87 S124 exterior lamp failure mod bypass?

[ObsoElitist]

I have an 1987 124 wagon. The only symptoms are the stock USA left leadlamp unit works wrong. Turn on parking lights, the left headlight and fog turn on. 3 of 4 wires from the harness are hot with switch in parking position. Turn on the left turn signals with, headlight off; the headlamp flashes. headlamp on; no left turn signal. Mice HAD eaten the insulation of some wires in the left front corner, but there were no signs of shorting when I replaced them. After studying the schematic (and not having a schematic of the module itself) I believe that the headlamp current flows through the module, rather than just getting signal from taps. Is that correct? With late 70's Honda Goldwings the modules always malfunctioned and a series of jumpers allowed operation with module removed, relying on the operator to check the lamps occasionally, like good drivers do anyway.
Have any of you bypassed the module, or do you just buy new parts and hope it wont happen again?

[jay_bob]

Yes the lamp failure module is a series connection to each external bulb. It actually measures the current the bulb is pulling.

There are some cases where there is 1 input and multiple outputs.

Note that the left and right parking lights are separately fused, and there is an independent fuse for each high and low beam filament on each side for the headlights.

I had a bad connection under the failure relay that was killing my right front marker light.

Note that there is a nice clean copy of the wiring diagram at www.w124-zone.com, better than the one on the MB site.

If you sit down with the diagram you should be able to reverse engineer what is going on inside the lamp monitor. It is pretty straightforward, the only quirky circuit is the turn signals. That one does have a tap from the flasher relay, in addition to the bulbs.

I would look at the diagram and figure out the paths for the affected lights. Then pull the relay out of the socket and jump the appropriate pins for each affected bulb and see if two things light up at the same time. That would narrow down if it's wiring or the relay.

But first I would confirm the ground in the front left corner. You can have all kinds of screwy action with a bad common ground. Current can go through 2 or 3 bulb filaments before finding another ground.