Mounting & Wiring Fog or Driving Lights

Joe & J.B.[and others!],

Here's one method for wiring fog and driving lights: Use one relay for the fog lights, and another for the driving lights, both fed from the same fused lead from the battery connection on the solenoid. Then, use a three position switch (readily available at most any auto parts store for around $5), wired as follows - with the switch in the up position, the fog lights would be on if and only if the parking lights were on. With the switch in the center position, neither the fog nor the driving lights would be on. With the switch in the down position, the driving lights would be on if and only if the high beams were on.

Reasons for this?

1. You won't have to remember to turn off another switch when you turn off the main lights, as the fog or driving lights will automatically go off.
   
2. No worry about blinding an oncoming driver, as the driving lights will go off when you dim the main lights.
   
3. If you really need the driving lights, you also need the high beams.
   
4. In a very heavy fog, even the low beams may be blinding to you, reflecting off the fog, so you want to be able to have only the parking and the fog lights on.
   
5. You will never need both the fog and the driving lights on at the same time.
   
6. One switch will do the function of two, which may make it easier to find a place to mount it.
   
7. One fused lead will do, as both lights will never be on at the same time.

When you mount your lights, remember the differences in the light pattern from the two types. A fog light has a very sharp cut off, limiting the light to a narrow band just above the pavement. This allows the light to go under the fog, eliminating glare from the light bouncing off the fog. For this reason, the fog lights should be mounted as low as possible.

Driving lights, on the other hand, are intended to have a long, penetrating beam, designed to light up the road as far ahead as possible. For this reason, they should be mounted as high as possible.

Below is the schematic for the fog and driving light circuit described above, along with a wiring/connection diagram as well. The wiring/connection diagram gives the physical details for wiring. Actually, there are two sets of diagrams; one switching the ground leads to the relays, and the other switching power to the relays. Functionally, the two circuits are the same, but one or the other will be easier to install, depending on where you mount the relays.

Ed. Note: This version of the document has inline pictures which may cause inconveniently long download times. If this occurs, you may click here to go to a version which has the images as links rather than inline graphics. -J.B.

In general, you should choose the one that limits the length of the "powered" wires to the relay coil. That is, if you have the choice of long powered leads and short ground leads, or short powered leads and long ground leads, choose the location that gives the latter. A short on a ground wire will do no harm, whereas a short on one of the power leads can burn a wire or blow a fuse. It is always good practice to limit the exposure of wires that have power on them.

A double pole, double throw switch is shown for both schemes, although you could get by with a single pole, double throw switch for circuit #1. The reason for this is because that is the most likely type of switch you will find in the auto parts stores. If you can find a SPST switch, and prefer to use it, no problem at all - just ignore one half of the switch in the diagrams.

Some general comments on the installation:

1. The relay can be mounted any where that is convenient. The only criteria that is of any concern (other than protection from physical damage) is the TOTAL length of wire from the Brown wire to the relays and then from the relay to the lights. This length should be kept short, but as long as you use the proper size wire, it is not really important (assuming you don't intend to mount the relay in the trunk!).
   
2. The fuse in the lead to the Brown wire MUST be placed as close to the connection to the Brown wire as possible. If you do this, the remainder of the wire will be protected, and routing becomes less critical. Rather than connecting to an existing brown wire, I recommend using a new brown wire, and connecting it as close to the battery as you can. If you do connect to an existing brown wire, make sure it is large enough to handle the existing load as well as the added loads of the lights. HINT: If you detest un-necessary splices as much as I do, you might try this trick. I buy heavy-duty in-line fuse holders from the auto parts store and modify them to suit my purpose. I cut the leads off to about an inch and strip off all the insulation. Next, I remove the fuse contacts, and the wire, from the holder. I place the contacts in a vise and spread the wire strands out in a fan shape. Using a pair of needle nose pliers, I pull the center strand out of the crimp on the contact. After a few of the center strands are removed, the rest come out easily. Once all the wires are removed, I spread the crimp just a little, and insert the end of the wire I wish to use and re-crimp, followed by soldering. This way, I get an in-line fuse holder with the correct color coded wires, and each wire long enough to reach the rest of the circuit without splices.
   
