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Jeremy5848 · #1 10-04-2012, 02:24 PM

The purpose of daytime running lights is to make the car more visible to oncoming traffic. No other outside lights or instrument lights need to be turned on. For my ’95 E300, I chose to use the “fog” or inner headlights (with clear bulbs) as daytime running lights because (a) they have a wide spread of light so they are more visible from the side, (b) the “regular” headlights won’t burn out as often, (c) you can still flash the high beams to get people’s attention. The following mod was specifically written for the W124 model but should be adaptable to most Mercedes vehicles with a few modifications.

The secret to this mod is pin “N” of the headlight switch. If you put +12 Volts on this pin, the fog lights (only) illuminate. If you accidentally turn on the fogs with the headlight switch too, nothing bad will happen (no smoke, no sparks). You can do this with any simple switch connected to the battery. The “lamp failure” module will still work normally but complications arise if you want to protect the car with a fuse, retain the stock appearance, and have some kind of warning so you don’t leave the fogs burning and walk away from the car. In my car the fogs are 55 Watt H3 clear bulbs. Those who use high-power bulbs in this position may have to make some modifications to my circuit to handle the higher power. In particular, I’m not sure how much current the internal contacts of the headlight switch can handle. YMMV and all that.

The DIY that follows is what works for me. The design uses existing features of the car and parts I had on hand, the lamps are fused, and they go off with the ignition switch. The mod is reversible (it can be removed leaving no damage to the car). There are many other ways to make this mod. I tried to keep this from being too wordy and used a lot of pictures.

This mod uses the “accessory wiring block X30,” which I describe in this thread. For convenience, I repeat here an image created for that thread.

The circuit diagram follows. It uses a surplus “rear dome lamp” switch, an “ice cube” relay, and some wire. The power from X30 is already fused so no extra fuses are needed. The switch mounts in one of the extra positions of the “limo wood” that I bought the car as a treat a few months ago. The relay mounts on a bracket behind the headlight switch.

Here is a picture of the switch and relay that I used.

The switch mounts in the “limo wood.”

I found a bolt behind the headlight switch, perfect for mounting the relay. The socket and bracket for the relay came from an unknown junkyard Mercedes.

Rather than take the headlight switch socket apart and risk fouling up the arrangement of pins, I simply wrapped the wire around pin “N” and pushed the socket on top of it. While crude and unprofessional, it was easy and it works. The half-moon cutout in the switch keeps the wire from moving around and a cable tie holds it in place.

The connection to X30 was made with an extra plug that I had from an outside temperature display (“Pin bushing housing,” part number 013 545 32 28 and “round pin bushing (2.5 mm),” part number 002 545 99 26). See my X30 thread for details. I used pin #1 (fuse C) to power the fogs and pin #2 (fuse B) for the relay. Pin #2 is powered only when the ignition switch is ON in position #2 (“Run”) so the fogs will shut off when you turn off the ignition. This prevents leaving the fogs on and walking away from the car.

Here is the finished installation. It’s simple and it works.

Jeremy