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New Here, Just Need Help With One Thing

675 views 5 replies 5 participants last post by  pooby88 
#1 ·
I'm trying to help my old man figure out his Mustang. He has a 1970 Mach 1 - 428- 4 speed.

The problem is: Upon turning the key in the ignition the engine fires up, but it dies as soon as you release the key. So, the coil loses power when the key is in the run position.

This car has an aftermarket wire harness installed. I believe it is an American Autowire product (if you know how to tell for sure let me know).

The fuse for "gauges" blows as soon as you turn the key on (not cranking, just turning the key to the first click.)

This car has a Ford HEI ignition setup, so it has some 1970 stuff (like the ignition switch), some late 70s era stuff (like the ignition module and distributor), and the aftermarket harness.

One of the wires tied to the gauges fuse seems to be grounded, but I haven't been able to figure out where. I can't find a wiring diagram for the mystery harness and I'd rather not pull the whole thing out and reinstall it.


Any help is greatly appreciated.
 
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#2 ·
Not exactly an easy problem, being a mix of parts like that. It is imperative that a customizer document and map out wiring changes, or the next person to fix it will have huge problems. It will add value to the car to do this, so talk with the installer to get whatever notes he has.

The only clue that I can offer here is that in the original factory ignition system, "run" powers the coil through a pink resistor wire (buried in the harness under the dash), while "start" bypasses that wire. So the coil sees about 5 or 6 volts during run but a full 12 during start. The reason for this is to keep the coil running cool.

Since you have a newer technology HEI setup, it needs 12 volts all the time (actually 14.2 or so when the alternator is charging). Measure the voltage at the coil with the key on run, and if it's not 12, then the pink wire needs to be bypassed.
 
#4 ·
The 70 front end is my favorite on the Mustangs.
Electrical issues can be a pain, especially if you are trying to figure out aftermarket stuff.
Take a look at this 70 diagram I have, hopefully you can figure it out.
It probably came out upside down, just save the image and rotate it.
Good luck and try not to get a headache looking at the diagram, I know the image quality sucks.
 

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#6 ·
Thank you for all the replies!

1. If throwing a new ignition switch on it and seeing if it fixes it were as easy as it should be, I'd just do it. Unfortunately the switch on this car no longer has its plug. It is spliced straight into the new wire harness. So simply plugging in a new one to see what happens isn't an option and I can't return a switch if it fixes nothing after I've hacked it up.

2. From what I can tell, my grounded wire is the pink one that runs to the tach. That wire also connects with another pink wire and the cluster connector. Both wires are attached to the same pin at the connector. Incidentally, while I was testing that wire, I bumped the turn signal switch and the ground on the pink wire flashed on and off. Given that trying to read those old diagrams makes my eyes cross I'm not sure if that is ok or indicates a problem.
 
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