition resistor. If it is not feeding fuse 1 OR the ignition resistor, you will loose ignition. If it's not feeding fuse 5, you will loose dash instruments and indicators.
    Check in order:
    Its brown wire should have 12V at all times.
    Its black wire should have ground at all times.
    Its two white wires should have 12V when the key is turned to run.

  8. #8
    absotively posilutely bytes311's Avatar
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    Thanks guys. This gives me a place to start.

  9. #9
    absotively posilutely bytes311's Avatar
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    I pulled fuse #5, which is the fuse for the dash instruments, while the motor was running and it reacted EXACTLY like it did when I lost my tach signal the other day. I revved the engine and it sputtered and wanted to die. It would idle just fine though. I reinserted the fuse and everything worked again like normal.

    The alternator light stayed on for a few seconds and then switched off, which tells me the car was running off the battery and not the alternator. Makes sense if the alt light is out in the dash.

    My battery puts out 14v, however, it is coming up to 7 years at this point. I'm also using a 10 amp fuse for #5 while online it shows a 20 amp.

  10. #10
    "Former Delorean owning Guru" Spittybug's Avatar
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    Listen to Ron and Bitsyncmaster, they knows their proverbial 'excrement'..... David, while good at waxing philosophic, is generally short on solutions.

    CLEAN EVERY CONNECTION & GROUND. And I don't even own one of these things anymore....
    Owen
    I.Brew.Beer.

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