Specs on the truck: 1973 F100, 390, Auto transmission, Factory A/C, 130k miles.

I'll just go ahead and apologize for the length of this post.

Okay, so I'm solving issues on the truck one day at a time, finding problems, and fixing them. Today I fixed a few things involving the vacuum system, then fell into a new set of issues while trying to diagnose the issue of why my horn doesn't work.

1. Most Urgent: While tracing the horn wire, we found a wire coming off the hot side of the starter relay that has the insulation BLOWN off of it. The wire is 14ga, the wire is completely exposed, and the insulation is blown off of an 8 inch section of the wire and is carbonized. Being an amp builder and repair man, my first instinct is to pull the wire completely out and replace it. It appears that the wire might go to the alternator, but it disappears into a wire bundle, and being that it is always hot, I haven't cut the bundle apart yet. One mechanic suggested splicing in a new piece of 14ga wire and soldering it, then heat shrinking or "wrap it in lots n lots n lots of that rubberized 'lectrik tape, then wrap that in some of that standard 'lectrik tape." Seems like a viable option, I'd like opinions though.

2. Kinda Strange: I found a switch today...it ain't factory. The switch is under the driver's side dash area and is a pull switch--you pull it, it makes a connection, push it in, it breaks it. The wire going to it comes from the back of the compressor clutch, passes through the firewall, and goes to the switch. The other side of the switch goes to the bundle of wires immediately beneath the steering column. I thought it might have been a kill switch for the clutch, but it's not. **Here's part 2 of this strange wiring sequence**: There IS a wire that has a disconnect plug on it that connects to the same wire about a foot from the connection of the compressor in the engine bay, it has the female end of the disconnect plug is on the compressor side. The male side of the disconnect plug is one of two wires that goes to something that looks like a sensor between the grill and the radiator on the driver's side. When the male and female are joined and the A/C is turned on, the compressor clutch turns. When disconnected, it stops. The switch inside the cab doesn't seem to be doing ANYTHING at all.

THIS SWITCH IS CONFUSING ME...Any help would be greatly appreciated!

3. Horn Don't Blow. Well...yeah, the horn doesn't blow. I took the steering wheel off, replaced the light that illuminates the shift indicator, and tried to figure out what the heck was going on with the horn on the same time--no clue how the horn system works, so no clue what's going on. I took a jumper wire from the battery to the terminal on the horn, and it does actually work--just not with the horn button. PO replaced the steering wheel with a Grant GT wheel. While I had the steering wheel off I tried shorting the terminal that the horn button wire goes to directly to ground (I was assuming the switch was simply completing the ground path of the horn). Nothing happens when it is shorted in the steering column--it only blows when the battery is directly connected to the terminal on the horn. I'd like to get the horn functional, even if it's on a momentary push button switch like we use in our effects pedals and isn't in the actual horn button. Any info as to how the horn circuit functions would be WONDERFUL.

Again, I'm sorry for the length of the post--these are the last of the issues other than the exhaust leak and super dim lights (I believe that is caused by the worn out light switch) that I need to solve I THINK...at least for the time being.

Thank you guys so much for any help!!! Now it's time to grab a cerveza and relax, so I can get up in the morning and work on this beast.

Ng

 

First, that wire is probably a fusible link. DO NOT splice a piece of 14 ga wire into the circuit! If it is the fusible link it should be tagged as such and will be protected by a 16 ga fusible link not 14ga.
Go here 1973-1979 Ford Truck Wiring Diagrams & Schematics - FORDification.net for the FACTORY wiring diagrams, they don't have the '73 yet, but the '74's are nearly identical, you can use those. This should help some.
Second, I'd dump the switch under the dash. That sensor in the front of the grill is the ambient temp sensor, it's used on '73-mid'74's.
The horn is a grounding circuit. There is a horn relay near the battery. It has 3 wires going to it. One is hot at all times, one goes to the horns and the third goes to the horn switch. The way it works is when you press the horn button you're grounding the horn relay. This makes the relay close and energizes (blows) the horn. So grounding the wire at the steering wheel didn't do anything, check for voltage at the relay,if there's voltage ground the terminal that goes to the steering wheel and see if it blows, if yes, the wire to the column is bad, if no, probably the relay..
I have a '73 also, 390, fac a/c etc. Here is a picture of the alternator wiring harness for mine. That sounds like the wiring in your problem #1.

