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HEI and factory tach.

HEI and factory tach.
D

duke94

Jeeper
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Location
Ann Arbor, MI
Vehicle(s)
77 CJ5, 83 CJ7
Is there any way a factory Tach can be made to work with a HEI dizzy? When I plug the tack into the HEI, the engine won't start.
 
The factory tach is triggered from the pulse on the pos. side of the ignition coil.
Hei ignition tach is driven from the trigger side of the hei module. The pulse is neither long or strong enough to trigger your stock tach.The stock tach is wired inline to the coil IE batt/tach/coil pos. when you hooked up your coil to the hei you were back feeding the module a voltage level high enough to maybe kill it.
 
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And still neg driven per JS, the pos side supplies voltage, the neg side of the coil is tapped for the tach impulse.
 
Tinman22 said:
The factory tach is triggered from the pulse on the pos. side of the ignition coil.
Hei ignition tach is driven from the trigger side of the hei module. The pulse is neither long or strong enough to trigger your stock tach.The stock tach is wired inline to the coil IE batt/tach/coil pos. when you hooked up your coil to the hei you were back feeding the module a voltage level high enough to maybe kill it.
Click to expand...

Thanks,
I guess I was lucky, the engine ran after disconnecting the Tech wire. Still confused. So do I connect the 12V lead and Tech wire together and plug them into the 12V terminal on the dizzy?
 
Just went through the same thing when installing an 86 Tach after Painless harness install and HEI. If your tach has heavy gauge red and red/tr wires, it is intended to be installed on pos side of coil, not neg side of HEI. So, I recommend you get a modern rendition of the original tach which is designed to install on neg side of coil and you'll be fine.
 
Interesting and at first I thought it was very simple. The positive end of the coil gets a constant 12 volts, no pulses, it's just tied to the ignition switch to get 12 volts when the key is turned.
I've read the posts and links saying to use the positive side of the coil. I don't understand that.
coil.jpg

The HEI distributor has a terminal for tach. Can't you just use that? The OEM tach is different? That pulse won't work as well as the positive end of the coil? :confused:
The coil is in the cap of an HEI but I realize the "Tach" terminal is from the ignition module. I thought I knew this topic but now I am completely confused.
So does the HEI coil get pulses on the positive end?:confused:
 
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All I can tell you is I followed the Painless instructions to re-wire an old tach to the HEI and it caused major disruptions in my ignition. I pulled the tach plugged and she purred just fine. Fortunately I didn't damage anything, but others have roasted coils doing this. Maybe you can use an old tach with the heavy gauge wires, but I could not. I bought a repro for $60 and it worked perfectly with the HEI.
 
DANsCJ7 said:
All I can tell you is I followed the Painless instructions to re-wire an old tach to the HEI and it caused major disruptions in my ignition. I pulled the tach plugged and she purred just fine. Fortunately I didn't damage anything, but others have roasted coils doing this. Maybe you can use an old tach with the heavy gauge wires, but I could not. I bought a repro for $60 and it worked perfectly with the HEI.
Click to expand...
What Repro did you buy and did it fit in the hole in the dash?
 
Might be a little late, but i was perfectly able to use the stock tach with my HEI DUI upgrade...

let me know if you still need some help...

:chug:
~ Jr
 
JR74CJ5 said:
Might be a little late, but i was perfectly able to use the stock tach with my HEI DUI upgrade...

let me know if you still need some help...

:chug:
~ Jr
Click to expand...
so you used this tach connection on the cap?
coil.jpg
 
basically, yes. Though the DUI cap is laid out slightly different, same basic principal, and has worked fine.

Off the top of my head, I don't remember needing to do anything except remove the wire from the negative post on the coil to that cap "tach" location. Didn't mess with anything power wire wise at all.

I can confirm that tonight or tomorrow (wedding "honey-doo" project tonight first...)

:chug:
~ Jr

To clarify I meant I moved the wire for the tach from negative side of coil to the tach hookup on HEI / DUI cap.
 
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perhaps the differance is my using a DUI "HEI" vs a GM style HEI???

:confused:

All I know is mine works great!

lol
 
I have heard some are successful with stock tachs with the thick red wires and some burn out modules and coils. Did not work for me despite very careful wiring. I do wish you luck. My hei is a dui, so I dont think this matters. Good luck and be careful!
 
Honestly, I'm not sure how you really would unless it was hooked up wrong... But a nice new tach would be pretty sweet anyways! :cool:

:chug:

~ Jr
 
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