Zoom!

There are a variety of wiring problems you may have if you've got a 30 year old car like I do. Some result from the poor design of the wiring in those cars and some just come from old age.  My headlights were not as bright as they should have been. Measuring at the headlight connector, less than 10 volts was available! My turn signals flashed way to slow when my headlights were on and not at all when the engine was off. And with the engine off the horn just squeaked.

I applied my engineering geekyness to the problem to found the cause: one 30 year old 14 gauge wire was trying to carry ALL the current to my fuse block and it just couldn't do it.  This is bad for a whole lot more reasons than just a slow turn signal blink. The high resistance that caused a transmission voltage drop means my wires and connectors were generating heat. In an extreme case hot wires can melt insulation. 'Nuf said.

Ford ran one +12V line from the hot side of the starter solenoid to the fuse block under the dash and distributed EVERYTHING from there. Earlier Fords (pre '68???) didn't even protect this supply with a fused link.  With no protection any short between the battery and the fuse block will pull enough current though the wire to melt the insulation quicker then you can say classic-car fire.  Even when the wiring was in perfect shape that one little 14 gauge wire couldn't carry enough current to run everything at once. When you have your lights on AND your wipers running AND your heater fan on, not to mention your ignition on, that one little wire has to carrying all the current.

If you're car is like mine the wiring harness isn't new anymore. (Boy, do I wish I could get a new one. But unless your older car is a very popular model you are probably out of luck.) Corrosion gets into older wires where the insulation ends at the connectors. And the connectors themselves accumulate tarnish that makes them inefficient.  Wiring harness can be rebuilt, but it isn't as easy - or as cheap - as you might think. If you want to go that way try an aviation mechanic. They're experts at it.

Modern cars typically use multiple fuse blocks, one under the dash and one under the hood. That way you can feed each one with its own supply wire. And both +12V supply wires are protected with a fused link right at the source.

My upgrades include: 1) adding safety with fused links, 2) offloading the old harness with a headlight relay, and 3) augmenting the primary battery to fuse block lead. The whole thing should cost about $50 and take an afternoon. If you don't have any of my symptoms you're probably OK.  But, with your engine off and your headlights on, if your turn signals blink slowly or your horn squeaks this will fix your problems.

First, we need to take some load off the single factory fuse block under the dash. The best way to do this is to add a relay to feed current to the headlights. You'll need a two circuit relay (J. C. you-know-who sells 42ZX0002W for around $19) or two 12V 15A relays from Radio Shack. 42ZX0002W is a better idea because it is internally fused. Find a place to mount the relay(s) beside your original wiring harness and as close to the headlights as possible but where they won't be splashed too easily. This has to be between the fuse block and the first headlight it runs to, usually the driver's side lights.

OK, disconnect the battery. (If you didn't think of this first yourself, maybe you don't want to be doing this.)  Mark the wiring harness where it runs next to your new headlight relay and unbolt the fuse block to separate it from the wiring harness connector at the firewall.  Carefully remove the wiring harness from the car.  There are lots of connections, bulbs, etc. to be disconnected so add some labels if you need to keep track of what's what.

If your +12V primary lead from the starter solenoid doesn't have a fused link, this is a good time to install one. Just cut off the existing ring connector and crimp on a 14 gauge fused link. Get a "Ford" one with a ring connector that you can bolt right onto the hot side of your starter solenoid. Next, attach two 14 gauge fused links to two new 12 gauge automotive primary wires using the crimp connectors on the links. You need to add these two new wires into your existing harness so they start at the starter solenoid. One is going to go to the headlight relay, the other all the way through your firewall and to the fuse block.

This is the time to figure out which wire in your harness is +12V for your low beams and which is +12V for your high beams. A good wiring diagram with color codes for your car comes in handy about now, but you can figure it out with just a continuity tester if that's your only option. Go ahead and cut both these wires about one foot from the marked point on your harness near the new headlight relay, but nearer the fuse block and switch. Splice in a couple of extra feet of wire to both the high beam and low beam wires from the side connecting to the switch. The idea is when you rewrap your wiring harness you should have 5 wires coming out of the harness that can reach your new headlight relay: the 12V feed from the battery, the switched lead for the low beams, the switched lead for the high beams, the wire to the low beam light bulbs, and the wire to high beam light bulbs. Be sure you've coded the wires so you know what's what.

The other new supply wire is going all the way to your fuse block, plus a couple more feet.

You can either just wrap the new wires to the outside of your old harness with black tape, or you can remove ALL the old black tape first and rewrap the entire harness if you are really into neatness. Either way, don't leave any loose wires that can be snagged! Wrap up every inch. Then bolt the original primary lead and your two new fused links to the hot side of the starter solenoid and reinstall the wiring harness. Be certain the wiring harness is connected to the car in all the places the factory intended. If you need any extra motivation, imagine what might happen if something came loose and your wiring harness ended up wrapped around your fan blades. Don't reattach the fuse block connector yet and leave the battery DISCONNECTED for now.

At the headlight relay crimp on the appropriate connectors to your 5 new wires and connect them to the relay following the wiring instructions that came with the relay. Most automotive relays will pick up the ground from being bolted to the car. If your relays need a separate ground just add a jumper to the screw you used to attach the relays to the car. And if your relay doesn't have internal fuses you should add one inline on the +12V primary wire from the battery. Of course, you may decide the fused link at the battery is good enough, but I wouldn't recommend it.

We're going to complete this little project by supplementing the existing +12V primary from the battery to the fuse block with the new wire we added. We don't want the splice before the wiring-harness-to-fuse-block connector since that connection may be making poor contact. Instead pull the new wire though the firewall in some appropriate hole. Here's the tricky part: There is probably one unfused, large gauge wire leaving the fuse block carrying +12V. (If you have a complete wiring diagram you can confirm this wire exists and maybe even find out what colors it should be.) You need to find that wire. I found it by inspection since the large gauge size made it obvious and I confirmed it by popping out all the fuses and checking continuity. Splice your new primary lead to this wire as close to the fuse block as you can get it using one of those nifty 3M connectors.

OK, button everything up and start your tests. It is a good idea to just touch the battery cable to the battery at first to be sure you didn't introduce some tragic short.  For my Ranchero, I was rewarded with a horn that honked and flashers that flashed, even when the car was off and my lights were on. With 12.1 volts at the battery, before these modifications my headlights were only getting 9.5V.  After my mods they were receiving a full 11.9V. 

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