 

Here's the horn relay, between the battery and starter solenoid.

 

Where can I buy a reliable, not terribly expensive replacement harness for the alternator and associated bundle of wire? I've seen plenty of full truck kits, but I can't locate just that section of wiring...I'd like to rewire the whole truck, but it's gonna have to be done one section at a time--saving up money so we can pay for our wedding. I'm sure that the future wifey is gonna freak a bit when I tell her I need to use part of those savings for a wiring harness hahaha.

Ng

 

You'll have to go factory for the wiring harness. The alternator harness contains a shunt for the ammeter. The one in the picture is for my own project and took me a year of searching to find.
I wanted to do the same thing, completely rewire the Pumpkin with NOS factory harnesses. I was fortunate, I was able to locate all the harnesses except the main underdash harness and the frame harness that ran to the rear along the frame rail.
Unless there is something wrong with the shunt portion of your harness you should be able to repair it. It will probably be cheaper too. When I finally located the alt. harness for mine, it cost me $175.
If you do repair the harness, the best way is to solder all connections and use heat shrink tubing to protect them.
Another tip that doesn't make any difference now but you'll thank yourself years down the road is dialectric grease. Use a dab of it on all electrical connections whether the screw, bolt or plug in.
I'll do a search for the harness for you but I think I pretty much got the last '73 alt. harness out there.
Yours has an ammeter correct? I saw in another post you have an oil pressure gauge.
As a side note, if I can't locate one (no one reproduces them) Mustang harnesses ARE reproduced. Might be able to adapt one. They share a common electrical system.

 

Wiring harness, alternator to voltage regulator
'73 F250-350 w/ ammeter & oil pressure gauges w/o diagnostic system
D3TZ-14305-B
Unfortunately, no dealer or obsolete supplier have any.

 

It does have an ammeter.

I don't mind repairing the existing harness. I have the tools to do so here in my shop, but wiring high voltage guitar amps and such is different from wiring low voltage, high current sources in the trucks. My question is this--how should I do the repair? I know that Auto Zone has solderable fusible links, I'm assuming I could splice into the existing wire where the wire and insulation are good with new wire, solder that connection, place a fusible link in line with the new wire, then put a new terminal on the end where it connects to the hot side of the starter relay.

As for the switch, I am planning on dumping it, but what in the world it was there for is beyond me. I suppose I could simply take the two wires off, strip back to good, non oxidized wire, solder, wire nut, electrical tape, and be done with that. I always tend to go overkill on securement and insulation of soldered joints--I don't want a truck fire.

Ng

 

Here is a pic of the wiring in question.

 

In that picture it looks like there is a fuse holder in that bundle too. It's possible that's the circuit that used to be the fusible link and the burned wire in question isn't a fusible link.
It would probably be best to go to the link I posted in post #2 and positively identify the cicuits in question. That way you can repair them properly.
And please.......NO wire nuts!

 

Ok...no wire nuts. I usually utilize those to cap off a soldered connection, then heat shrink--I NEVER rely on them to make a connection or keep one intact.

I am looking at the schematic, it does not appear that the battery to alternator connection is a fusible link, though in some schematics that I have seen in the past for different vehicles, I've seen them utilized. I wonder if I should install one or simple replace the wire. I believe that it is the connection to the alternator that I am seeing with the blown insulation. There is a fusible link on the schematic, but honestly the schematic confuses me. The fusible link appears to have a 37 beside it.

I'm going to try to get it running tomorrow when it's not raining...just trying to figure out what the heck I need to go buy.

Ng

 

The wire that goes from the alternator to the battery is black w/ red stripe and it will be a heavy gage wire, IIRC it is a 10 or 12 ga. The wire with the fusible link is black w/ yellow stripe and is 14 ga.

 

Got it fixed. I put in a 14ga fuselink, soldered the connections, and it fired right up.

Ng

 

Thanks for the update.
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