Coil not getting 12 v, help please

[HHStang]

My 72 is driving me insane. I bought a car in pieces and am now trying to get it running.

It has a 429 in it with a Mallory Unilite and aftermarket coil. I can start the car and it runs jumping the starter relay directly to the coil.

I can't get 12v (any volts!) out of the wire that is supposed to feed the coil. On this car it's red with green.

Does anyone have any ideas or how I can diagnose?


Thanks!

 

[Max Power]

I don't believe that your are supposed to have 12v there anyway, probably only 8 or 9v. Ford used a factory resistor wire under the dash rather than a ballast resistor under the hood. I believe they were still doing that in 1972.

Check all your fuses, and if they are all good, start at the coil end and work your way back. You might make it all the way back to the ignition switch.

Or you can use any source under the hood that is 12v in the ignition on position and 0v in the ignition off position.

 

[HHStang]

Thanks! I've tried every wire in the harness and none give me 12v in the start position. At the risk of getting flamed, I'm not sure what the fuses would have to do the coil wire. It seems that the 12v from battery ends up directly at the swith. I could be way wrong, so any help will be appreciated. I have another column with igniion switch and I'll try that after while.

Admitedly, I'm an idiot, but the diagram on this car shows a red/green coming off the starter relay and then joining with red/green from ignition switch. My guess is red/green from starter relay runs back up in loomed harness and then just turns around to merge.

Anyone?

 

[69gmachine]

I would highly recommend getting a wiring diagram for your car. The 12 volts comes from the alternator/battery as a black wire with a yellow or orange stripe, that goes through the firewall at the fuse junction box. From there it goes to the ignition and is red with a green stripe. From the ignition it goes into the resistor wire (usually pink) where it drops the voltage down to typically 9 volts. Then it goes back thru the firewall and to the engine harness where it is once again red with a green stripe. In addition to getting a wiring diagram, I would also suggest switching to electronic ignition of almost any kind. Then you don't need the resistor wire and can provide a true 12 volts to the coil for a hotter spark. Use the brown wire that originates from the start terminal of the starter solenoid and terminates at the engine harness where it joins the ignition wire to the coil. The end that goes back to the solenoid can be capped and stowed. Use the coil end to pick up 12 volts somewhere before the resistor wire. Splice it under the dash so that it will go from the engine harness to the ignition wire after the key. Again, you really need to have a wiring diagram to make sense of it. Good luck!

 

[HHStang]

Thanks. I have about four of the wiring diagrams and have the basic idea of how it runs now. I tried a different column/switch and it didn't change anything. Any other ideas why I wouldn't have 12v (or even 8-9v) at engine harness red/green? at switch red/green?

 

[69gmachine]

First make sure you don't have a blown fuse. If that checks OK, it's time to get out the voltmeter or troubleshooting light. I prefer to use a voltmeter. Since you already know you have no voltage at the coil, You should back up to the ignition switch and verify there is 12 volts coming to it. If not, back up all the way to the main engine compartment loom where it connects to the alternator. Verify you have 12 volts there, and then just keep working your way toward the ignition switch. If you don't have 12 volts there, work your way to the alternator. This wire tees off to charge the battery from the alternator and also goes to the fuse block.

 

[ForceFed70]

This really shouldn't be that hard to figure out...

There's a wire that runs from the ignition switch to the coil. MAke sure the ignition switch is providing the +12V to the wire, then follow the wire to the coil.. Run a new wire with a ballast resistor if you have to.

 

[HHStang]

Thanks all. New problem now. I've got the 12v, but I have to hold the key in start position to keep it running (and yes the starter does keep spinning).

When key is in start position, what volts should "I" on relay be and what volts should "S" be? How about in run position on key?

 

[TOM B]

you got the 12 volts from the "Start " side of the ignition switch, thats ok for starting but when you let off the key it dies, you must find a hot wire thats hot when the key is on the run position, you can splice these 2 wires together or run both. If you have a after market electronic ignition "not points" you will want a full 12volts.
Do not wire the coil to a 12volt hot all the time wire or you will have a dead battery the next day and a very hot coil.

 

[mustangman1974]

Yes you will recieve 12 volts when you hot the ignition key to start. Ford Designed this to bump up the ignition voltage to increase coil output when starting. This is normal, but you should leave the key in the start position.

The resister wire has gone bad. I had the same thing happen a long time ago. The resistor wire may give you a voltage reading, but upon runing the increased current causes the resistor wire to short.

Check the wiring diagram and see which wire goes from the coil to the firewall connector. On the other side of the firewall there should be a wire that goes to the ignition switch. The older cars it was pink and look extra thick. Look closely at the connector pictured. They usually make a good representation on which wire passes through the connectiors, and yes many times they change colors at the connector. You will have to make a jumper wire to bypass the resistor wire. You can cut the resistor wire and use the harness.

 

[HHStang]

Thanks all. Turned out to be a couple pinched wires and loose connectors. New problem now to do with Mallory distributor. New post.

 

[HHStang]

Follow up. Problem (s) have been cured. The ballast resistor had a crack and ignition starter wire had a short. Fixed and running great now. Thanks all.