Engine cranks and starts but then dies

[DML]

Hi, I have a problem with my '70 Challenger. When I turn the ignition key the engine cranks and turns on, but then the engine immediately shuts off. First I thought the problem was the ignition key, so I changed the ignition lock cylinder and switch, but that didn't help anything.

When I connect a wire from the positive terminal on the ignition coil directly to the battery, then the engine cranks, turns on and stays on.

Any idea what could be the problem? Could it be a bad coil, ignition resistor or voltage regulator??

[Chryco Psycho]

Welcome  @DML
Generally this is due to a bad ballast resister as it bypsses the ballast during starting until it runs then it drops voltage to the coil .
I assume you are running a factory ign system , if i tis a MSD or other it may be wired wrong with the smaller red trigger wire only wired to the brown start wire & not the blue 12v run wire

[DML]

Thanks for the response. Yes, its factory ign system. I see you mention in another post that "you can jump the brown & blue wires together at the ballast temporarily if it starts you know that is the problem", I guess the same applies in my case?

[Burdar]

 

The ballast is bypassed in "start" mode.(when the starter is engaged)  The ballast kicks in when you let go of the key.(run position)  A bad ballast will cause the exact symptoms you describe.  To test the system...

Disconnect the yellow wire that is connected to the starter relay.  This will disable the starter while you test.  The brown wire coming from the ignition switch gets power in the "start" position.  It should only have power when you turn the key to "start".  The blue wire coming from the ignition switch should only have power in the "run" position.  Test these two wires at the ballast on the firewall.  If you have power on the blue wire in the "run" position on one side of the ballast but not the other, then there's your problem.

If you don't have power on the blue wire at all, you need to check some other connections.  The blue wire goes through the bulkhead connector.  You could have a bad connection there.  That would cause the same symptoms.  The blue wire also goes through the ignition switch connector at the base of the steering column.  You can also have a bad connection there.

I had an issue with my car where it just died while going down the road.  It ended up being a bad connection at the ignition switch connector. I had to squeeze the female terminal so the male end fit tighter.(on the blue "run" wire)  Hopefully your issue is just a bad ballast.  Keep a spare in the glove box.

[DML]

Thanks a lot! Much appreciated! I did suspect the ballast so I will try that first.