Erratic Tach - Vintage Mustang Forums
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post #1 of 10 (permalink) Old 08-30-2016, 06:09 PM Thread Starter
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Erratic Tach

Hi,

My 8000k tach seems to be jumpy as I accelerate. It is fine on start up and at low speed but as I step on it and it begins to reach 3-3.5k it begins to become jumpy...and eratic and seems to stop at the 4-5k mark and will not go above this.

Any suggestions on causes and fixes

Thanks
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post #2 of 10 (permalink) Old 08-30-2016, 06:17 PM
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Had a similar issue recently - my coil was going bad, causing weird un-explainable jumpy issues and misreadings with the tach. Then one day, the coil just died. Replaced the coil and now the tach is just fine. Could be a similar issue with yours. Good luck!
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post #3 of 10 (permalink) Old 08-30-2016, 06:26 PM
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Is the tach original equiptment or aftermarket ?

Z
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post #4 of 10 (permalink) Old 08-30-2016, 08:07 PM Thread Starter
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Original tach, with petronix
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post #5 of 10 (permalink) Old 08-30-2016, 08:52 PM
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Quote:
Originally Posted by Mustangmaster1967 View Post
Original tach, with petronix
Someone else with better luck than me with Pertronix will have to chime in.

Z
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post #6 of 10 (permalink) Old 08-30-2016, 10:23 PM
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The signal for the pertonix is to high for the tach input. I added a 47k 1/2W resistor in series. Between the coil and the tach. This fixed the issue.

On my Dads 450sl, that we converted to Pertronix, we ended up using a 8.2K resistor.

So the absolute value may depend on the individual tach.

You could buy a 50K pot from Frys or Radio Shack, connect it in line (center tap to out side) turn the pot till the tach works correctly. Disconnect the wires and measure the pot. Then add the fixed value in line.

Hope this helps

Gary
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post #7 of 10 (permalink) Old 09-01-2016, 01:56 AM
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Quote:
Originally Posted by gtonavy View Post
The signal for the pertonix is to high for the tach input. I added a 47k 1/2W resistor in series. Between the coil and the tach. This fixed the issue.

On my Dads 450sl, that we converted to Pertronix, we ended up using a 8.2K resistor.

So the absolute value may depend on the individual tach.

You could buy a 50K pot from Frys or Radio Shack, connect it in line (center tap to out side) turn the pot till the tach works correctly. Disconnect the wires and measure the pot. Then add the fixed value in line.

Hope this helps

Gary


Does using an inline potentiometer work on any tach? Just hook it inline on the coil feed and adjust it until the tach is stable?

My tach is stable at lower RPM but bounces a bit above 3500. I'm figuring it's just a worn-out tach but I'd try this.


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post #8 of 10 (permalink) Old 09-01-2016, 04:12 AM
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Quote:
Originally Posted by gtonavy View Post
The signal for the pertonix is to high for the tach input. I added a 47k 1/2W resistor in series. Between the coil and the tach. This fixed the issue.

On my Dads 450sl, that we converted to Pertronix, we ended up using a 8.2K resistor.

So the absolute value may depend on the individual tach.

You could buy a 50K pot from Frys or Radio Shack, connect it in line (center tap to out side) turn the pot till the tach works correctly. Disconnect the wires and measure the pot. Then add the fixed value in line.

Hope this helps

Gary
I've got a Pertronix coil and it works fine with my tach. I know this to be true because I told the guys who chassis dynoed the car to check the tach to make sure it was correct.
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post #9 of 10 (permalink) Old 09-01-2016, 08:19 AM
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There is one other possibility, just to possibly complete the tach diagnosis picture. Original D'arsenval tach meters are by nature easily damaged and definitely do not like being contaminated by any metal shavings inside the meter. I had this happen with my 6K tach years ago. Don't ask me where the shavings came from, but somehow they got into the tach. They made the needle hesitate above 2k rpm at several places and then actually get stuck partway around the dial. If I tapped the gauge it "released" the needle. Useless for driving!
Hopefully the OP's case is related to the Pertronix and the signal being properly calibrated when it reaches the tach. Otherwise it needs rebuilt. Tachman is a good rebuilder, there are others out there.

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post #10 of 10 (permalink) Old 09-01-2016, 07:56 PM
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I had the same problem with my pertronix ignitor. Bought this from rocket man TACHS but summit also sells it.
https://www.summitracing.com/parts/cin-sn20
Worked like a charm.

1967 S-code GT built Dec 13, 1966
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