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Old 07-03-2002, 02:38 PM
Don Atienza
Posts: n/a
Question How do you install a headlight relay on w123?


I just bought a set of high-wattage halogen bulbs for my 1976 w123 200D. I have the 'newer' version rectangular headlight assembly (the original was the one with the big round lamp for the headlights and a small round on for the fog lights, but also in a single unit). Problem is, there wasn't much of a difference as regards brightness. I was told that this is because my car model was not equipped with headlight relays. What EXACTLY are relays and how will it help brighten the headlights? Is one relay just good as another or are there "specific" relays (one kind for headlights, another kind for the aux. fan)? How do I install one (hope it is a DIYer)? Thanks in advance!
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Old 07-03-2002, 05:30 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
A headlight relay won't make your lights brighter. They will only be as bright as your battery has voltage.

The point to using a headlight relay is that if you really want some light you will use some form of higher wattage bulb or combinations of bulbs. Originally all the current goes through the headlight switch and the combination switch. The combination switch will have its life reduced by carrying more current than MB intended. To use a relay one would power the bulbs straight from a fused battery lead using the original headlight wire to only turn on the relay. This will bring the current going through the combination switch to less than a half amp. The original set-up places 6-10 amps with each settings. Higher wattage bulbs could take that current to 15-20amps. The switch won't like this but relays are designed to take that much current easily.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 07-03-2002, 09:16 PM
Registered Diesel Burner
Join Date: Aug 2001
Location: Northern Virginia
Posts: 2,911
It's possible that a headlight relay could make your lights brighter if it handles the electrical current with a lower voltage drop than the normal circuit (through the headlight switch) does. Meaning the relay has more current-handling capacity and less resistance.

Before you go to all the trouble of buying and intalling a headlight relay, you can run a simple test to see if it will help. Carefully hook up a temporary jumper wire directly from the battery to the headlight terminal. You might even be able to do this with the regular connector in place - I don't know for sure since I haven't tried it on a MB. But the idea is to directly provide a good circuit from the battery to the headlight and see how much brighter the light gets. It's similar to just using a voltmeter at the battery to see how much voltage is being delivered to the headlight.

Also, the other part of the electrical circuit is the ground. I've seen people spend countless hours and money trying to improve the voltage being delivered to the headlights, when they had a bad ground. You can test the ground too by just jumpering a wire from the headlight directly to the car's frame.

Naturally, to do all this, you have to know which pins are + voltage and which are - voltage. You don't want to create a short!

Just about any 12V relay with the proper current handling capacity will work as a headlight relay. I think you would want one that is sealed in a plastic case somehow. Two 100-watt bulbs will draw something on the order of 17 amps, so you need at least a 20-amp capacity relay. You use the existing electrical circuit that is switched by the headlight switch to throw the relay. The switched relay terminals have one side connected directly to the battery and on side to the headlights. (You may only need one relay for the high-beam circuit - or you may need a second relay for the low-beam circuit also if its really dim.)

A previous owner burned out the headlight wiring in my W124, possibly with high-wattage headlights. The dealer charged him a pretty penny to fix it. If I ever go with new high-wattage headlights, I'll be using a relay to protect the stock circuit.

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Old 07-04-2002, 12:35 PM
Don Atienza
Posts: n/a
Thanks as always!

I will try that test and if installing them relays wouldn't hurt the car, I'll do that too. Thanks again!
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