73' F100 Electrical Issue
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73' F100 Electrical Issue
Disclaimer: I'll start by saying I probably wouldn't even qualify as a shade tree mechanic, so please bear with me.....
My F100 (390CI) has developed an intermittent problem with being dead at the switch.... I've tried some of the obvious things (ie, new battery, starter solenoid, battery cables) all to no avail....... Yet to replace ignition sw.....
Sometimes it will turn right over & sometimes not...... Any input would be greatly appreciated!
Trapper
My F100 (390CI) has developed an intermittent problem with being dead at the switch.... I've tried some of the obvious things (ie, new battery, starter solenoid, battery cables) all to no avail....... Yet to replace ignition sw.....
Sometimes it will turn right over & sometimes not...... Any input would be greatly appreciated!
Trapper
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Joined: Apr 2002
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From: Waterloo, Iowa
Welcome to FTE
Do the dash lights and headlights still work when this problem occurs? These old beasts sure benefit from clean grounding, new cables at firewall, block and frame. Ignition switches fail in that way. Believe you are on the right track.
Do the dash lights and headlights still work when this problem occurs? These old beasts sure benefit from clean grounding, new cables at firewall, block and frame. Ignition switches fail in that way. Believe you are on the right track.
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I don't have any headlights/dashlights when this occurs...... Guess I'll try the ignition sw next............
Post Fiend
Joined: Apr 2002
Posts: 19,311
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From: Waterloo, Iowa
Didn't word that well. Don't replace ignition switch yet, though after 40+ years it wouldn't actually hurt. Headlights are on their own circuit. This points to a loose or corroded connection somewhere?
On older Fords the wiring harness has large rubber moulded connectors, maybe called junction blocks? Anyway at the firewall. These are prone to bad corrosion. A small wire brush and a solvent like WD40 will clean them up nice. It's important for lowest possible resistance at every connection, as the wire runs are long and wire gauges are only just barely adequate to begin with.
On older Fords the wiring harness has large rubber moulded connectors, maybe called junction blocks? Anyway at the firewall. These are prone to bad corrosion. A small wire brush and a solvent like WD40 will clean them up nice. It's important for lowest possible resistance at every connection, as the wire runs are long and wire gauges are only just barely adequate to begin with.
Post Fiend
Joined: Apr 2002
Posts: 19,311
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From: Waterloo, Iowa
That's your horn relay, and start solenoid. You knew that already I suppose, if you replaced it. Make sure solenoid is grounded well to firewall though, and engine has good ground strap to firewall.
At least on older Fords there should be some pinned wiring harness bundles that go through the engine firewall. These are the corrosion prone connectors was talking about. You can try "hot wiring" your solenoid the next time it won't crank, this would bypass the ignition switch making it the suspect. Make sure it's in neutral/park, stay to the side, don't run yourself over, etc. It won't hurt anything to clean those two SAE connectors, either. Many people don't pay attention to connectors till they fail altogether.
At least on older Fords there should be some pinned wiring harness bundles that go through the engine firewall. These are the corrosion prone connectors was talking about. You can try "hot wiring" your solenoid the next time it won't crank, this would bypass the ignition switch making it the suspect. Make sure it's in neutral/park, stay to the side, don't run yourself over, etc. It won't hurt anything to clean those two SAE connectors, either. Many people don't pay attention to connectors till they fail altogether.
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New User
Joined: Feb 2015
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That's your horn relay, and start solenoid. You knew that already I suppose, if you replaced it. Make sure solenoid is grounded well to firewall though, and engine has good ground strap to firewall.
At least on older Fords there should be some pinned wiring harness bundles that go through the engine firewall. These are the corrosion prone connectors was talking about. You can try "hot wiring" your solenoid the next time it won't crank, this would bypass the ignition switch making it the suspect. Make sure it's in neutral/park, stay to the side, don't run yourself over, etc. It won't hurt anything to clean those two SAE connectors, either. Many people don't pay attention to connectors till they fail altogether.
At least on older Fords there should be some pinned wiring harness bundles that go through the engine firewall. These are the corrosion prone connectors was talking about. You can try "hot wiring" your solenoid the next time it won't crank, this would bypass the ignition switch making it the suspect. Make sure it's in neutral/park, stay to the side, don't run yourself over, etc. It won't hurt anything to clean those two SAE connectors, either. Many people don't pay attention to connectors till they fail altogether.
Trying to picture these pinned wiring harness bundles.... I'll look again.....
I've owned the truck for 35+ years & it was my only transportation for 6 - 7 years following a nasty divorce, so I had to learn how to replace solenoids,starters,water pumps, brakes, alternators, u joints, tune ups, etc, etc (all with some very basic tools) but was never much force when it came to electrical stuff.... Still aint.......
I will try hot wiring it......... That could eliminate a whole bunch of stuff...
What are the two SAE connectors you are referring to?
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Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
Basically ANY connector that can be separated needs the corrosion cleaned off by now. Trucks really operate in brutal conditions being exposed to brine and pollution and high temps. FORD (and everybody else) viewed copper wire as a semi-precious metal and only grudgingly allowed it's use when they absolutely had to, and then used the minimum gage size they could get away with. It works when new, but corrosion starts to cause problems with voltage drops, each connection is where the problems start. As the voltage drops, current draw goes up and components get smoked. You can feel them get hot when really bad.
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Cargo Master
Joined: Jul 2005
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From: Kansas
if your headlights (or horn) don't work, then the ignition switch is not the problem. I would start on the small wires that come off the large positive battery post on the solenoid...especially that blue butt crimp connector...pay attention to the fusible links (or look for more butt connectors where a fusible link has possibly been replaced...
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