..picking up an old thread...

I've just done major fiddling with my 87 w124 headlight system, much brighter now. These are the USA DOT 9004 POS design. I know I could have got Euro assemblies to replace these.. $$$ if I was lucky on ebay etc.

But the mods I've done apply to either kind of assembly, so there is cumulative benefit to be had if I do change over to Euros. For now, I've got probably the best light output possible for 9004's.

Here's how:
1. clean the inside of the lens assemblies. Take them out of the car, take bulb out, get a little dish soap and hot water in there, half full and shake and swish and risk with hot water. Air dry in hot sun.

2. new bulbs, you pick a brand lots of other threads on these. I chose some over-wattage 80/100W bulbs. (DANGER! these get hot.) (Warning! these are illegal on the street, so now I just use my w124 for you know, off-roading.) Pro-tip: In order to safely use 80/100W, without risk of burning up your wiring harness, sockets, headamp switch, etc... the next step is mandatory, not just optional. Not only is it mandatory, you'd be wise to substitute high temperature sockets for the 9004's to replace the stock ones. Don't get the made in china ones off ebay buy real ones. I found USA made ones and compared with the china ones that fell apart in my hands, can you say cr*p? Hands down winner: USA brand. That's not patriotism because I'm Canadian, that's just a fact. Now then....

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!

4. oh-oh... bulb out indicator on dash is now rightly confused for headlights for both low beams and high beams and comes on when you use headlights. This is that pesky "N7 Exterior Lamp Failure Monitoring Unit" (a.k.a. that huge "relay" in the back of the fuse box, you have to unscrew the lid to see). It's not a relay, it's a pair of circuit boards that you modify - giving it just enough of a frontal lobotomy to forget about low beams and high beams, but continue to serve its useful purpose for every other exterior bulb on the car. Here's the deal, one of the reasons MB has voltage drop in the headlights is N7. Voltage must flow THROUGH it for lamps to work. There are copper foil traces handling full headlamp power, a short resistor wire, and the contacts of the N7 all fighting against and adding to voltage drop, not to mention the thinnish gauge wires going to and from N7 to the headlamps. Yuck. But none of that matters now for headlights, because it's only powering and monitoring your relays - which are happy to get even minimal power. To make the world more blissful it's time for that lobotomy, open up the shell. Once N7 is out of the car, it can easily be pried apart it with a flat tool chasing around its joining edge - it's snap together fit. A picture tells a thousand words, so I'll post a couple... in short, you cut two traces (one on each circuit board) then put it back together and into the car. Easy, and just like a real lobotomy.

5. Now, looking at your dash alone, you will never know if your headlights are out... but if you happen to look out the windshield at night it will all become pretty obvious, duh! On the bright side, happy motoring! Aside from using good electrical practices throughout, making good connections, motherhood, apple pie, at-your-own-risk warnings, etc, that I can never understand why people put in DIY forum threads (!?) this is the best you can do with USA DOT 9004 design lights, for pretty cheap.

__________________
Cheers!
Scott McPhee

1987 300D