Ignition issues
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Mountain Pass
Joined: Nov 2007
Posts: 245
Likes: 4
From: Ponca City, OK
Ignition issues
'79 F100 with a 302
Driving down the road it'll just die. Step on the clutch, bump the starter and it'll fire back up. I can also step on the clutch for a second and let it back out and it'll fire up. I've swapped coil, pickup and ignition module. No change. I got to chasing voltage and only have 7.9 volts at coil. At connector by firewall I have 8 volts. For fun I ran a jumper wire from coil to battery and it doesn't miss a lick. So I replaced ignition switch thinking that might be my voltage drop. No change.
Shouldn't there be 12.7 ish volts at coil??? Haynes wiring diagram bites. Wire colors etc. don't match but I can see no reason for a voltage drop in the schematics.
Driving down the road it'll just die. Step on the clutch, bump the starter and it'll fire back up. I can also step on the clutch for a second and let it back out and it'll fire up. I've swapped coil, pickup and ignition module. No change. I got to chasing voltage and only have 7.9 volts at coil. At connector by firewall I have 8 volts. For fun I ran a jumper wire from coil to battery and it doesn't miss a lick. So I replaced ignition switch thinking that might be my voltage drop. No change.
Shouldn't there be 12.7 ish volts at coil??? Haynes wiring diagram bites. Wire colors etc. don't match but I can see no reason for a voltage drop in the schematics.
Posting Guru
Joined: Feb 2009
Posts: 1,088
Likes: 4
DO NOT RUN 12 VOLTS TO THE COIL. There is a resistor wire that runs to the coil. It sounds like you have proper voltage. I would suspect a loose connection or a wire grounding out. My '76 460 had a small break in the insulation that would periodically ground against the back edge of the intake manifold and kill the engine.
One other thing to check: with the engine running, wiggle the key around in the ignition. A worn out ignition cylinder can cause the engine to die. If you wiggle the key and the engine cuts out or dies, you've found the problem.
One other thing to check: with the engine running, wiggle the key around in the ignition. A worn out ignition cylinder can cause the engine to die. If you wiggle the key and the engine cuts out or dies, you've found the problem.
Thread Starter
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Mountain Pass
Joined: Nov 2007
Posts: 245
Likes: 4
From: Ponca City, OK
I got it! Just trying to wrap my head around how it worked. I was going to start trying to chase a chafed wire or something. I've been chasing this for days. If the darn thing would go ahead and break I'd be golden. It's a pain chasing ghosts!
Posting Guru
Joined: Feb 2009
Posts: 1,088
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I would start by tracing the wiring from the distributor back to the firewall, paying special attention to the wire that feeds power to the coil. I'll bet you have a bare spot on that wire that is grounding out, from the sound of the problem.
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Mountain Pass
Joined: Nov 2007
Posts: 245
Likes: 4
From: Ponca City, OK
The plug at the fire wall looked like crap. I repaired that a few days ago. The rest of the harness looks surprising good although I haven't removed the electrical tape from it yet. This afternoon I ran a test wire from the firewall connector to the coil bypassing all the "suspected" wiring. Drove it around quite a bit since and so far so good. I'll leave that wire until next weekend and if my ghost doesn't reappear I'll yank out the harness and rebuild it from the firewall out.
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MSEE
Joined: Apr 2004
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From: Austin, TX
Did you check the ignition switch as previously outlined? It's much more likely to be the cause of the issue than wiring, since it has moving parts. It's probably the most common cause of such issues, behind the ignition module and pickup, both of which you have already replaced.
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