69 Tach and HEI - Team Camaro Tech
Electrical & Wiring Troubleshooting electrical.

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post #1 of 30 (permalink) Old Aug 31st, 03, 12:59 PM Thread Starter
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I have a repop tach and a large body HEI. The tach works fine until I rev it up to about 3000 RPMs and then it jumps around. I have gone through everything that I can think of and nothing fixes it. Any suggestions before I declare the tach junk?

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post #2 of 30 (permalink) Old Aug 31st, 03, 02:13 PM
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Do you have a tach filter in series with the feed wire from the HEI to the tach? Corvettes with HEI and electronic tachs had an in-line filter, and without one, they do the same thing. [img]graemlins/thumbsup.gif[/img]

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post #3 of 30 (permalink) Old Aug 31st, 03, 03:34 PM Thread Starter
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Really?!?!

No I don't have a filter, do you know where I would get one?

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post #4 of 30 (permalink) Old Aug 31st, 03, 04:56 PM Thread Starter
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I found in my "extra" electronics stuff I have a 47nF capacitor, a 3k resistor and a 10k resistor.....I think that's all there is to the filter!!

[ 09-01-2003, 02:34 PM: Message edited by: paulm ]

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post #5 of 30 (permalink) Old Sep 1st, 03, 10:16 AM Thread Starter
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Do you think that this would work?

From dist....3k resistor to 47nF capacitor, 47nF capacitor to ground, 47nF capacitor to 10k resistor to tach feed.

http://www.stratagaz.com/TachFilter01.JPG

http://www.stratagaz.com/TachFilter02.JPG

[ 09-01-2003, 02:34 PM: Message edited by: paulm ]

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post #6 of 30 (permalink) Old Sep 1st, 03, 10:40 AM Thread Starter
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Wooooohooooooo!!!!!!!!!!!!!!!!!!! It works!!!!!!!

That tach never worked right until today!!!!

JohnZ, THANK YOU, THANK YOU, THANK YOU!!!!!!!!!


If anyone has problems with their tach and an HEI distributor, one solution may be to try the "tach filter" in my pictures above. I am not much with wiring schematics and such, but with the pics and my description you should be able to figure it out!

You will need:

1) 1 3k resistor
2) 1 10k resistor
3) 1 47nF capacitor
4) 6 inches of 16 awg wire(brown for filter in/out)
6) 6 inches of 16 awg wire(black for ground)
5) 4 butt connectors
6) 1 female spade
7) 1 male spade
8) 1 ring terminal that a 5/16 bolt can fit through


I found this diagram:



[ 09-01-2003, 02:34 PM: Message edited by: paulm ]

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post #7 of 30 (permalink) Old Sep 1st, 03, 11:07 AM
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Hey PaulM which is it 3k resistor or 5.6 resistor?

This is so much fun I can't believe it's legal.
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post #8 of 30 (permalink) Old Sep 1st, 03, 11:33 AM Thread Starter
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Duh.....I used an Orange/Black/Red, which is a 3k!! I tried one with a 5.6k, but it didn't work.....let me fix my info!!


Sorry about that!

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post #9 of 30 (permalink) Old Sep 3rd, 03, 04:03 PM
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If you're going to install a bounce filter, making the connections inside the tach housing makes for a clean install. Also, not all tachs will respond to the same combination of resistors. Trial and error will find the proper value. If you don't have a pocket full of resistors, install a 40k adjustable trim potentiometer and adjust until you find the resistance that stabilizes your tach.
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post #10 of 30 (permalink) Old Sep 3rd, 03, 05:32 PM Thread Starter
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I thought about using a potentiometer, but played with a few different resistors instead. Do you think that the capacitor could need to change as well?

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post #11 of 30 (permalink) Old Sep 4th, 03, 12:33 AM
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No, the cap is fine. All you want to do is to reduce the peak-to-peak voltage of the incoming signal to an usable amount.

The cap levels out the peaks, as in the definition of a "filter," a selective network of resistors, inductors, or capacitors which offers comparatively little opposition to certain frequencies or to direct current, while blocking or attenuating other frequencies.

What you have made is a "T-section" filter.

You're lab score is a 98........Good work !!

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post #12 of 30 (permalink) Old Sep 4th, 03, 04:04 AM Thread Starter
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Woohooo a 98!!!

Maybe I'll wire one up with a 40k pot and snap some pictures as an example for people who may encounter this in the future.

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post #13 of 30 (permalink) Old Feb 12th, 04, 11:46 AM
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OK...I am encountering it now./ I bought a 50K potentiometer...now what?

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post #14 of 30 (permalink) Old Feb 12th, 04, 03:01 PM Thread Starter
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I think that you can wire it the same way that I did mine, just replace the 10k resistor with the 50k pot. You will have to mess with the adjustment until it is right.

One thing that is not apparent in my diagram/pics....Do not hook the wire from the capacitor to a ground, just let it hang free. I tried grounding it at first, but it didn't work.

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post #15 of 30 (permalink) Old Feb 12th, 04, 07:36 PM
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So I'd still have to use a capacitor and a 3k resistor? And no ground?
What is a butt connector?
And which end is which...in to the tach or in to the distributor???
When it comes to stuff like this, I'm totally lost....and I could fry one of these components. I just can't do it.

How about you make one for me to plug in and I'll pay you for it? or does SOMEBODY make one???
This sucks...brand new motor and no tach!

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