The MGA With An Attitude

By now you may know I'm not a big fan of installing unnecessary relays. I'm sure I said that in a prior article. But sometimes when you add electrical gadgets and need to do some odd functions, a relay can come in handy for logic switching. This is one of those "magic" applications to handle multiple functions with a single relay, and in this case also a single switch. I post this article because someone seriously wanted to do this, and it is a different approach to installing old period lighting. This is for when you want to install one driving lamp, and one fog lamp, which was fairly common, back in the day.

This setup is also predicated on the idea that you actually may be legally allowed to do that. Back in the day when MGA cars were new, it was allowed to use a single Fog lamp or a single Driving lamp. (Not very long before that some cars had a single tail lamp and single brake lamp). I don't recall exactly when a new regulation was introduced (perhaps someone will remind me), but sometime after MGA was out of production it became illegal to install asymmetrical lighting. This may have been around 1968 with a large list of new safety regulations, and at a time when alternators were coming into common use to allow additional electrical loads in a motor vehicle. After date of the new regulation all new cars had to use exterior lamps in pairs, like two Driving lamps instead of one. I might speculate it was to make it easy to spot cars with a non-functional lamp, or maybe a safety feature so if one lamp quit working you still had another one to run on. In any case, MGA cars were built when single lamps were still legal, so the cars are grandfathered in the law, meaning if it was allowed when the car was new you can still do it today.

You might also consider the modern day circumstance when you may be running your vintage car with asymmetrical lighting, and you might be stopped by a cop who thinks you have an equipment problem (non-functional lamp). While your car may be entirely legal under the grandfathered law, this traffic stop could be an unnecessary inconvenience, and you may occasionally spend some time convincing a cop that your asymmetrical lighting is actually legal. In other words, the asymmetrical lighting may be an attractive nuisance. If you can live with the idea that you may be pulled over occasionally for no good reason, then you might like to do this.

The first idea (and request) here is wanting to use a standard lighting switch in place of the original Fog light switch, giving an extra switch position. Then use this three-position switch to control both a Fog lamp and a Driving lamp without drilling another hole in the dash for a second switch. In this case the (duplicate) lighting switch will not directly do what you want, but installation of a single relay can make it work. Additionally, the relay can enable multiple functions that might otherwise require multiple switches and some conscientious manual control to avoid doing something bad. It can also reduce electrical current draw to some standard acceptable level even when you add two lamps up front. So let me post the electrical circuit first, followed by the explanation. You may click on the circuit diagram to open the image in a separate window so you may have it handy while you read the rest of the text.
Relay circuit to switch between fog and driving lamp

5 amp miniature relay fron Radio Shack HOW TO INSTALL:

  • 1. Change Fog switch to 3-position headlight type switch. Connect power input (Red) from Lighting Switch S1 to Fog Switch "A".
  • 2. Add Double Pole Double Throw relay, Radio Shack number 275-249, 1" long, 1/2" wide (about $6 in stock). Tie wrap it to anything handy for mounting.
  • 3. Connect original R/Y wire (going to Fog lamp) to relay Normally Closed-left.
  • 4. Run new wire from Fog Switch S1 to relay Common-left.
  • 5. Run new wire from Fog Switch S2 to relay Coil-left, and ground relay Coil-right.
    When the relay is very close to Fog switch, color of wires in between doesn't matter much, but new colors like Pink and Light Blue might be nice. Ground is always Black. (Avoid Red or Green, as there are lots of those behind the dash).
  • 6. Run new Blue/White wire from Dipper Switch to relay Common-right.
  • 7. Run new wire from relay Normally Open-right to Driving Lamp.
    It would be nice to be Blue with a different color stripe (Blue/Yellow or Blue/Green for instance), but plain Blue may do as it is the only blue wire running to front of car. There is only one other Blue wire in the car, running from Lighting switch to Dipper switch.


  • a.) Lighting switch in 1st and 2nd positions, and Dipper switch work all parking lights and headlights as original, as none of that wiring is changed.
  • b.) There is No power to Aux lamps when Lighting switch is off.
  • c.) Lighting switch on (1st or 2nd position) puts power to Fog switch, as original.
  • d.) Fog switch to 1st position puts power onto relay COM connecting through relay NC to power up the Fog lamp.
  • e.) Fog switch to 2nd position adds power to S2 which triggers the relay. This disconnects the fog lamp and connects the Driving lamp.
  • f.) Driving lamp is effectively a third high beam headlamp, but it will only work when the relay is triggered. When Driving lamp is switched on, it will go on and off with high beams when kicking the Dipper switch.
  • g.) Since Fog lamp and Driving lamp cannot be on at the same time, there is no additional load on the electrical system or on the lighting switch. Original system was designed to operate one Aux lamp in front. -- Note however that the Dipper switch and its input wire must now carry the electrical load of three front lamps on high beam, up to 15 amps. You might want run an additional 18 gauge wire in parallel to the original Blue wire to handle the additional 5 amps (more on that detail below).
  • h.) View 1: The primary purpose of the relay is to inhibit use of both Fog and Driving lamps at the same time. A bonus is that the relay also works logic magic so you can substitute a standard Lighting switch in place of the original Fog switch and work both new lamps with a single switch.
  • i.) View 2: The relay (and substitute switch) takes the place of adding a second switch for the Driving lamp, and also inhibits use of both new lamps at the same time.
  • There is an ALTENATE SENARIO available by swapping just two wires. Run the Fog switch S2 wire to relay COM-right, and run the Dipper switch high beam wire to the relay coil. Then the Driving lamp takes power from the Fog switch and from the parking light circuit as original (same as the Fog lamp), and the Dipper switch only has to handle two headlights as original (plus the relay trigger). Difference in operation is that the fog lamp cannot work on high beam, but kicking to low beam will bring the Fog lamp on when the Driving lamp goes out. That's a little quirky, but it would work (without adding another relay).

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