Fires but shuts off when key is released

DARIN RIDDLES

MEMBER
1980 JEEP CJ7
4.2L
6 CYL
4WD
MANUAL
200,000 MILES

Information:
80 CJ7 with 83 4.2, Webber carburetor, Motorcraft ignition with the square coil and the flat box control module on the passenger fender.
Painless harness.

Background:
Went to breakfast the other morning and had a hard time starting when we left.
Got it home and parked it but left a switch on and drained the battery.
Tried to jump it last night after letting it charge a bit, needed to use it to drag my trailer.
That is when the really weird problem started.

Problem:
Hit the key, engine cranks, and fires but as soon as you let off the key it shuts off.
It is like the key is being turned off because it loses all ignition to the point if you are trying to rev it will backfire.

Ignition switch?
Wiring problem?

All gauges work in the run position.

3 Replies

DARIN RIDDLES

MEMBER
1 POST

Found the problem, bad resistor on the firewall.

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Monday, February 12th, 2018 AT 7:49 AM

RENEE L

ADMIN
1,257 POSTS

Hello DARIN RIDDLES,

So happy to hear you found the problem. We appreciate you posting the solution.

Thank you and have a great day!

Renee
Admin

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Monday, February 12th, 2018 AT 9:46 AM

CJ MEDEVAC

MECHANIC
11,004 POSTS

Hello DARIN RIDDLES!

I just stumbled onto your question months later.

I wanted to maybe give you a heads up on ballast resistor failure.

I am not saying that these puppies are fail-proof, as any one of them can fail.

Many are open in the back and the coil is exposed, water dirt, and bouncing around will shock the little coil and cause it to break or burn in half.

Below are two screenshots of the same ballast resistor (front and rear views). Notice how on this particular one they have filled the trough with ceramic material. This really helps out on the life of the device.

As another "Get me back in" trick if you experience Ballast resistor failure is to "temporarily" Jump the terminals with a jumper wire, lay and tape a metal object over the terminals, or simply snatch both wires off of it and tape the "females" together. This will get you "in", replace it as soon as you can to avoid other ignition problems.

The Medic

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