My driver's side corner light is having issues. The turn signal filament of the bulb works fine (can signal turns), but the running light filament works only intermittently (it's not burnt out). I tried sanding the contact points, and got the whole bulb working, but an hour later I checked and it had gone off again -which leads me to believe it's temporary function was a coincidence.

Is there another point I might try cleaning?

I've read that the hazard light switch in the shifter console controls the turn signals but does it also control the running corner lights or there a separate relay?

Did you try tapping on the housing? I swear, once you get light bulbs working nicely in an old MB you should never touch them again.

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'84 190D 2.2 5MT (Red/Palomino) Current car. Love it!
'85 190D 2.2 Auto *Cali* (Blue/Blue) *sold*
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I seem to have isolated the problem. Oddly enough, it appears to be the lightbulb itself.
Tried the other (passenger side) socket and bulb and then just bulb alone on the driver's side and everything worked fine.

Both filaments appear to to be intact, so I'm not sure what the issue is... I guess these bulbs get old?

Maybe the filament has a hairline break.

The bulb/age problem I see most often with these cars is that the bulbs get dark, like really dark. The glass turns black and dims the bulbs.

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'84 190D 2.2 5MT (Red/Palomino) Current car. Love it!
'85 190D 2.2 Auto *Cali* (Blue/Blue) *sold*
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If I remember correctly, under the hood on a W123 directly behind the front marker lights/turn signals there is a black plug - in wire connector for these lights. You might pull apart these connectors and clean the connections.

Clean and condition the ground wires located on the right hand body behind the headlight (according to the FSM).

Flaky, intermittent electrical issues are usually due to bad grounds.

I had a similar issue resolved by cleaning and conditioning the turn signal ground point. See post 3 on this thread:

http://www.peachparts.com/shopforum/diesel-discussion/375603-w116-right-front-turn-signal-electrical-gremlin.html

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Mannys,

The main reason that older bulbs become dark, is that a small portion of the tungsten in the filaments boils out over time and plates out on the inside of the cooler glass wall. I suspect that this is partially responsible for the filaments becoming brittle with age. All due to the good old 2nd law of thermodynamics, which basically states that everything tends to seek a 'lower state' ... ie goes to heck !!!

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150

That's a great point, I'll have another look in the morning. I didn't see any lines that looked like a "ground" though. All of the wires seem to be contained within the various wiring harnesses.

In any event, the driver bulb wouldn't work on the passenger side either, while the passenger bulb worked perfectly on the driver side. That seems to suggest a bulb issue, no?

Initially, you reported an the drivers's side running light was only intermittently operating, which is consistent with a poor electrical connection (including a failing bulb filament).

Test the suspect bulb with a direct connection to the battery. Connect a negative test lead from the base of the bulb to the negative post of the battery. Then connect a positive test lead from the positive battery post to one of the two positive filament connectors on the bottom of the bulb. If the filament is good it should light up. Next test the other positive filament connection on the bottom of the bulb. If the low wattage filament no longer lights up any more, then yes I agree it is a bulb issue and time for a new bulb

Still, it's not a bad idea to periodically clean and condition the electrical connections once a year or so.
I got a little more info from the FSM.

If you have AC and/or a right hand horn, follow the brown ground wire(s) to where they attach to the body. This will also be the ground point (G101) for the RH front turn signal, RH front parking light, RH headlamp, and RH brake wear sensor.

The LH front parking light ground point (G102) is located behind the center of the instrument cluster (with about 15 or so other ground wires).

Turn off your radio (to protect its microprocessor) before you disconnect the negative battery cable just prior to cleaning and conditioning the grounds with your favorite products (I like CRC QD electronic cleaner, DeoxIT cleaner, and DeoxIT Gold conditioner).

If the bulb is good and the problem persists after the grounds are clean, then the likely suspects are fuse 3 and/or the rotary headlight switch itself.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Ground wires on Mercedes are brown in color.

Thanks to everyone for the responses. I'm going to have another stab at cleaning the contact points before tracking down a new bulb (scientific method points to bulb, still I'd like to be thorough).

Alec300Sd -"G102 is behind the instrument cluster," meaning I'll need to remove the instrument cluster? Or behind it so to speak, meaning closest point to the instrument cluster under the hood?

Yes, you need to pull the intrument cluster to access the (G102) ground location.

FYI the online FSM for your car is here: Mercedes-Benz Model 123 - Disc 2

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Before you pull the instrument cluster you might want to use a jumper wire from the light bulb socket to ground. If your light then works ok then the problem is a ground. Pulling the instrument panel can be a little tricky without the proper puller hooks. If you do pull the panel make sure and mark the wires that you remove from the back of the panel. Another tip is when replacing the panel the speedo cable can be a bit tough to screw on to the back of the speedometer because of the length of the speedo cable (not much slack). Open the hood and near where the left back of the engine and transmission join together you will see the black speedo cable. It will be held in plastic by a plastic clip. Located near the front of the transmission. Remove the plastic clip from the small metal bracket and this will give you more slack in the speedo cable. Once everything is re-installed and is working properly re-install the plastic clip for the cable.

Well, turned out it was just the bulb after all. I tend to forget occam's razor when it comes to these cars. For good measure though, I decided to clean the ground point behind the instrument cluster. Wasn't actually too difficult to remove after loosening the speedo cable from within the engine bay, and, it was a little corroded.

Bwhitmore -great advice
Alec300SD- cheers for the link, super useful

Glad it was the bulb.

Lycoming, that makes perfect sense. Thank you.

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'84 190D 2.2 5MT (Red/Palomino) Current car. Love it!
'85 190D 2.2 Auto *Cali* (Blue/Blue) *sold*
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