Electrical mystery - PeachParts Mercedes-Benz Forum

Jim H · #1 09-18-2004, 06:03 PM

I had the following electrical mystery occur last week.

86 300SDL, 241,000 miles.

As I'm driving back from the job site, I notice that the overhead courtesy lamp and the underdoor lamps are on. Well, glowing, not completely on. The courtesy switch is in Auto.

I discover that when I open my door or the passenger door the lamps go off, and they come back on dim when I close the door! now that's just not right!

Playing with the courtesy lamp switch reveals:

Auto - lamps are dim, go off when doors are opened.
Reading - light comes on full bright, door lamps go off.
Off - turns all lamps off.
Courtesy and Reading - lamps come on and the doors go off.

Switching between settings operates the lamps, and curiously, the Fasten Seat Belt warning in the overhead energizes and flashes, as if a door had been opened, but without opening a door.

I give up, and pull up to my favorite Chinese restaurant for dinner.

In the parking lot, I open the doors, remove the switches, find nothing amiss that I can see. No apparent bare wires, broken wires, etc.

Then I wonder if someone messed up the wiring in the front passenger door when the power window motor was replaced. I discover that one of the hooks on the door panel hasn't been engaged, but no wiring problem that I can find.

I discover that the lamps go off when I turn off the key. Good, I don't ahve to worry about running the battery down overnight.

Then I find that the driver's lock operates but no others, since apparently the central locking system has now quit.

More bummer!!!

I call my wife for sympathy, since I spend several days a week away from home in the line of work. Dinner was consumed in a bit of a foul mood, as might be expected, and didn't quite taste as good.

During dinner I wonder how these could be related. I decide to check the fuses, figuring that a blown or loose fuse could easily explain the non-functional locks. After dinner I open the fuse box lid, look and see no blown elements. For grins, and because I can, I touch and rotate each and every fuse one in its holder. Nothing wrong that I can see.

Son Of A Gun!

The central locking is restored, and the courtesy and door light function returns to normal. I'm extremely happy, relieved, and puzzled at the same time.

So, I'm now wondering how the apparent loss of power to the central lock motor caused a backfeed, or a sneak ground, on the lamps. I guess I'll have to research the wiring diagrams.

Oh by the way, my fortune cookie said "Your tide of fortune will be turning" and it did, so I now have to start believing in fortune cookies, too! :p

I love a good puzzle, and I hope you all do too!

Best regards,
Jim

P.S. I'm an electrical engineer by profession, which makes it even more interesting...

whunter · #2 09-18-2004, 06:31 PM

Hello Jim H
I would suspect the switch and the door wiring in the hinge flex point.
Have a great day.

Jim H · #3 09-18-2004, 06:50 PM

whunter, thanks for the reply.

I forgot to include that the central locking did not work at either door or the trunk.

While nothing was apparently wrong at the switch when I removed it, I won't say it isn't bad.

I don't want to mess with the wiring cover at the hinge just yet, maybe when it happens again and doesn't repair itself?

Perhaps I will be able to stop by your place of business when you're around and meet you in person.

Best regards,
Jim

pberku · #4 09-18-2004, 07:31 PM

Last week, one of my headlights stopped working. Checked the bulb and it was good. Checked the fuse for that headlight (with an ohmmeter), and it was good. Gave the fuse a little turn while in place, and the headlight came on.

The morale is that the fuse can be good, but its socket may be slightly corroded. I cleaned the fuse contacts, as well as its socket in the fuse box and so far so good.

In your particular case, with a corded fuse contact, the current may have found an alternate path, causing the symptoms that you noticed.

Try to duplicate the problem by removing one fuse at a time.

Phil

phantoms · #5 09-18-2004, 09:21 PM

And when you remove them, look at the ends of them. Sometimes the fuses can look okay, but the ends can be corroded or worse yet, worn off. I've run across a fuse that had a hole worn in the contact area on the end of it.

BrierS · #6 09-18-2004, 09:57 PM

JimH
It was great reading about your electrical plight. Having read many postings involving the lack of good contact with these fuses I started cleaning, rotating and even using dielectric grease on the ends. Finally I replaced all of the fuses and still give them a twist now and then. It has become part of my maintenance routine . . . no recent gremlins.