Protection and convenience wiring

1986 f250 xl sclb 6.9l

Having issues with flowing fuse #7 which marked as the courtesy lamp fuse. Seems to be shorting and I cannot figure out where. I get a reading of 13.5v on the black/orange wire which is on one side of the fuse, and 0.0 ohms on the light green/yellow side of the fuse. The wiring has been altered due to a previously blown headlight switch. It looks to me as though there is supposed to be a lg/y wire coming from fuse 7 to the headlight switch. Then another lg/y wire from the same terminal on the switch, to splice 1103 (protection and convenience). I have continuity throughout the wiring in s-1103. Using a 15 amp fuse as prescribed.

Also lost all running lights the other day. Headlights, reverse, turn and brake lights all function but no marker lights. Reading only mV at the 5 amp fuse (F-1).

Could this all be due to another bad headlight switch? I can't seem to find the steps to test the switch itself anywhere.

Hoping this is all just me being somewhat unedggumicated in the world of wiring but thats why I'm turning to you all.

Thanks in advance!

Hey Keaton.
Just to avoid confusion - this is fuse 8 (instead of 7).

The dome light wiring travels under the driver side sill plate and is often subject to rough conditions there.
But if you've had previous headlight switch issues and modified wiring in that area, that sounds like a good place to start. Plus if you are having issue with running lights then it's a common piece to both circuits.

Did the running lights come back or did you have to replace a blown Fuse 4?

If you put a fuse in 7 does it blow immediately?

Just a thought to add.
It's not entirely uncommon to have a door jamb switch short out
So if  you don't find a raw wire beneath the drivers sill take those switches out to have a look

So the switch tests good.
The wiring I replaced is the tan/white wire from fuse 4 to post A in the switch as well as the lg/y wire from fuse 8 to post D1 on the switch.
In the EVTM, it shows two lg/y wires coming from post D1, one to the fuse block and the other to splice 1103. In the wiring diagram, it shows 5 wires at that splice, whilst mine has 6. I have checked and I have continuity to the 5 components shown in the diagram. Wondering what the extra wire goes to.

Fuse 8 will either blow immediately or close the circuit for a second or two, causing the warning buzzer to turn on, and then blows. When I jump the fuse, the buzzer turns on as well as the dome lights, however the circuit begins to get quite hot.

I have not had a blown fuse 4 yet.

The driver's side door switch looks good but the passenger side looks like this...



I don't think it's the entire issue but I'm betting it part of it.

I stripped down a harness not long ago and found some deviations with splices also. I'll check later to see if that was one of them.

I would start unplugging connectors/connections.

Any info would be awesome on deviations!

Put a new door switch on the passenger side, and found if I held the drivers door switch it wouldn't blow a fuse. Even got my clock, dome light, and cargo light to come on! When I would let go of the drivers door switch, it would blow the fuse. Fiddled with some things and found this wire had been grounded at some point.

It is a black wire with a light blue tracer. I'm assuming for the driver side courtesy light under the dash?
I disconnected it from the ground and the fuse doesn't blow.
 
Now the issue is the cargo and dome light wont turn off without pulling fuse 8.
Fix one thing to find another issue right?

Still no running lights or dash illumination lights as well.

My head is starting to hurt from staring at diagrams haha.

Great find!
Yes - that's exactly what that connector is for. It connects to the light and the light is grounded via the screw connection to the dash structure.

Check to see if rotating your headlight switch takes care of the dome/cargo light issue. Muscle memory takes over but I think rotating it counter clockwise until it 'clicks' turns them on. 50/50 shot.

Sadly I won't be much help on the splice for that circuit besides saying that the one out of my 86 parts truck also has 6 wires in it. I cut some of the wires out of it when I was measuring various circuits so can't say what they all go to but I'll do some digging.
The splice I was thinking of was the one on the marker lights circuit that doesn't match the EVTM.

Messed with the dimmer and doesn't seem to do anything. The dimmer switch checked good when I tested it.

Just to make sure - talking about the dash light dimming feature via rotating the headlight switch and not the foot activated 'dimmer switch', right?

Correct, the headlights (high and low beams) work as they should. Rotating the dash dimmer does not turn of the dome/cargo light.

Rotating the dimmer switch past full bright should override the door switch and turn on the dome light.
I think Scott is after whether the dome light wiring is shorted out somewhere in the sill or B pillar.

Okay, I think I was just too narrow focused last night... the dome light turns off when I shut the door.. XD

Still no running lights or dash illumination lights though.

Thanks Jim. Yeah, I just wanted to make sure the headlight switch wasn't rotated enough to make the dome/cargo be on constantly. "Dimmer switch" could refer to that as well as the foot switch so wanted to ensure we all weren't talking about different things.

Keaton - glad to hear it and stuff like that happens  Easy to focus on one path and forget about other factors.


The only connection running/marker lights should have with the dash lights is via the headlight switch.

Is fuse 17 still good?

Figured out the running lights!
There was a bad connection on the backside of fuse 4.

Thank you both for the assistance! Been chasing this one for a few years now. Had the dash on and off too many times. One step closer to leaving it on.

That's great news!  

Is your radio stock, or did someone miswire it?
(Not sure where you were saying the radio only works in 'run' and not 'accy' went?)

Glad to hear it!

Oh, what about the dash illumination lights? Are they still not working?

jbritt wrote

 You know, a replacement pigtail for the headlamp switch might help a lot.

Not sure if your switch is 8 or 9 pin.

https://www.repairconnector.com/products/Ford-Headlight-Switch-Repair-Connector-8-Wire-Pigtail.html

https://www.repairconnector.com/products/Ford-Headlight-Switch-Repair-Connector-86-thru-97.html

The second one says it fits '86-'97, but I think the switch changed with the '87 model year (which could be an '86 build date)