Odd tach problem - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

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post #1 of 5 (permalink) Old Apr 5th, 19, 10:48 PM Thread Starter
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Odd tach problem

On a 72 with SS gauges, I have an intermittently working tach. Sometimes it behaves normally. Sometimes not at all. But often at idle the tach will display about negative 200 RPM (the needle rest below zero. Increasing engine speed will increase tach, but always reads lower than it should. When I shut off the ignition, the gauge will jump up to the O mark and stay there. That is the needle will properly point while the engine is off. All other instruments requiring a good ground seem to be okay.

I just replaced the printed circuit board if that might have anything to do with it. BEFORE the replacement I had the same intermittent operation, but had never seen the negative rpm behavior.

Thanks for any help!
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post #2 of 5 (permalink) Old Apr 6th, 19, 11:07 AM
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Re: Odd tach problem

The electronics in that tach are nearly 50 years old, it is probably just about shot. Probably has a few weak solder joints inside or some of the components are intermittently shorting out. Either replace it or have it rebuilt with new electronics.
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post #3 of 5 (permalink) Old Apr 6th, 19, 1:27 PM
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Re: Odd tach problem

Do yourself a favor and send the tachometer to Redline Gauge Works. I got mine back quickly. They can also convert it to a very accurate digital unit. Don't worry you cant tell by looking at it
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post #4 of 5 (permalink) Old Apr 9th, 19, 9:41 AM
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Re: Odd tach problem

I also have had good luck with Red Line... Al
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post #5 of 5 (permalink) Old Apr 9th, 19, 9:55 AM Thread Starter
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Re: Odd tach problem

Thanks guys. I'm just going to replace the entire unit. This is my daily driver and I will likely have to pull the dash to replace it and I don't want to wait a couple of weeks. I'll wait until I have the replacement and then just swap out.
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