w126\electical\gremlin - PeachParts Mercedes-Benz Forum

TX76513 · #1 01-15-2007, 08:42 AM

The 300SD has been put up for several months but this weekends high 70's prompted me to take her for a spin. The car was put up with no mechanical issues, serviced, washed, battery disconnected and battery maintainer attached and car covered and stored in garage.

When the battery was reconnected the #18 idiot light (battery) illuminated (with key out of ignition).

Car started up promptly but the headlights and taillights all came on (headlight switch in off position) -then #18 idiot light goes out

I didn't have time to do a lot of nosing around but:
Battery condition is good
Alternator charge is good.
V Belts at correct tension.

The engine bay is immaculate and free of oil and dirt, everything appeared to be as left in October.

:fork_off: My only suspect right now is the headlight switch.

Has anyone experienced this anomaly

Brian Carlton · #2 01-15-2007, 09:03 AM

If the all of the following illuminate:

1) headlights
2) front marker lights
3) taillights

The only possible source for all those items is the main headlight switch. But, those symptoms don't seem like a switch failure as a result of lying dormant for a few months.

Best thing is to disconnect the switch........see if all the lights will extinguish. I don't immediately see any other possibility.

TX76513 · #3 01-15-2007, 09:21 AM

That was my thought process also - with the exception of the idiot light. Going to look over the schematic later as see if there is any relation between the two. Looks like those switches are hardwired instead of a coupler - odd.

R Leo · #4 01-15-2007, 09:29 AM

Quote:

Originally Posted by TX76513

That was my thought process also - with the exception of the idiot light. Going to look over the schematic later as see if there is any relation between the two. Looks like those switches are hardwired instead of a coupler - odd.

That's correct, the headlight switch wiring is connected with terminal ends and screws, not a plug-type connector. Very difficult to disconnect for a simple test.

Is the symptom still present? If so, I'd exercise the switch through a few rotations and see if the problem still exists. If it persists after that, I believe I'd be looking for rodent damage in the wiring or corrosion at a ground terminal connection.

TX76513 · #5 01-15-2007, 09:31 AM

Quote:

Originally Posted by R Leo

That's correct, the headlight switch wiring is connected with terminal ends and screws, not a plug-type connector. Very difficult to disconnect for a simple test.

Is the symptom still present? If so, I'd exercise the switch through a few rotations and see if the problem still exists. If it persists after that, I believe I'd be looking for rodent damage in the wiring or corrosion at a ground terminal connection.

I drove it around thinking maybe some bouncing around might undo the problem, but it didn't

I also went through some hard rotations, pull fogs, etc. I did notice the switch positions are not as "crisp" as the SDL for what ever that may be worth.

No rodent damage the SD lives with boat and that area is tight as a drum and temp controled.

rrgrassi · #6 01-15-2007, 12:28 PM

What about insects? Ants like to wreak havoc as well.

TX76513 · #7 01-15-2007, 01:25 PM

Quote:

Originally Posted by rrgrassi

What about insects? Ants like to wreak havoc as well.

I hope not - we do have the bug guy come through once a month, but learned some time ago never to say never

SD Blue · #8 01-15-2007, 01:46 PM

Hmmm........the last time I saw the batt. light on without the key was with a failed alternator diode. You might check to see if there is any mild corrosion in such places as .....eng-chassis ground strap or alternator-engine block mating surfaces.

TX76513 · #9 01-15-2007, 02:02 PM

Quote:

Originally Posted by SD Blue

Hmmm........the last time I saw the batt. light on without the key was with a failed alternator diode. You might check to see if there is any mild corrosion in such places as .....eng-chassis ground strap or alternator-engine block mating surfaces.

I will look around the ground straps - diode is new Bosch. Looking at schematic it looks like the battery light gets it test feed from the 30A red wire that feeds the rotary light switch. Since the rotary switch is always hot, (speculating

) if it failed with a closed circuit if possibly could feed backwards.
Looks like fun to get that switch out