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Thread: Tachometer needle jumps sometimes - cause?

  1. #1
    Senior Member 82DMC12's Avatar
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    Tachometer needle jumps sometimes - cause?

    I have noticed sometimes that my tach needle generally works smooth and steady but periodically it will jump up for a moment like it has a seizure or the throttle was barely blipped, and then it is fine again. This has been observed both at idle, like at a stop light, and while accelerating especially at higher RPM's. I can't predict it but it does it randomly during pretty much any drive. I don't notice any change in engine performance during this split-second jump.

    I assume it means the signal is getting interference somehow but it's so brief that the needle doesn't jump more than say 20 RPM - where should I be looking? I did replace the impulse coil with a NOS coil a couple months ago when I had my distributor out.

    Ignition coil is an Accel Super Stock coil and perhaps 20 years old. I have a new Bosch rotor, DMCMW repro cap, DMCMW ignition wires and coil wire, and NGK plugs, all installed this year and properly gapped.

    Here's a video showing the tach jump at about 5 second in.

    https://youtu.be/hBG6znVDcBU

    What is the cause?
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  2. #2
    Senior Member Bitsyncmaster's Avatar
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    That is going to be hard to find the problem. What I would do is check the DC voltage from fuse #5 with an O-scope to see if the DC voltage is related to the jump. If it happens enough you can test if loads (headlights, AC etc.) turning on or off cause it.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #3
    Gone Fishin' Ron's Avatar
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    You might try connecting a handheld tach and see if it also jumps -- Could be a loose connection feeding the dash tach (but no problem to the distributor).

  4. #4
    Senior Member 82DMC12's Avatar
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    Thanks guys. It was also suggested a loose PCB or the screw that holds it on. I'll actually be pulling my binnacle off perhaps next weekend for replacement so I'll check that.

    Sent from my Pixel 3a using Tapatalk
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  5. #5
    Senior Member
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    I would start by replacing the coil. Many owners had problems with "performance" coils. This could be an indication of the coil beginning to fail.
    David Teitelbaum

  6. #6
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    Quote Originally Posted by 82DMC12 View Post
    Thanks guys. It was also suggested a loose PCB or the screw that holds it on. I'll actually be pulling my binnacle off perhaps next weekend for replacement so I'll check that.

    Sent from my Pixel 3a using Tapatalk
    I'd also like to know because mine does the exact same thing in exactly the same way you've described it. I also think it's a loose connection or ground up at the console or the printed circuit, but I haven't gone back in and taken apart all of that to find out. Yet.


    Sept. 81, auto, black interior

  7. #7
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Jonathan View Post
    I'd also like to know because mine does the exact same thing in exactly the same way you've described it. I also think it's a loose connection or ground up at the console or the printed circuit, but I haven't gone back in and taken apart all of that to find out. Yet.
    I'm going to get a new OEM ignition coil and see how that goes first. It could definitely be the coil because this spring I realized it was hooked up backwards (!!!) but interesting to report I never had a problem all this time. This spring I reversed the wires to make it "right" and seemingly no change but I am noticing this random tach jump. Maybe related to reversing the coil? My idle hasn't been as smooth ever since I reversed the coil but I've made a lot of other adjustments lately too....

    If that's not it, in about a week I'll be inside the binnacle for the first time in about 20 years so I'll see what I find there (I did replace the PCB board back then though!)

    Sent from my Pixel 3a using Tapatalk
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  8. #8
    Gone Fishin' Ron's Avatar
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    Quote Originally Posted by 82DMC12 View Post
    I'm going to get a new OEM ignition coil and see how that goes first. It could definitely be the coil because this spring I realized it was hooked up backwards (!!!) but interesting to report I never had a problem all this time. This spring I reversed the wires to make it "right" and seemingly no change but I am noticing this random tach jump. Maybe related to reversing the coil? My idle hasn't been as smooth ever since I reversed the coil but I've made a lot of other adjustments lately too....

    If that's not it, in about a week I'll be inside the binnacle for the first time in about 20 years so I'll see what I find there (I did replace the PCB board back then though!)

    Sent from my Pixel 3a using Tapatalk
    $.03

    Reversing the polarity on the coil will not hurt it, but it will reduce the spark. And, if the coil is causing the needle to jump, you should be able to hear the engine miss.

    With all of the changes you've made, I'd suggest you go over all of the basic setting and tests.
    If the (handheld;-) tach needle doesn't jump and there in no skip when using the timing light on each of the plug wires, I wouldn't purchase a new coil, except as a last resort stab in the dark.

  9. #9
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Ron View Post
    $.03

    Reversing the polarity on the coil will not hurt it, but it will reduce the spark. And, if the coil is causing the needle to jump, you should be able to hear the engine miss.

    With all of the changes you've made, I'd suggest you go over all of the basic setting and tests.
    If the (handheld;-) tach needle doesn't jump and there in no skip when using the timing light on each of the plug wires, I wouldn't purchase a new coil, except as a last resort stab in the dark.
    How do you hook up an external tachometer? I've never attempted such a thing. I see I can get a Bosch tach on Amazon for like $33.99.

    Sent from my Pixel 3a using Tapatalk
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  10. #10
    Senior Member
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    Hooking up a coil "backwards" makes the spark jump from the ground electrode to the center electrode. It can cause the engine to not run as well as it should. Make sure it is correctly hooked up now, maybe it was right and now it is wrong. I would replace the coil even if it is good, too many stories of non-OEM coils causing problems and no one has said an aftermarket coil gave them any noticeable improvement. If it was backwards for a long time I would clean and regap the plugs or maybe just replace them (depends on how worn they are).
    David Teitelbaum

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