W116 right front turn signal electrical gremlin

[Alec300SD]

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.

[Alec300SD]

A machine screw holds the white plastic end of the harness to the headlamp assembly.
The screws holding the headlamp assembly to the body must be removed to allow removal of the turn signal harness.
Since I had access to the headlight connections, I also cleaned all the connections with CRC contact cleaner and conditioned them with DeOxit Gold.
I also replaced the broken plastic connector for the left front turn signal plug.
On the right side, I replaced the newly broken cover with one I scavenged from my 83 W126 300SD.


A secondary job was to replace the washer motor and its broken keyed plastic connector plug.
One of the plastic nipples on the pump broke off when I dropped the alternator on it during the AL129X alternator upgrade.
(See the last sentence on post #1 of that thread).
The first Pick-n-Pull replacement motor was not tested and of course was seized up.
The second Pick-n-Pull replacement motor was tested to verify it worked prior to purchase.
I used a drill motor battery to power it up and a water bottle as the reservoir.

[Alec300SD]

After buttoning everything up, I tested the wipers could now use the windshield washer squirters again.
No front right turn signal though.

I searched for the front grounding point and found it just behind the left headlight, close to the left horn and below the AC lines.
I removed the sheetmetal grounding screw and cleaned and conditioned the terminal rings.

This simple cleaning of the ground connection restored the right front turn signal function back to normal.
I tested the supposedly ‘bad’ harness I had just replaced and it worked just fine.
Now I have a good spare.

The moral of this story…if you have weird electrical symptoms, check your grounds.