Tach gets interference from headlights, brakes, and horn - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

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post #1 of 6 (permalink) Old Dec 27th, 18, 6:46 PM Thread Starter
 
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Tach gets interference from headlights, brakes, and horn

Hello, i have a new Autogage Tach with some pretty distinct issues that im sure one of you fine people could easily help me diagnose (im ok but definitely not good with electronics/wiring)..

HEI dist.

Sometimes, it seems on cold startup, tach works fine, but after about 2 minutes, one touch of the brakes, headlights, or horn and the needle will start bouncing around, but only when one of those three is active.

Other times, the needle will be bouncing around from startup, but if I turn on the headlights or hit the brakes, it settles down and acts as it should (opposite issue..!!)

I've researched the interwebs and read about the importance of a great ground for tachs in particular, moved the ground to a clean unshared wire straight to the engine block, tested it, same issues..

Which leaves me with the power and signal wires...

(story continued in first comment)..

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post #2 of 6 (permalink) Old Dec 27th, 18, 6:47 PM Thread Starter
 
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Re: Tach gets interference from headlights, brakes, and horn

...continued...

My signal wire is bundled with the rest of the tach wires for about 9 in. and passes thru the firewall in a tight grommet with ~5 other wires, could there be a disturbance in the force from being so close to these other wires, and giving it space/insulation help? (Seems to make sense that once the wires heat up just a bit they could start affecting the signal wire..?)

My power wire is spliced in with the radio on the ignition switch, could fluctuations in the voltage thru here be affecting the needle, and would it be smoother getting power elsewhere?

I've read about people using resistors to cure bouncing tach needles, but none of the issues described seemed exactly like mine, and unclear on the type and where it is supposed to placed (some say signal wire at the dist, some say bridging the signal and power wire close to the tach..??) Is this something that would help in my case? If so which type and where?

Those are the only ideas I got, really appreciate any assistance you are able to provide!

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post #3 of 6 (permalink) Old Dec 27th, 18, 7:34 PM
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Bruce
 
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Re: Tach gets interference from headlights, brakes, and horn

Which Autoguage Tach -- is there a module for the Tach? Which HEI (GM or Aftermarket)? Find the factory instructions for the Tach and reinstall it according to the instructions. With all respect a tach is pretty simple, only the wire from the HEI (there is a special attachment point for the Tach you know) and the ground count.

Try the tach on another engine/car...
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post #4 of 6 (permalink) Old Dec 28th, 18, 5:33 AM
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Geoff
 
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Re: Tach gets interference from headlights, brakes, and horn

I would connect a voltmeter across the battery & check it once the engine has fired. Look for any over voltage [ higher than 14.5 v ] or voltage spikes. If you get any of these, the wiring in the charging cct should be checked for bad connections &/or alt & volt reg checked.
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post #5 of 6 (permalink) Old Dec 28th, 18, 7:48 AM
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Rick
 
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Re: Tach gets interference from headlights, brakes, and horn

Try a temporary replacement signal wire that is separated from the run alongside the other wires that could be causing your interference problem. If this solves your problem, move the signal wire away from these offending wires.

If you cannot move the signal wire away to solve this, try to add a few split toroids made out of Type 43 ferrite material at each end of the signal wire. The ferrite material has high absorbent permeability to these kinds of offending signals while allowing the proper signal to pass through unchanged. Simply wrap the signal wire through the split toroid and then snap the toroid case back together.

With the Radio Shack company now gone, it's harder to find these but the Internet helps. Two companies will have these for a few bucks each. Try Digi-Key or Mouser.

Rick

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post #6 of 6 (permalink) Old Dec 30th, 18, 8:07 PM Thread Starter
 
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Re: Tach gets interference from headlights, brakes, and horn

Thanks all! Ultimately the solution ended up being a resistor in-line on the signal wire, works wonderfully now

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