W116 right front turn signal electrical gremlin - PeachParts Mercedes-Benz Forum

Alec300SD · #1 02-14-2016, 02:02 PM

My 78 W116 300SD developed a strange behavior where the right front turn signal would go out every couple of days.
All the fuses were fine. Removing and reinstalling the bulb would always allow it to work normally…for a few days.

When it went out, the running light filament would flash when the lights were on.
So I could get a feeble right turn signal with the lights on.
The main filament would not flash with the lights on or off.
The main filament would flash when the hazard button was activated whether or not the lights were on or off…strange behavior indeed.

I thought there was a poor connection in the socket as the rear plate of the socket was askew and barely crimped in place.
It looked like the spring (between the rear plate and the movable terminal plate) wasn’t pushing the terminal plate evenly.
The bulb contact terminals on the plate had mushroomed and one terminal was shiny and the other was dull.
I put two more crimps in the bulb holder body to better stake the rear plate in its proper place.
The same strange behavior persisted.

I tried a German bulb with the wider cat-eye style contacts.
No change in the right front turn signal behavior…at all.
It seemed to be time to replace the harness.

Pick-n-Pull yielded a good used left front turn signal harness.
The same part is used on both sides.
I also got both parts of the plastic connector for the left front turn signal as the snap-on cover was missing and the wires were held in the cracked base with clear silicone caulk.

In the engine bay, the old plastic in these cars dries out and becomes brittle.
The snap-on cover of the connector for the right turn signal broke off went flying off when I tried to unplug the connector.
The broken cover ended up adjacent to the washer motor.

Accessing the right front turn signal wiring harness requires removal of the battery and windshield washer reservoir to remove the black metal access cover.