Daytime Running Lights DIY

[Jeremy5848]

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

[compu_85]

Instead of wrapping the wire around the headlight switch pin I would suggest taking the headlight plug apart and solder it to the connector shell. There is generally enough space in the pin shells for 2 wires.

If you wanted to be really fancy you could ground the relay to the parking brake switch, and then run it through the dome lamp switch... then you can turn the lights on and off, but the lights will be off when the car is parked.

-J

[gsxr]

As usual, nice work & writeup, Jeremy!

For the record, I use DRL's all the time on all my cars, but I do it manually by turning on the fog lights. The difference is, my method also turns on the instrument cluster lights, and the marker/parking lights. Jeremy's setup turns on the fogs alone.

[sixto]

Can you send reduced voltage to the fog lights so they don't burn as bright in DRL mode? Would that trip the warning relay?

What's the risk of turning on the fog lights using the headlight switch with DRL mode engaged? Does bridging N and the fuse C create an undesired loop?

I wonder if I can pump 15-25 watts through the city lights in Euro headlamps as DRLs.

Sixto
87 300D

[Jeremy5848]

Originally Posted by compu_85

Instead of wrapping the wire around the headlight switch pin I would suggest taking the headlight plug apart and solder it to the connector shell. There is generally enough space in the pin shells for 2 wires.

I agree that your suggestion is superior, I just didn't want to pull that dam' plug apart. Want a job???

Originally Posted by compu_85

If you wanted to be really fancy you could ground the relay to the parking brake switch, and then run it through the dome lamp switch... then you can turn the lights on and off, but the lights will be off when the car is parked.

-J

Would this require the parking brake to be set in order to turn off DRL? 'Cause not all of us use the parking brake. My wiring the relay through ignition switch position 2 ensures that the fogs will turn off with the ignition switch.

J.

[Jeremy5848]

Originally Posted by sixto

Can you send reduced voltage to the fog lights so they don't burn as bright in DRL mode? Would that trip the warning relay?

What's the risk of turning on the fog lights using the headlight switch with DRL mode engaged? Does bridging N and the fuse C create an undesired loop?

I wonder if I can pump 15-25 watts through the city lights in Euro headlamps as DRLs.

Sixto
87 300D

(a) It would take some kind of voltage regulator IC and a heatsink able to handle 10 Amps or more (depending on bulb wattage). What the bulb-out module would think of that I don't know. You don't have to reduce the current very much to dim the lamps so my guess is that the warning system would be OK.

(b) No risk. BTDT, see the third sentence in my second paragraph.

(c) Can you find a bulb for that much power with the tiny base required of the city light socket? Would you change to a bigger socket?

Jeremy

[compu_85]

VWs used a very long resistance wire for the DRLs... I have one someplace for a project like this.

[sixto]

Originally Posted by Jeremy5848

(b) No risk. BTDT, see the third sentence in my second paragraph.

Does no risk mean a short in the fog light bulb circuit won't fry fuse C?

Sixto
87 300D

[Zulfiqar]

how about using a resistor or GM type resistive wire to drop voltage before this splice into the headlamp switch, That way you can override it with full strength fogs when fogs are actually needed.

very nice Jeremy... Ive noted this into my list to do but Im thinking of using the alt exciter light to work the system. btw does this terminal N go dead once the main headlamps are switched on?

[Jeremy5848]

Originally Posted by sixto

Does no risk mean a short in the fog light bulb circuit won't fry fuse C?

Sixto
87 300D

"No risk" means you can turn on the fogs with the headlight switch AND have them already on with the new switch and nothing bad will happen. A short will blow Fuse C, as planned. In any case, I should probably write "reduced risk" or "low risk" rather than "no risk," which is asking for trouble.

BTW I already (today at K-Mart) left the new switch on and walked away from the car -- but the fogs turned off because the relay tripped out when I turned off the ignition so no harm was done. It works!

Jeremy

[Jeremy5848]

Originally Posted by Zulfiqar

very nice Jeremy... Ive noted this into my list to do but Im thinking of using the alt exciter light to work the system. btw does this terminal N go dead once the main headlamps are switched on?

I suppose it depends on which model car and which headlight switch. My '95 (switch part number 000 545 53 04) turns on the fogs if you turn the switch one click (parking lights) or two clicks (headlights) and then pull out one or two clicks. The second pull-out also turns on the rear fog, of course (although my car doesn't have a rear fog -- I stole that position of the taillamp housing for a second taillamp bulb on each side).

Jeremy

[benedict]

Very good Jeremy - drawn like a true EE!

I always liked the daytime runners that Volvo once upon a time did in their early 'air bags in a box' cars.

[oldiesel]

I did a similar mod for day running lights, only difference i fed the relay pull coil from an ign switch wire and the ground from the relay coil connected to the headlight wire at the headlight switch so with the key off the drl lights are off and when you turn the headlights on the drl s are off,with the key on and headlights off the drl s are on, if there is a drawback its that the drl s are on before the engine is started while the glow plugs are activated so in extreme cold weather(not here in Florida) the added current draw might affect starting.The drl s canstill be turned on by the normal switch. Did it to both the 87 and the 91 pretty simple mod with one little relay and a few wire connectors.

[sixto]

Originally Posted by Jeremy5848

I suppose it depends on which model car and which headlight switch. My '95 (switch part number 000 545 53 04) turns on the fogs if you turn the switch one click (parking lights) or two clicks (headlights) and then pull out one or two clicks. The second pull-out also turns on the rear fog, of course (although my car doesn't have a rear fog -- I stole that position of the taillamp housing for a second taillamp bulb on each side).

Jeremy

If I understand, and that's a stretch, headlights on, fog lights off, DRLs on means headlights and fog lights are on, correct?

Do the 86-up fog lights turn off with high beams as on the up-to-85s?

What does the 500E have to switch the aux lights in the bumper?

Sixto
87 300D

[Jeremy5848]

Originally Posted by sixto

If I understand, and that's a stretch, headlights on, fog lights off, DRLs on means headlights and fog lights are on, correct?

Do the 86-up fog lights turn off with high beams as on the up-to-85s?

What does the 500E have to switch the aux lights in the bumper?

Sixto
87 300D

Depending on the headlight switch, it's possible to have both headlights and foglights on at the same time. DRL in my case now means foglights only, no headlights, taillights, or instrument lights.

Jeremy