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Discussion Starter #1
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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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.
 

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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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Discussion Starter #6
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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Discussion Starter #8
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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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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Discussion Starter #10
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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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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OK...I am encountering it now./ I bought a 50K potentiometer...now what?
 

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Discussion Starter #14
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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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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I guess the repop tachs are made differently than the originals. My original tach works great with my GM HEI. I hope it works with my new MSD HEI?
Jim
 

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Well...I found out what you all probably already knew...all Fastburns come from GM now with MSD HEI, and the tach DOESN'T work with it!
I emailed MSD tech 2 times...no response. Let's see...a GM tach, and a GM motor...not a match!Guess I'm just outta luck.
 

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Steve
I don't understand why the MSD HEI wouldn't work with a stock tach. My stock GM HEI that came in my first Fast Burn motor worked great.
Jim
 

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Well...mine doesn't. Maybe there's a difference between the 68 tach and the 69, I don't know!
Now I've sent 3 emails to MSD and not even one reply from them. I emailed GM and they said contact the dealer. Contacted the dealer and they said they don't know. Contacted Classic who says Mallory might have a filter that MAY work. Emailed Mallory thru Mr Gasket and they said it might be the ground from the distributor thru the engine(?). Emailed Sallee (because of their work w/ the zz430) and they said to contact MSD. I'm really getting frustrated with this mess. I'd hate to think I have to install some aftermarket tach on a hose-clamp after spending a small fortune on the console gauges and tic toc tach and close to 5 grand plus installation on this "fastburn" motor. Yeah...fastburn your money! This is ridiculous. I have a GM car with a GM motor and a GM tach, and they're telling me they dont work? Dammit!
 
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