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  Mustangsandmore Forum Archive
  '69 to '73 -- The Musclecar Mustang
  72 351w DUI HEI and factory Tach

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Author Topic:   72 351w DUI HEI and factory Tach
72_Mustang
Journeyman

Posts: 4
From:
Registered: May 2006

posted 05-01-2006 11:34 AM     Click Here to See the Profile for 72_Mustang        Reply w/Quote
Hello,

I upgrade my distributor to an after market (basically a GM) HEI that has a built-in coil. It has a tach lead which my after market tach work with. I have not swapped out the speedo and dummy lights for a speedo and tach. This OEM tach will not work with the old tach feed from the HEI distributor.

It looks like for ford tach require lead from the coil or tach lead and an igition wire. When we connect the wires to the tach the motor dies.

If I find a wire with 6 to 8 volts on it and replace it for the ignition 12v the tach will work, but it is not accurate.

Does anyone know how to get my OEM tach to work with this after market hei distributor.

Thanks,
Scott

Hemikiller
Gearhead

Posts: 726
From: Killingworth, CT
Registered: Feb 2002

posted 05-01-2006 06:10 PM     Click Here to See the Profile for Hemikiller        Reply w/Quote
The original 72 tach is a current driven unit. It generates a signal from the current coming from the ignition switch, going through the tach to the (+) side of the coil. It is wired in series, like shown below.

(BATT +)-------(IGN SW)-------(TACH)------(COIL +)

With the DUI, the coil is powered directly from a switched 12V source.

You may be able to run the DUI off the Tach, like the factory ran the original coil. I would bypass the pink resistance wire, as HEI's won't run a 7 volts. Locate a switched 12V source from the fuse box, run it to the tach, then run a lead from the tach to the DUI and see if it will work properly.

72_Mustang
Journeyman

Posts: 4
From:
Registered: May 2006

posted 05-01-2006 08:22 PM     Click Here to See the Profile for 72_Mustang        Reply w/Quote
Hemikiller,

Thanks for the reply. I was currious if I was the only one ready my post.

I have tried what you have suggested and with the motor running it dies when I attempt to connect the tach.
If the motor is not running and I try to start it with the tach connected it does not start. As soon as I disconnect the tach it fires right up.

I am told by a reliable source www.tachman.com that the HEI distributor that I have is not compatible with the factory tach. He can make it work with it for a modest price.

He suggest going back to the factory distributor and replacing the points and condenser with a Pertronix Pertronix Ignitor Electronic Ignition part#1281.

Which is a great plan execpt I toss out the old parts.

Never the less I am looking for a old distributor that I can upgrade again and use my old factory tach.

The good news is I have a really nice and slighty used D.U.I. HEI distributor forsale that will fit small block Fords.

Thanks,
Scott

69 Sportsroof
Gearhead

Posts: 2814
From: Valley, Alabama, USA
Registered: Mar 2002

posted 05-01-2006 11:31 PM     Click Here to See the Profile for 69 Sportsroof        Reply w/Quote
If you're going to have to round up the old parts, skip the pertronix and go with a Ford Duraspark unit. Do a search on this site and you'll get quite a few hits with lots of info on converting it over.

72_Mustang
Journeyman

Posts: 4
From:
Registered: May 2006

posted 05-01-2006 11:48 PM     Click Here to See the Profile for 72_Mustang        Reply w/Quote
69 Sportroof,

Thanks for the info. What do you think the chances of me finding a working Ford Duraspark and control unit in a junk yard.

Are there better models of ford to look for?

I am all for finding a better solution that will work with my mach 1 tach. Are you sure I can run my pre 73 ford tach on this setup?

I will do some searchs, I just though I would ask you about it and why you thought the other stuff was junk.

Thanks for you time.
Scott
72_Mustang

SteveLaRiviere
Administrator

Posts: 48752
From: Saco, Maine
Registered: May 99

posted 05-02-2006 07:15 PM     Click Here to See the Profile for SteveLaRiviere        Reply w/Quote
I think you'll find this thread interesting:

https://mustangsandmore.com/ubb/Forum12/HTML/007820.html

------------------
'70 Mustang Mach 1 - '70 Mustang Convertible - '72 Mustang Sprint - '94 F-150 XL

Hemikiller
Gearhead

Posts: 726
From: Killingworth, CT
Registered: Feb 2002

posted 05-03-2006 05:56 PM     Click Here to See the Profile for Hemikiller        Reply w/Quote
quote:
Originally posted by 72_Mustang:
[B]I have tried what you have suggested and with the motor running it dies when I attempt to connect the tach.
If the motor is not running and I try to start it with the tach connected it does not start. As soon as I disconnect the tach it fires right up.

You are missing what I'm telling you. You need to power the the dizzy THROUGH the tach, to the power lead for the DUI. You can't be doing this if you have the motor running and -then- connect the tach, or if the motor will run if you disconnect the tach.

The sole source of power for the DUI (the heavy red wire) MUST be coming through the tachometer. If it doesn't, the tach will never work properly.

Splice the tach inline to the power lead and try it that way. If it still doesn't work, then you'll need to get an MSD tach adapter, or switch to a Duraspark setup.....

BTW, reman'd Duraspark dizzy's are $55 at any discount parts stores. The control boxes are $20 or so. you'll need to source a harness from a junkyard.

72_Mustang
Journeyman

Posts: 4
From:
Registered: May 2006

posted 05-03-2006 11:36 PM     Click Here to See the Profile for 72_Mustang        Reply w/Quote
Hemikiller,

You are so right I did not understand what you are saying. I will try what you suggested. I also deciede to go the duraspark route as well.

I will post my what I am sure will be my success either way.

Thanks,
Scott
72_Mustang

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