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1987 F350 460 Dually 4 doors 2WD.
When I pull light switch first notch I have no lights. When I pull second notch I have headlights only. When I pull slowly from first notch to second notch I have side marker lights/tail lights at a narrow 'sweet spot' with sometimes the headlights too. New light switch did not fix it. I am also looking for a fusebox diagram/light switch diagram to understand what is going where. My Haynes is not too helpful there.
Without thinking too much about it (because I don't know more things to think about!) I would have to say it sounds like the switch itself.
Yes, you can get weird goings-on with bad grounds, and your metal frame around the fuse panel makes it look like your truck has spent some time where it gets wet, but the fact that the different positions act differently when you jiggle the switch would say it's the switch to me.
I'm sure there are tests you can perform on the switch connectors when it's unplugged, and you can always benefit from just cleaning and re-tightening all your grounds anyway, but you might have to do them all anyway.
I'm leaning towards the switch still though.
Oh, and since at least some of the metal on your truck is fairly well crusted over with surface rust, I would take a few minutes to remove all the fuses, breakers and relays from the fuse panel, make sure all contact points are clean, and then re-install them all.
Just the act of pulling and replacing the electrical components cleans the contacts for better electrical flow.
As mentioned, a new switch still does it. I did pull all the fuses too before and replaced the busted ones. The frame of the truck is in a much better shape than that fusebox frame actually. Sound advices still, thank you. I think the "sweet spot" mentioned is when the shuttle valve inside the light switch is hitting both the first notch and second notch contacts. If you never saw the inside of one, I'll gladly attach a picture of the old one I opened up.
Looks like the fusebox I have. I need to find a wiring diagram for the light switch so I can test what is getting power as I pull the light switch.
Yeah, caught the bit about the new switch too, but kind of ignored it (temporarily) still thinking it sounded like a switch. Must be something else though, as you are thinking. It's unfortunately very common for the new parts, especially electrical parts, to be bad right out of the box. Happens WAY too often these days.
But to have the same exact odd symptoms? Not very likely. Maybe still possible of course, but not very high on the list.
Maybe it's still a grounding thing. But as odd as they act sometimes, I'm having a hard time correlating your specific pattern with a bad ground.
And the other "most common" thing to go south on many Ford and GM headlight switches of older vintage than your '87 are the wires in the connector getting burned, corroded, or even melted. Compromising the connection right at the interface.
But looking at yours, I don't see any of the tell-tale darkening of the wires right at the connector/strain relief housing.
Can you see anything with the connector that might cause some funny business?
Your theory about the sweet spot makes sense. My best guess is that the markers and headlights have separate power feeds going into the switch (separate fuses), and the feed for the markers isn't working. With the switch in just the right spot, the contacts inside bridge from the headlight circuit to the marker circuit. I'd double-check that #4 fuse and make sure it's getting power, then check the wire between there and the light switch plug.
I chased a no power circuit in my truck that tested out fine from the box to the device.
Turned out that when plugging in a new fuse, one blade was not making contact because heat had weakened the plastic of the box enough to allow the receiving clip to be pushed out the back of the box.
The tip of my meter probe would go deep enough in the slot to read power on the hot side & continuity on the other, but putting a new fuse in still left the circuit dead.
Had to take the box loose & feel around behind it enough to get a finger on the wire & hold it enough to push in new fuse enough to make contact.
This may not be the case on yours, but these thing are aging out as to do all sorts of things "not in the repair manual".
The "sweet spot" is where you are sending headlight power (wire 38, BK/O) to the marker lamp circuit,
In other words you are not getting any power from the fuel box to the switch over the T/W 195 wire powered by fuse #4.
The markers do not use the head light power from the fuse link at the starter solenoid.
After testing all sort of wires I followed you guys advise and went back to the fusebox. I replace fuse #4 with a new one and all works now. Old fuse visually looked not burned nor broken, yet a fuse tester says it's no good.
Hey guys, I want to bring back this somewhat old post..my 86 F250 is doing this exact same thing. Iv repacked the switch twice as the OP did and had no luck. Now I have checked the fuses briefly but was wondering what exact fuse I should check on my bullnose? Any help would be great, thanks
Oh hell I didn’t realize the bullnose had the same fuse panel. So I checked mine and sure as heck, that fuse was stuck in the panel and pretty messed up. It had slightly melted to the panel. The contacts are still there in the fuse panel though. I wonder if it would be safe to clean it up best I can, and put another fuse in it and try it?
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