I'd be suspicious of the connections under the hood... electricity in the cab comes from a few wires up at the starter solenoid on the battery side, I would inspect & clean all connections up there, maybe replace the battery cables if their condition warranted it.

 

Ok...your black and orange wire comes directly off fusable link "L". Off of the link, the B/O wire splits...one goes directly to the headligh switch, the other goes to the fuse panel (there it powers fuses 4, 8, 12, & 16). Fuse 4 is eventually becomes your tan/white wire. So...they both come from the same source...fusable link "L"...one through a fuse, the other not.

I would go check your fusable links. They are up on the passenger side fender by the starter solenoid. Look for the one with the B/O wire going from it...will be a Yellow wire going TO it.

May have to take some of the tape off to find these. It is common for these to get corroded. Look for other bad connections up there as well.

I will get some diagrams up that shows all this

 

This is page 15 & 16 out of the EVTM. On 15, you will see the fusable link "L" that has the yellow wire coming into it and O/B coming out. You can see how it splits going directly to the headlight switch and also goes to page 16 to the fuse panel. From there you will see fuse 4 and the tan and white wire.

Trace all that back and see where you have a bad connection.

I'm assuming you have good voltage on your battery? Check all those connections as well.

 

Thanks again for these diagrams -- very helpful. So here's what I've found:

Voltage across battery is good -- 12v

I started checking voltage at the fuses you listed for the Black/Orange feed 4,8,12,16 and results have me a little confused:

Fuse 4 -- 3v
Fuse 8 -- 12v
Fuse 12 --3v
Fuse 16 --3v

Not sure why fuse 8 would be reading ok while others are low, figuring if problem was somewhere between solenoid and fuse block that voltage for all 4 would be low given that they're coming from same feed.

What are the 'S' designations in the diagrams...splices ? Do the numbers indicate location? I was going to try and investigate S220 and S101,S102,S103 and S202. I followed the feed from Fuse Link J off the solenoid and I can see the Black/Orange go into a bundle that crosses over the engine towards the driver side. I haven't started untaping anything yet.
Thanks again for any ideas.

 

Yes...those are splices...this is page 17 that lists the location of the splices.

If you have different voltages at the fuses, first off, make sure you're measuring the supply side of the fuse, not the load side. You can check both actually, if you see a difference across the fuse, maybe it's bad fuses.

Check the voltages at the fusable link. You're loosing voltage somewhere, and it just might be bad connections is the fuse block itself.

 

Update - got things working again. I did 2 things - not sure which fixed it.

I disconnected a big connecter that had the big yellow wire and a bunch of others as well. There was a bunch of dried blue/green corrosion (i'm assuming) so I cleaned that up and put it back together. Should have tested things at that point but I found something else that looked suspicious and started looking at that. After untaping a few things and cleaning dirt off the wires I found where the original fusible link L should be in the orange wire and found that it was missing but there was a black wire spliced in that had the word fusible link printed on it but no big rubber thing like the other original fusible links I see. I took that out and temporarily twisted the 2 orange wires directly together. Tested and and everything was working . . lights, horn, radio.

The spliced-in black "fusible link" wire just looked like a regular wire and did not have a big rubber piece --- so is this really a fusible link? I was under the impression (maybe incorrectly) that the rubber piece was the "fuse" part.
Should replacement fusible links have that rubber piece or is there something about the wire that makes it "blow" like a fuse ?

There was connectivity end to end on the spliced-in fusible link wire, so I am thinking the problem was with that big connector or some other jiggling that was going on. I can splice that wire back in but I want to make sure I have the fusible link properties.





Thanks again for posting diagrams and pointing me in the right direction. I feel much more confident navigating the wiring.

 

So someone had already replaced the fusible link and it blew again. I would replace the fusible link with a fuse. In the electric section at your auto parts store will be a fuse holder with wires. I would splice that in instead of another fusible link.

 

The fusible link was replaced, but I don't for sure that it blew again. There was connectivity across it and it was spliced in well. I haven't put back to test it.

I like the idea of replacing with a fuse. It seems like it would be much easier fix if it blows.

Is there a down side to using a fuse? any recommendations on amperage?

 

Fusible links are smaller pieces of wire that will melt instead of the main wiring harness. The advantage of them over fuses is that a fuse holder/fuse combo has connections that can get dirty and cause problems where the FL is usually sealed in. But, many vehicles now use fuses under the hood so that approach obviously works.

If you fixed the corrosion then you fixed the problem. That was giving you a high resistance which dropped the voltage delivered to the accessories. You can put the FL back and prove that - or replace it with a fuse.

On amperage, you could add the value of the fuses it supplies and then drop back about 25%. (EDIT AGAIN: Wrong! Looks like those fuses add to about 80 amps, 75% of which would be 60 amps and that is waaaay too much. See below.) Or, does the fusible link or the EVTM have a value?

EDIT: I've now switched to a computer from the iPhone and I still can't read the EVTM for sure. I think it says 16 but it could be 18 ga. According to Wikipedia a 16 ga fusible link would be used to protect a 12 ga circuit. Following the link in that link you'll find that a 12 ga wire is good for 25 amps, although my remembrance tells me that should actually be 20 amps, although I'm thinking of house wiring. So, you could go with either a 20 or 25 amp fuse, although I would probably opt for the smaller value and go up if needed. EDIT AGAIN: Since that link/fuse powers several things that have 15 to 20 amp fuses, you are probably going to need at least a 30 amp fuse to run those items - especially if you have power locks.

 

You have to be careful here, Ralph (81-F-150-Explorer) and Ted (oddshot) have explained this before.

Fuses act quicker and wear out faster than fusible links.

Also, the black/orange is in a circuit connected directly to the alternator; if the alternator ever needs to supply more than 20/25/30/whatever-fuse-value-you-use amps, your fuse can/will blow.

There are special considerations if your truck has gauges and not idiot lights but I forget the details.

Some stuff to read for details:

https://www.ford-trucks.com/forums/8...ine-fuses.html
https://www.ford-trucks.com/forums/9...lace-with.html