I have an '87 300 TD wagon that has a short or break in the circuit to the passenger (right) side low beam headlight. The high beam works fine, and I have checked the fuse box and the fuses are okay for all the headlights low and high beams.

I am going to try tracking down any break in the circuit for this low beam light, but in looking at the wiring diagram there is a "light control unit" between the lights and the fusebox. Is this just a relay - or something else? And where is it located?

I think the diagram I have has enough info I could track if the break is between the lighting control unit and the fusebox or lighting control unit and headlight. I just have to find everything.

Any other advice on tracking this down appreciated as well.

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Jimmy M.

87 300TD 230k mi's

Yep, the fusebox is your friend when tracking down wiring issues. The low and highs are on different fuses as well as left and right. Usually, there are companion circuits on each headlight fuse so... if you notice the companion circuits are also having issues then the fuse could be bad or the whole circuit protected by the fuse is suspect.

Does the fuse to that circuit burn out when you activate it? In other words, activate the device that sits on the same circuit with the faulty headlight. Does it work? If yes then the fuse is ok. You can then check the wire that goes from the fusebox to the headlight. I'd start at the headlight itself and definately rule out a bad headlight by swapping to the other side. Then checking the connector that plugs into the headlight and working my way back.

Get a cheap multimeter and have fun tracking it down. I did. It's a great feeling when all the little lights and gizmos are working right.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

Codifex Maximus - Thanks for the response -

I had noticed before how they grouped varying things to fuses so it would be hard not to notice something else not working when one thing went out - indicating a blown fuse.

In this case I have checked and rechecked fuses - and no it doesn't burn out - so I am thinking there is a break somewhere. I have had an rpm sensor either get knicked by a rock or chewed on before. So I know breaks in the wiring happens.

With the headlamp - the inconvenience is driving somewhere at night and getting pulled over and getting a warning citation because the light is out.

You don't happen to know if the "light control unit" is an extra fusebox or a relay do you? I have a relay listing document, showing location on the car - but I didn't see it on there. I looked up "light control unit" at an online parts store and it came back with a light control relay - a 34 pin plug-in unit (17 pins a side). It somewhat matches up to the wiring diagram I have which has 13 connections a side - just extra pins - but the breakdown of pins is very similar to the wiring diagram.

I will start tracking down the break from the headlamp unit back to the fuse.

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Jimmy M.

87 300TD 230k mi's

The illumination module is in the aft section of the fusebox. It's the longest module in there at about 4 times the length of a cube relay. AFAIK all it does is monitor exterior bulbs and trigger the illumination warning light in the dash. Can't do without it, though. The one in my 87 SDL was flaky until I had a friend reflow the solder joints. My 87 300D has the same problem as you're experiencing with a dead passenger side low beam. As an aside, I bumped the headlight lens to see if it was a bad connection. My arm is little more than a chopstick and I used the soft bottom of my fist yet I managed to crack the cheap @$$ TYC lens. Argh!

Sixto
87 300D

Thanks Sixto!

So the circuit for the headlight goes from the fuse to the relay module and then to the light socket?

I have the perfect time to check this out - took out the turbo last night to send it off to get rebuilt so plenty of room to check out that headlight and if necessary rerun some wiring.

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Jimmy M.

87 300TD 230k mi's