Sunday, December 26, 2010

Ford truck headlight wiring upgrade

I've had the 1975 Ford F-350 for about a year now. It has plenty of problems (some of which I listed in my previous post), but it's what I expect from a 35-year-old truck.

One problem I've been ignoring for a while is the headlights. While driving at night, they would turn off and on. Intermittent electrical problems are difficult to diagnose, and in this truck the wiring is nuts. Three-and-a-half decades of dirt, decay, and home "repairs". It's often hard to tell what color a wire is, and who knows if it will match the wiring diagram? The only good thing I can say is that the truck lacks many of the modern features that would add complexity (a chime when you don't put on your seat belt, or ABS brakes, or a "MAINT REQ'D" indicator).

I studied the wiring diagram, but it didn't tell me much: wires go from the headlight switch to the headlights, and don't do much else. Duh. I tried wiggling the wires that I could see, to trigger a loose connection, but it didn't work. I held up a multimeter near the truck, but it didn't care. I tried looking around the internet, but couldn't find anything that matched my symptoms.

I ended up just avoiding driving after dark. Since I didn't use the truck very much, that was OK.

In the fall I started using the truck more, just as the days were getting short. Right now our sunset is around 4:30pm, so the "don't drive at night" tactic is very limiting. So, I went looking for help on the internet, again. This time I found something.

The problem:

The stock headlight wiring sucks. The wire is quite small (#18) and it follows a very long tortorous path from the battery to the headlight switch, down to the dimmer switch and then back thru the engine compartment to the headlights. There is several volts lost thru all this wiring and switches, so the lights only get maybe 10-11 volts instead of the 13-14 they should have.
The other problem is that all that headlight current heats up the headlight thermal breaker, which eventually dies from the well known "flashing Ford headlights" syndrome.
His solution is to run a new circuit to supply the headlights, and use the existing wiring to control a relay on that circuit. This is pretty basic automotive wiring. (It's right at the edge of my comfort zone, which tells you something about how I am with automotive wiring).

I went to the local auto parts store and bought some supplies:

-  A couple relays, with sockets:

- A small fuse panel and fuses:

- A tube of dielectric grease
- 14 AWG wire - big enough to carry the headlight load
I also used a some 18 AWG wiring, crimp connectors, and heat-shrink tubing I already had.

I mounted the fuse box on the side of the engine compartment. Heavy steel was tough to drill. 2 holes to mount the box, plus one for ground screws.

I used the fuse box instead of in-line fuses for two reasons. First, the in-line fuse holders I tried fell apart in my hands. Second, the starter solenoid already had 5 wires attached to one stud (1 from the battery, 4 to various systems) and I would be adding 2 more. A fuse box can do the distribution, instead of everything being bolted on the stud. I only put these two new circuits (high beam / low beam) on the fuse box for now, but I may move the others over later.

I reused the existing headlight sockets, but leaving 3"-long pigtails to splice in to. They're only 18 AWG, but at that length it'll be OK.

I put silicone grease in every connection to keep out water & air, so it won't corrode. Solder + heat shrink tubing would have been more secure, but I think this will be good enough.

The headlights seem brighter now, although I don't have good before-and-after comparisons.

While I was looking around the engine compartment, I noticed a bunch of disconnected wiring on the right side. There's a space for a battery that isn't there, a ground cable for that battery, and what looks like a second starter solenoid, but is actually a battery isolator relay. This truck used to have a slide-in camper long ago.

The reason I started working on this headlight project was because I got stuck while working on the motorhome. Its battery isolator was dead, and I was having a trouble finding a replacement. I was at the auto parts store looking for one, which they didn't have, so I bought the bits for this headlight project. Yay, got my isolator for free. :-)


S said...

great story

Chuck Stevens said...

This is some awesome advice. I have an older Ford truck and the electrical wiring is definitely starting to go. I have been picking up different Ford parts here and there but it's definitely though to figure everything out on my own!

Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.