New Here, Need Help!

Pooby88 · Aug 25, 2016

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.

tos · Aug 26, 2016

That's tough without being able to see it. If you can post pics of different views it would help. Pics of the start relay, coil and ignition switch would help.
Look for a label on the harness or an individual wire that might give the manufacturers name and we can find a diagram.
Your color codes however should match the original so you should be able to use any 70 diagram.
I would start by flipping the key on and find the power lead going to the coil and check for volts. you should have power at the coil with the key on. If you don't have volts at the coil with the key on then check any other wire that may be going to the coil for voltage, it could be miswired. Next check the two small terminals on the start relay (if you have that style relay), they sometimes get reversed. The I terminal if I remember right should have a brown wire and this would be the supply to your coil - this terminal should be showing voltage with the key in run position and start position.. The S terminal is red-blue? and is connected to your neutral safety switch - This terminal would have no power in run position and power in start position only.
If there are additional wires on either of those terminals or if the coil supply wire is on the wrong terminal it could cause your start problem. If they are connected to the proper terminals then go to the ignition switch and check for miswiring there.

Pooby88 · Aug 26, 2016

tos said:
That's tough without being able to see it. If you can post pics of different views it would help. Pics of the start relay, coil and ignition switch would help.
Look for a label on the harness or an individual wire that might give the manufacturers name and we can find a diagram.
Your color codes however should match the original so you should be able to use any 70 diagram.
I would start by flipping the key on and find the power lead going to the coil and check for volts. you should have power at the coil with the key on. If you don't have volts at the coil with the key on then check any other wire that may be going to the coil for voltage, it could be miswired. Next check the two small terminals on the start relay (if you have that style relay), they sometimes get reversed. The I terminal if I remember right should have a brown wire and this would be the supply to your coil - this terminal should be showing voltage with the key in run position and start position.. The S terminal is red-blue? and is connected to your neutral safety switch - This terminal would have no power in run position and power in start position only.
If there are additional wires on either of those terminals or if the coil supply wire is on the wrong terminal it could cause your start problem. If they are connected to the proper terminals then go to the ignition switch and check for miswiring there.

Thanks for the reply. The harness was installed a couple years ago and the problem just appeared recently, so it isn't an issue of something being wired wrong.

tos · Aug 26, 2016

Ok that's a little different, however I would still follow the same procedure of verifying power at coil and if no power there then at relay terminal.
Was the car sitting for awhile? Possibly gotten some mice in it chewing on some wires under the dash?

horse sence · Aug 26, 2016

it sounds like you have the key wired to run on the start position but not in the run position on the key, so the coil turns off soon as the key is released The coil should have power in run and start position .

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Pooby88

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tos

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horse sence

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