Bulbs are good, dash 'bulb out' lamp is on. Checked fuses... all good.

Does the combination switch handle all of the headlamp current or is there a relay somewhere?

They mystery module does not contain anything that would handle the hi beam current that I can see. Just a couple of ICs.

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1992/300SE/SYSTEM%20WIRING%20DIAGRAMS/4219.pdf

I'm not too keen on replacing the comb sw.

I'm an analog refugee lost in this age of digital spew.

I see in the module some resistive metal strips. Perhaps the module looks at the voltage drop across these to indicate a failed bulb. Theres no low beams with the module removed so current must pass thru this device.

The MB CAN(not)...

First thing to do is measure the voltage at fuses 13 and 14 (both ends) with the headlights on and combination switch on high beam.
Questions: does the high beam indicator work? Does the 'flash to pass' work with the ignition switch on?
There are NO relays associated with the headlight basic function. I assume that the mystery module you are talking about is the lamp failure module in the fuse block. You are correct, the module is looking for a voltage drop across the resistive strips in the module. I have seen a lot of problems in this type of system. As in analog circuits, you have to trace the voltage to find where it stops. This is basically an analog circuit with only 0V and 12V. I recommend every one wanting to fix this type of problem get a 12v test light. It is easy to use and read. Hook it to ground and probe a circuit that you know works. In this case, turn on the low beams and check the fuses for the low beams. If the light doesn't work, check it at the battery to make sure it works. If everything is working, now turn on the high beams and check the fuses for the high beams. Let us know what you find and we can go from there.

Paul

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84 500 SEL (307,xxx miles)

Paul, Using a voltmeter I have nothing at the hi beam fuses, either side. Hi beam indicator on the dash does not come on and the operation of pulling back the combination stalk does nothing. Bulbs are good, I tested them with my bench power supply.
I fear it's the combination switch which has failed. Not looking forward to that job.
I am working on this car for an elder widow friend. She has been taken advantage of over the last couple of years by shady mechanics. I've had the car for over two weeks and have repaired numerous issues. Headlamps are the final job before I return it to her. I have been trying to encourage her to get rid of it as it has become a money pit. Although, she loves the car so I'm keeping it going best I can.

It does look like the combination switch is bad. If you could get to the connector X1 for the combination switch you could jumper from X1-1 and X1-2 pins and see if the High beams come on. If they do, the switch is bad.

Paul

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84 500 SEL (307,xxx miles)

Where is X1 located?

I can't believe MB relied on the switch to carry the headlamp current considering the complexities of the W140. How much would another relay have cost them???

I'm sorry, I don't know where X1 is in a 140. Looking at a picture of the switch it looks like it has a cable and plug that would reach behind the dash somewhere.
As far as using a switch or a relay, they are the same thing except one is intergized manually and the other electrically. The actual contacts that close are the same. If the switch can be designed for the amperage needed then only one thing in needed to turn on the headlights (in this case, switch low to high). It becomes a cost factor.

Paul

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84 500 SEL (307,xxx miles)