Installing Driving (or Fog) Lights in an old British Car Without Destroying the Wiring Harness or Annoying Oncoming Traffic

First, make sure what sort of light you have. Hooking them up in the garage is probably not the best way to determine this, unfortunately. A fog light should have a very distinct upper cutoff (probably a horizontal edge) where the light stops - the point of a fog lamp is to keep the light low, so it can't reflect off the water droplets and back into your eyes. A driving lamp won't have such a cutoff - it's just a bright beam, tuned for long distance rather than breadth.

Common sense (and the law, in some more enlightened climes) dictates that you want to wire driving lights in parallel with your high beams - they'll only be on when your brights are on (and you've selected them); if you dip your beams, the driving lamps will turn off, too. I like to wire fog lights in parallel with the parking lamps, so that if the conditions get really bad and/or the cutoff on your low beams isn't all it could be, you can run the fogs alone.

This is pretty easy. You need a switch of some sort to turn them on/off, a relay, and a fuse. The discussion proceeds with the (slightly more complicated) driving lights, with commentary on fogs at the end.

Please note that many of the details below are oriented towards installing these lamps in a Triumph TR4A - because the person that prompted me to write this up asked specifically about that car (and because I have just a bit of experience with that model). The wiring colors and most of the details of finding places to insert in the harness should apply to most British cars, and the general principles to most any car.

A lot of people like to use a lighted bat-handled toggle switch for this application, because then you have a visible reminder in the car that the things are switched on. If your lamps are amber instead of white, you might not care about this; some of the lighted switches are pretty bright inside the generally dim British cockpit. Your choice. The Mini and Morris Minor of the mid- to late-60's used toggle switches on the dash; I know Holden makes a repro of this, and you might well be able to get them from Mini City in the East or Mini Mania in the West.

For a more modern interior, you might want to use one of the lighted square push/push switches that Dave Bean sell, complete with the international symbol for "driving lights".

Or just pick up a switch you like from Radio Shack or the local auto parts store.

All vendors mentioned in this article can be contacted by information found in The Monster List.

Figure out where you want to mount it - I have mine mounted on the radio blanking panel. That's not a horribly convenient place to reach, as it turns out, but it's not awful. I chose a switch that has too small a handle, too, so it takes some groping around to reach.

For this kind of application, I really like the Marchal 514 relay; it's bombproof, and it has an internal fuse. I buy them from Dave Bean Engineering; get a couple of the unique fuses, too. Not cheap, though. (If you object to having French electricals in your British car, Lucas makes the fairly good all-metal 28RA relay, but you'll need a separate in-line fuse holder, and it will all be more of a nuisance to mount. Whatever you do, make sure you use a fuse! You might also find an old-style 6RA relay in various configurations.) Marchal makes a lovely chrome toggle switch, too.

This relay is easy to mount, since there's just a single tab with a pre-punched hole. Pick some reasonably accessible spot under the driver's side dash; you want to be able to get at the fuse easily, and all your wiring is there. Chances are that there's a bolt or screw that you can undo and use to mount this without drilling any holes.

If you want to do this "right", you want to get properly colored wire! You can get this from British Wiring. The wire running out to the lamps should be blue with a yellow tracer, 12 gauge (28/0.30 outside the US). You'll also want a few feet of thinner blue with white (16 or 18g will do, 9/0.30), the same of blue/yellow, and some 12g brown with blue.

The idea is that you will use the brown/blue to supply the main current to the relay; you get this from the same place the lighting switch does. The lighting switch has a similarly colored wire coming into it; if you trace this back, you will find a Lucas barrel connector (under the steering column). If you replace this two-hole (what Lucas calls a "single") sleeve with a "double", you can now connect four wires together, of which you need three - the original two, plus the one running to the relay. Connect to the terminal marked 30/51 (#1 on the 514 relay). (Please see note at end for a warning and alternative.)

Both British Wiring and The Roadster Factory sell the sleeves and bullets. Lots of people dislike them, but I find them to be quite reliable when installed correctly - either soldered, or crimped with the proper tool (British Wiring sells this, too). If this is not something you plan to do often, just solder the connectors with rosin core solder and a good soldering gun; otherwise, the crimping tool is a good investment, and you probably just want to buy a selection of bullets and sleeves.

OK, you've got power to the relay. At the other end, you need to run power to the lights - that's the blue/yellow wire. You can run it out the firewall along the main harness; if you want to keep it out of sight, run it underneath and use a loop of black electrical tape every 6 - 8" to support it. It will be mostly invisible. You can add another double connector at the top of the radiator with the rest to split the wires to the two lamps, just like all the others. Connect the other end to the terminal marked 87 (#2 on the 514).

Now, you need power for the switch. You only want to have power on this switch when there is power to the high beams; that way, the driving lamps get turned off when you dip the mains. Find the wire that feeds the dashboard high beam indicator and trace it back (it's blue with white) to a connector. Again, you should be able to substitute a single sleeve connector for a double, and run the extra (blue/white) wire to the switch. Run the thin blue/yellow from the other terminal of the switch to the coil terminal on the relay (marked 86, or #3 on the 514).

Depending on how you mounted the relay, you may need to run a separate ground from the other side of the relay coil (85) to the frame; the easiest way to do this is with a short black wire run to bolt on the frame, probably the same one that's holding the relay. The true Lucasite would buy a ground clip to place under this bolt, and use a bullet connector on the end of the black wire. The 514 has an integral ground terminal, #5, so you can jumper that to #4, which is the 85 terminal.

If you are wiring things up with an older Lucas relay that doesn't have these designations, you are probably looking at terminals marked W1, W2, C1, C2 or some subset of that. W stands for "Winding", apparently, and C for "Contacts". So W1/W2 should be connected as 85/86 above, and C1/C2 as 30/87 above. If you only have three contacts, then one of them is providing power to both the winding and the contacts - this is a useful thing in some cases, but not here.

For fog lamps, you do much the same, except that you get power for the switch from the parking light circuit (red) and run red/yellow wire from the relay to the fog lamps. The sizes and hookup are otherwise identical.

That's it! Now that they're working, please aim them.

I would probably hook up the switch and relay first, and make sure you can hear the relay click on and off when you actuate the dip switch and the dash switch. Then run the 12g wires to/from the relay and lamps...

Note that when you've got all these lights running (plus heater and wipers) you may well be exceeding the capacity of your stock alternator and the supply wiring to the lighting switch. Do the math (8 amps per 100W of lights) and check under-dash wiring. You may wish, instead, to supply power to the relay(s) directly from the battery (with a fuse, of course!). This won't solve the capacity problem, but will keep you from melting the wire in your car. The only possible downside is that the current drawn by the auxiliary lamps won't be properly indicated on the ammeter ...