Indicator electrical problem 1987 300D - PeachParts Mercedes-Benz Forum

Ravenswift · #1 03-01-2019, 01:15 PM

I've got a 1987 300D W124 that I have just put back on the road, but my drivers side front turn signal isn't working and I can't track down the issue.

Here's what I know.

-new bulb
-running light works
-indicator does not (neither with hazard switch or turn signal)
-fuse is fine
-solid 12v at fuse, but nothing at plug going into lens
-no easily visible evidence of any wire being chewed through
-pretty sure I may have a couple mice that I haven't been able to get rid of yet

Where should I continue from here to try and diagnose the problem? Any suggestions would be greatly appreciated!!

Mxfrank · #2 03-01-2019, 01:35 PM

With a problem like this, the first thing to do is to see whether it’s power or ground. So first take out the bulb and see if there’s power at the socket.

Father Of Giants · #3 03-01-2019, 01:57 PM

You have an open on the power side, your ground is fine. An open ground will not cause you to have 0 volts at the lens.

You'll have to trace backwards to find the open. Wires fray and corrode internally even IF the insulation looks fine.

You have 2 options,
Hook a test light to ground , you'll stick it in the back of a piercing probe, use a piercing probe in conjunction with the test light to find the open.
Or you can just hook the piercing probe straight to the multimeter.
Downside is you'll have to dab liquid electrical tape on every spot you pierce.

Option 2 is to use an open circuit finder, it's easy as pie to use.
https://www.amazon.com/Power-Probe-PWP-PPECT3000-ECT3000-Electrical/dp/B0719JV971
https://www.youtube.com/watch?v=RrA6HtLyNmw
Downside is, it cost a pretty penny.
Yes, you may have to cannibalize the harness by opening it up to find the open.

Ravenswift · #4 03-01-2019, 02:52 PM

It's power not ground, I know that much. Dang I really wish I had one of those testers laying around! That's pretty handy. I wonder if i can make use of my datacom wire tracer in this scenario

Also considering just running a completely new wire for it and not even bothering to try and find the break/problem. Only issue with that is I don't know where the wire actually goes...im assuming it goes from the fuse box to the relay and then to the fixture itself? Not entirely sure which relay it is though (much less where it is...under steering wheel or behind fuse box is my assumption)

Father Of Giants · #5 03-01-2019, 03:17 PM

Here's a video on the DO'S and DON'TS of bypass testing.
https://www.youtube.com/watch?v=-4oCAelzrWM

You can feed 12v to the POWER side of the light bulb with a fused jumper wire connected to Battery positive or a multimeter set to Amperage connected to Battery positive.

Diseasel300 · #6 03-01-2019, 08:25 PM

Do you have the yellow "bulb out" idiot light on the dash? It could be the bulb monitoring module that's bad. They're known to develop cracks in solder joints and lead to intermittent or no lights.

jay_bob · #7 03-01-2019, 08:55 PM

Good suggestion except they ran the turn signals directly from the flasher/switch to the bulb. They just tapped a sensing line from the flasher to the relay to monitor the flash rate on the line. These are the only external bulbs on the car that do this. Every other exterior bulb has a series connection through a sensing element in the bulb monitor relay.

If one of the turn signal bulbs went out the fast flash function (current monitoring) on the flasher relay made the remaining bulb on that side flash fast. The monitor picked up on this fast flashing and triggered the bulb warning indicator.

dieselmania · #8 03-02-2019, 07:20 PM

Quote:

Originally Posted by Diseasel300

Do you have the yellow "bulb out" idiot light on the dash? It could be the bulb monitoring module that's bad. They're known to develop cracks in solder joints and lead to intermittent or no lights.

This is what fixed all my lighting issues on my 87, I re-soldered all the joints and everything started working again. Easy fix

dlssmith · #9 03-10-2019, 12:37 PM

Quote:

Originally Posted by dieselmania

This is what fixed all my lighting issues on my 87, I re-soldered all the joints and everything started working again. Easy fix

Where is the bulb monitoring module? I may need to look there in my 87. Thanks

dieselmania · #10 03-10-2019, 12:50 PM

Quote:

Originally Posted by dlssmith

Where is the bulb monitoring module? I may need to look there in my 87. Thanks

It's in the fuse/relay box http://www.w124performance.com/service/w124CD1/Program/ETM/relay_control.pdf

Ravenswift · #11 03-17-2019, 04:54 PM

Sorry for the late response.

I checked the combo relay and didn't see any corrosion, cracks, or obvious damage of any sorts, so I'm back to tracing the wire...

@jay_bob did you say the wires go directly from the turn signal switch to the bulb? or does it meet up with the relay first? At this point I think i am just going to run a new wire, but i need to know where it goes first.