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Thread: Bouncing / fluttering tach problem solved!

  1. #1
    Senior Member
    Join Date:  Jun 2011

    Posts:    4,686

    My VIN:    3937

    Bouncing / fluttering tach problem solved!

    Well, giddyup. Today I figured out a small issue that had been around on my car for a while.

    My tach needle would bounce from time to time. Flutter even. It wasn't what the engine was doing though and it wasn't idle hunting. This was the gauge dancing a little while you were driving. I say it looked like it was bouncing because of how light that needle is inside the gauge and if it got a weird signal, it might look like it was spazzing out a little.

    I figured it was a ground connection of some kind and had pretty much convinced myself it was something on the back of the cluster in the printed circuit. When driving, it would dance usually only when your foot was off the gas and then not always. An intermittent thing of course, which makes finding it a challenge.

    A few nights ago I went to go for a drive and it was shuddering, like it wanted to stall almost and giving it gas in first (automatic trans) felt like there was no power. The shuddering kind of felt like it might if you've ever got the engine wet after a wash and maybe one of your spark plug wires is loose or full of water.

    In any event, I never left my neighborhood and instead went home and parked it for a look. I got looking at the spark plug wires and ballast resistor and when wiggling things realized touching the ground wire that is located below the ballast resistor made my engine compartment light flicker. And it was repeatable.

    So I wiggle some more and see that the ground wire that goes from that bracket across like 6 inches to the engine block right near the idle speed motor was loose. Not the screw on either end, but the crimped on connector I had replaced a while back. See, I took that bracket off a few seasons ago to clean it up and paint it and replace the resistor. I must have put a new connector on the end of the wire and it wasn't a very good one. Got brittle under the heat and wasn't making solid contact anymore.

    So I made a new wire altogether with two new connectors and I soldered them on and put heat shrink over the ends. And I go on a drive tonight and BAM! no more bouncing tach needle! Yeehaw! No more flickering engine bay light either. So another poor ground issue... surprise!! This little wire connects on the same screw of the ballast resistor bracket with two other small grounds which I believe are also tied to the ignition or idle circuit and then they all get tied to the engine block on the same jumble as the idle speed motor. And a chunk of that grounding system wasn't making good contact.

    So there you go. Another little fix in the troubleshooting toolbox.


    Sept. 81, auto, black interior

  2. #2
    Senior Member
    Join Date:  Nov 2019

    Location:  Pittsburgh, PA

    Posts:    216

    My VIN:    Yes.

    I have a similiarish problem. My tach is non-responsive, but sometimes it starts working with a tap. This started after last time I removed it to retrieve an LED I dropped inside it (don't do this).

    I'm pretty sure the problem is some grounding/loose connection, given the taps and messing with the rear. I guess I'll have to remove it one more time and make sure everything is seated properly.

    This is the very last thing in my cabin not working properly, so of course once I fix this, something else will go wrong.

  3. #3
    Senior Member
    Join Date:  Nov 2019

    Location:  Pittsburgh, PA

    Posts:    216

    My VIN:    Yes.

    Turns out that when I replaced the PCB I only did the nuts finger tight and many had wiggled loose. Tightened better and all working.

    Sent from my Pixel 3 using Tapatalk

  4. #4
    Senior Member
    Join Date:  Jun 2011

    Posts:    4,686

    My VIN:    3937

    False alarm on my end. It was back to fluttering the next time out. So it wasn't that grounding wire on the ballast resistor bracket. I suspect you're right about it being related to something loose on the back of the cluster. I haven't gone digging back there in a while. Guess I'm due.


    Sept. 81, auto, black interior

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