Factory tach with hei ignition

IICrew

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Mar 29, 2020
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I cannot find a definitive answer and wanted input from here before I burned up my factory tach. I converted the t top car to hei ignition. As I was searching for info on the conversion I read our tachs will burn out if used with an hei unit. The voltage jump from 20,000 to 50,000 is too high for our tachs. I don't know if that is true. In my mind that voltage does not hit the tach but I just don't know.

Currently I have a cheap column mounted tach but I want to make the factory unit work. Is anyone here using an hei ignition with factory tach? I know you need the msd adapter for some msd systems but mine is not msd. Do I still need their adapter?
 
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manicmechanic007

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Sep 26, 2017
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I have not heard anything on tach trouble using a GM HEI
Simply hook up the tach lead out of the cap to your tach
On a conventional early coil system you have a blinking 9volt light
Later ones (electronic ignition) it is 12 volts
If you have a 1965 tach or so and you are worried simply add a resistor to the wire and get the voltage down
Use Ohm's law and tell me what size resistor to get 9 volts?
 

extra_stout

Mustang Master
Aug 27, 2018
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I think an old factory tach should handle 12 volts or more even though they ran at 9
Exactly, because at start the voltage is increased to 12 V. Would make sense that it will handle this voltage.
But if is about electricity, there are only this 3 experts who can answer that:
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IICrew

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Should have known. We used to have bird feeders out front but pulled them down this year over all the bird deaths here. My squirrel friends used to hang out front all the time. I haven't seen them in a couple months now. Bet they could have gotten in contact with the Jedi Ninja Squirrels. Then I would have had all the answers.
 
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2Blue2

will be trying this sex one when I can find it
Mod Dude
Mar 5, 2019
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I got an HEI style ignition, i think all I did was make a jumper wire so it would plug into the wiring harness.
I needed the small cap style to clear the stock air cleaner I wanted to use.


This is what I picked for ignition.

s-l64.jpg


After determining the GM style HEI won't fit with the stock air cleaner. I got an small cap HEI, in blue.
130$ plus 15$ shipping so it was the right price for sure.

I handed the box to my son and his friends and talked them thru it but they put it in together.
Should of took a picture but it happened pretty quickly. I made them all hold the timing light and look at the timing marks.
A couple of turns of the screwdriver on the carb and it is dialed.
 
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7991LXnSHO

wanna catch the space herp
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HEI caps even have a tach terminal. Unless Ford did something screwy with the factory tach, it should be plug and play. Following for confirmation.
 
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IICrew

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Mar 29, 2020
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I have been looking more into it and cannot find where I got the info it would fry our tach. I can find references to earlier mustangs having the tach die but not ours. I most likely melded the two in my brain.

Thank You 2Blue. I will write it up when I get back to it. The next week or so I am busy getting ready for winter. My son moved in with us over the summer to rehab his old place and find a new one. He brought his chickens with him but didn't take them to the new place. My better half has fallen in love with them and now I get to build a chicken coupe.
 
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