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Originally Posted by scottmcphee
3. upgrade voltage to the headlamp. This is what this original poster was asking about 3 years ago. There is too much voltage drop through the MB + side of the supply to bulb, more on that later. Here's how to minimize it: bring both plus and ground from the battery posts to the lamp area on minimum 14 AWG wires, 12 AWG is better, these beefy new wires must be fused at the battery side. I used 20AMP per wire for a good reason. Pro-tip: avoid having to run 4 wires by running a common rail of 2 wires that passes behind by each headlight. Rotate the second side of the car's connections to the rails so it's high beam shares the low beam rail of the other side of the car. Both wires are active for low, and and high - just the roles reverse. When you do this, one blown fuse means one side of the car remains working on low beam and the other side of the car on high beam - but you'll still have some light at all times. If you notice this failure pattern it's probably your fuse and not two filaments in different bulbs going at the same time. Beware, it is possible to hold the light stalk just so (as if doing flash to pass in slow motion) to get ALL FOUR beams ON at the same time. This is why I chose 20AMP fuses for the 2 wire idea. If you're antsy run 4 wires. Back to the program... cut the three wires going to each headlight lamp (3 wires for 9004 bulb), leaving enough length on the socket wires to remain useful. Repeat this for each side of the car: wire the car-side of the bulb supply with 2 relays, one relay coil for low beam, other relay coil for high beam, using car-side bulb ground for the relays only. Now when you operate headlights you'll only get relays clicks and no light until you connect your beefy + supply from the battery through the relay contacts to the bulb socket. One rail and a relay for low beam, the other rail and relay for high. Attach the bulb socket ground to your beefy ground wire. You can't get lower voltage drop than this for any headlight style assembly or bulb type!
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OK, I know it's an really old thread, but Scott's solution above really intrigues me. I thought it would be an easy thing to do, so I took my headlights out already and even desoldered the plugs on the passenger side and connected relays to them (one for high, one for low and one for fog).
And right now, my relays click with no + power coming from battery, yay. About to hook up power, but then I reached the part about "rotating the second side...". I'm stumped. How would this work?
I searched some more and found another post by Scott that read:
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I brought out two heavy wires from the battery, separately fused, and passed both wires by each headlight cluster, buss style. On the left side of the car I tapped low beam relay to wire#1 and high beam from wire#2. On the right side of the car low beam on wire#2 and high beam from #1.
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I drew it out, and to my simple mind, it didn't make sense. So in essence, he's saying hook up terminal 87 to Low Beam of left headlight AND high beam of right headlight. So...when I switch on my lights, I would get one high and one low beam? I can't seem to wrap my head around this.
I PM'd Scott and now realize that he's sold his Merc and may not be on the forum anymore. Can anyone else understand this? Help!