Ignition problem
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Ignition problem
I have a 78 F150 351. I just bought this truck with a no start after it gets hot. I replaced the ICM and drove it 100 miles home thinking I fixed the problem. Now the truck dies after driving less than a mile. I found today that the red/green striped wire going from the ignition switch to the the two wire input harness on the ICM is getting so hot that it smokes. I bypassed the ignition switch just to rule that out, and also replaced the ICM a second time. This wire heating stops after I disconnect either the positive coil wire, the pickup, or the ICM. Has anyone experienced this problem?
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
That wire is the ballast resistor wire to the coil. If it's smoking (which means it will need to be replaced after the problem is identified), you're pulling too much primary current. The two most common reasons for that are:
1) Someone has swapped out the ignition coil with a "hot" coil (aftermarket, performance, etc).
2) Someone has tied an electrical load off the 'I' terminal of the starter solenoid.
Or, there's a short downstream. By disconnecting the items you've listed, you've interrupted the primary circuit's path to ground, and no current will flow through the ballast resistor, so your observations make sense.
1) Someone has swapped out the ignition coil with a "hot" coil (aftermarket, performance, etc).
2) Someone has tied an electrical load off the 'I' terminal of the starter solenoid.
Or, there's a short downstream. By disconnecting the items you've listed, you've interrupted the primary circuit's path to ground, and no current will flow through the ballast resistor, so your observations make sense.
Posting Guru
Joined: Sep 2009
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From: Suburbs of Chicago
I dont have my wiring diagram at the moment, I'll post back a little later when I am though.
Usually when a wire heats up, its because there is a voltage drop in the wire or in one end of the connection. Do you know how to do a voltage drop with a meter?
Usually when a wire heats up, its because there is a voltage drop in the wire or in one end of the connection. Do you know how to do a voltage drop with a meter?
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
The ballast resistor is supposed to drop voltage across it, 6 to 7 volts approximately, hence its name. Battery voltage divides almost in half across the primary winding and the ballast resistor. Voltage drop "in" something is ambiguous. Voltage drops across two points.
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
Then something else is responsible for pulling excess primary current. You'll need to investigate.
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An update on the repair:
I replaced the length of resistor wire with 12 gauge wire along with an aftermarket 1.4 ohm ballast resistor which I mounted in the engine compartment.
This made the problem worse.
I added a wire from the I terminal on the starter solenoid to the coil positive side. No change.
I added the factory length of resistor wire back in place of the aftermarket ballast resistor and now I'm back where I started.
I replaced the length of resistor wire with 12 gauge wire along with an aftermarket 1.4 ohm ballast resistor which I mounted in the engine compartment.
This made the problem worse.
I added a wire from the I terminal on the starter solenoid to the coil positive side. No change.
I added the factory length of resistor wire back in place of the aftermarket ballast resistor and now I'm back where I started.
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MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
As I've said twice now, you're pulling too much primary current. None of the steps you've described address this issue. Whether you're using the factory ballast or an aftermarket setup, nothing is going to change the fact that there is some sort of short downstream. As I said in my previous post, you need to investigate.
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I traced the short down to the ignition pickup inside the distributor. Replaced it with a new one. The engine ran like it should at idle, so I let it do that for about an hour with no issue. On the test drive it cut out completely a mile in. It restarts immediately, but does this a few more times on the way home. There is an improvement but the problem is still there somewhere.
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