3. The wires used from the Brown wire to the relays and from the relays to the lights should be sized to carry the rated current of the lights with a little margin. I would use 12 gauge -- good for 20 amps -- unless you are using very powerful lamps. Remember, only one pair will be on at a time, so the wires should be sized for the most powerful pair.
   
4. The fuse MUST BE NO LARGER than the current rating of the smallest wires used in 3).
   
5. When you connect to the Red wire, you can use ANY Red wire you find, whichever of one is most convenient for you to connect to.
   
6. Use at least 14-gauge wire for the connections to the Red wire, and you won't need to use a fuse, as the Red wire is already fused.
   
7. The physical configuration of the DPDT switch as shown in the connection diagram is not important -- only that it looks like that shown when the wiring is completed. Wired one way, the FOG lights will be on with the switch handle in the up position: wired the other way, the DRIVING lights will be on with the switch handle in the up position.
   
8. The wires form the switch to the relays in scheme one can be just about any size you want, as they carry only the limited current of the relays. If one of these wires should short to ground, the effect will be exactly that of turning on the switch - no harm done.
   
9. The wires from the switch to the relays in scheme two must be sized carefully. Although they carry the same load as those in scheme one, since they are hot under certain conditions, a short here will blow a fuse, provided the wires are adequate to match the fuse. If the wires are too small, they may burn before the fuse blows. The red wire is fused at 17 amps, so the wire must be capable of handling 17 amps. Normally, that calls for 12 Ga., but you could get by with 14. The wire from the blue/white wire can be any size you wish, as long as the fuse is rated no larger than the wire can handle.
   
10. The colors for the wires shown are only for existing wires. Use any color you wish for the new wires.

This is all I can think of right now. If you have any questions, let me know.

Dan Masters,
Alcoa, TN

[Dan wrote in with the following addendum:]

Gentlemen,

In the instructions for wiring the Fog/Driving lights, I said to connect to any Brown wire. This is correct, but after the recent thread on ammeters vs voltmeters, it occured to me that there could be a anomaly for those of you with ammeters. If you choose one of the Brown wires that are connected to the battery side of the ammeter, the lights will show up as a "charge" current on the ammeter when the engine is running. This is not a problem, but something you need to be aware of. You can either choose a Brown wire on the alternator side of the ammeter or just live with the charge reading. If you choose the latter, just remember that the ammeter reading for the lights will become your new "zero" reading related to "charge-discharge" on the ammeter. ie, if your lights draw 5 amps, and your battery is fully charged, you will read 5 amps on the meter. A reading of 0 amps will mean the battery is being discharged at the rate of 5 amps, while a reading of 10 amps will mean the battery is receiving a 5 amp charge.

Refer to the attached figure "ammeter.jpg" for further explanation:

Figure A: this figure is a simplified diagram of an ammeter circuit under normal operating conditions. All of the current to the loads comes from the alternator, and the alternator is supplying a small charging current to the battery to maintain it in a charged state.The small charging current is too small to register on the ammeter, so it shows zero amps.

Figure B: In this figure, the battery has been discharged. The alternator now supplies a heavy charging current to the battery, as well as maintaining the current flow to the loads.

Figure C: In this figure, the alternator is either not turning because the engine is off, or it has failed. In this case, the current to the loads is supplied by the battery, and the ammeter shows a discharge reading. A similar situation would occur if the alternator were working, but the total loads exceeded the alternator capacity. In that case, the battery would supply the additional current, which would show as a discharge. If this situation continues for long, the battery will run down.

Figure D: This is a more realistic representation of the circuitry often found in a Triumph. Some of the loads, such as the horns, are connected directly to the battery. When these loads are operated, the ammeter will show a charging indication, even though the current is comming from the alternator. In an early TR6, if you blow the horn with the engine running above idle, the ammeter will show a charge reading!

Figure D explains why is may be much harder to add an ammeter than to add a voltmeter. Without an ammeter, it doesn't matter where you connect the loads to the feed from the alternator to the battery. When you add the ammeter, all the loads you want to monitor have to be moved to the alternator side of the ammeter. On a Spitfire MKIV, for example, there are three wires from the solenoid/battery connection feeding the various loads in the car. Every one of these that you want to monitor will have to be moved.

Thanks,

Dan