My 300SD won't start; no power to anything, lights, starter, dash
lights. I briefly saw the dash lights blink, then heard a relay (?)
click and then nothing. Same with the headlights. So, I pulled the
battery out and got it load tested at the local auto parts store. It
had over 14 volts DC with no load and even with a 200 amp load it still
put out 11 VDC. The parts guy noticed their battery load tester getting
warm and smelling like burning electronics, so the battery is good.
That saved me from replacing a good battery but now I'm baffled .... Are
there any common problem areas with this model (126)? Does anything
very early in the car's wiring (past the battery) typically come loose
or corrodes? I tightened the negative battery cable but that didn't
help.

Any advice appreciated,
with thanks,

Robert W. Roe
Bethlehem, Pennsylvania, USA

Sounds like the battery (+) supply from the junction block to the ignition switch must be loose or broken. There are several RED wires (about 1/8" dia.) leading from the junction.

Thanks for the advice. Today with good daylight I used a wire brush to clean off corrosion from the negative battery cable where it
contacts the body as well as the positive wires leading from the junction block. It started right up. Also now I can see the direction
indicators on the radio when the tape is playing. Thanks again!

Now I wonder what is making my dome light and interior / trunk lighting blow the fuse. Any common trouble areas I can check? If not, anyone know where all that wiring runs, to the dome light, trunk light and rear reading lights? Do I have to remove the headliner or can I pull new wiring using the existing wiring as a fish?

Recently my SD has begun not to charge the battery once the engine is running. Are there any electrical contacts I can check before taking the car to the shop? Up until last week I was getting 14-15 volts at the cigarette lighter as soon as the engine started. Now I don't see hardly any change when I shut off the car; in fact the voltage increases by .2 volts with the key in position "1". I do usually see the battery light come on when I turn the key on, but I didn't see it last night when the engine was warm, in position "2". I've been charging the battery to keep it going but I know this isn't a long term solution. Any common trouble areas to check for... alternator connections, etc.? TIA,

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Robert W. Roe
1983 300SD 264K mi

Located on the rear of the alternator is an external voltage regulator. The power has to run through it to turn on or off the light for the batt. The regulator has brushes on it that do wear out. Replace it and you will more than likely solve your problem.

As for the fuse, Check to see if any of the door entry lights are contacting the inside of the door frame. I have seen many>

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Check the left trunk hinge area for broken wires that are possibly shorting the interior/trunk light circuit.

Voltage regulator is fairly easy to get to and is a common culprit. one other cause I have seen is a sticking glow relay. When these stick, the amperage draw exceeds the alternator output. The glow relay is mounted on the driver's side of the engine compartment and is black with a removable top. Check the voltage regulator first. Good Luck!

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Mercedes Star Technician,
Mercedes Master Guild Technician, ASE Master Tech. 18 years exp.

Thanks for all the replies so far. I checked my local MB dealer parts dept for these parts and was pretty surprised that the voltage regulator (pn 0021547206) was $77 and the "preglow time relay switch" (pn 0015459832) is $165 (!) Both were out of stock, so I figured I'd ask if these prices are right for OEM parts, and if any of these can be repaired cheaply; I recall someone replacing a couple capacitors in some part (I don't recall specifics) and saving big $$$'s.
Also, how can I check which part is bad? I charged my battery two days ago, but now I have less than 10 VDC while driving... and a bit more blue smoke while idling than I remember seeing. Also, revving the engine doesn't bring my voltage up at all. Could they both be bad? Could one bad part make the other part fail? Should I avoid driving my car until I resolve this? Strange observation: my tach needle , which hasn't worked in months (years?) began to jump up randomly today...

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Robert W. Roe
1983 300SD 264K mi

You can remove the regulator (2 screws) from the back of the alternator and take a look at the brushes. If they look worn low, replace the regulator. If not you would probably be better off taking it to a honest professional. Hope you find the problem.

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Mercedes Star Technician,
Mercedes Master Guild Technician, ASE Master Tech. 18 years exp.

I looked at the alternator today and have a question: is the regulator a small rectangular black plastic piece or the whole back half of the alternator? Can I pull the glow relay if it's stuck on? Thanks in advance.

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Robert W. Roe
1983 300SD 264K mi

Thanks to all who've helped me understand my car so much better. Now I am having the same exact problem with my 1984 SD. Wondering if it's a jinx with me and S-class electricals; anyway...

I've had the car four weeks. Two weeks ago, while vacuuming the car, I found the trunk lid light bulb and reinserted it into its socket. I'm happy because now everything *works" (except for my power antenna only going up a foot or so; my car is in pretty good shape for an '84.)

About one week ago Friday, my battery voltage died, below 10 volts. Recharge the battery, dies Monday. Replace the battery Tuesday (with a red top Optima 800U), now on Saturday it too is dead (below 10 volts.) In effect, I replaced a good battery; I charged my old one so now I have a spare/jumper battery. Anyway, what I'm trying to figure out is where to begin looking for an obvious problem before I turn it over to a mechanic.

Since replacing the battery, I've had a little DC voltmeter on the console, plugged into the lighter socket. I've had over 12 V all week while driving, sometimes going as high as 13. Revving the engine made it jump .3 to .5 volts. Even with the glow plugs preheating, I still had 11 volts. All week long I'm over 12 volts.
This morning, dead battery. I notice the amount of time for the fully charged battery to die was about 5-7 days. The first charge was only a two hour quick charge, and it died in 3 days.

Previous replies suggest that I check the following:

1. preglow relay (where is this thing?)
can it stick "on" and will it get hot if it's stuck?

2. Benzmac suggested to check to see if any of the door entry lights are contacting the inside of the door frame. Do I need to pull the door panels (ack!) or do they pry out from below? Can I spot problems from below?

3. Perhaps the trunk lid bulb is staying on. I haven't looked at this yet. Is there a mercury switch inside the trunk lid? Thinking maybe it moved a bit and is staying on or got pushed enough to short something out.

4. Possibility of a short (worn insulation?) on a wire near the left trunk hinge.

If I can't find an obvious problem I can foresee checking the ignition-off current draw from a (charged) battery, then if it's too high (over what, maybe .25 to .5 amperes at the *most*?) I'll debate whether to quit at this point and turn it over to a pro or pull every fuse and check each branch. (My last SD would blow the fuse for the dome light circuit and I never did get to troubleshoot that.)

I'm hoping it's a gradual depletion rather than an intermittent (yikes). Is the 126 notorious for electrical problems like these? Any thoughts? Thanks to any and all.

Robert W. Roe
1984 300SD 165,333 miles


[This message has been edited by Robert W. Roe (edited 08-29-1999).]

I think I've found my problem. I checked the trunk lid light bulb, and it was quite hot. Odd, I thought, for a 10 volt 5 second illumination, so I checked the little plunger switch that is supposed to turn off the lamp and it stayed on even when pushing on it. I surmise that this lamp has been on for the last two weeks +.

There appear to be three wires connected to it. Two appear to be "piggybacked" on the left side, another single wire is on the right. Has anyone ever taken one of these apart?

I'm quite happy to have found this simple problem. This may be my first order from PartsShop :)

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Robert W. Roe
1984 300SD 165,333 miles

1984 was the first year for alarm systems & that other wire(on the trunk lamp) feeds the alarm system. Sometmes the wires around the left trunk spring can be damaged by rubbing against the spring.

glad you fixed it, just a final note on your question of checking the preglow relay for staying on after starting up. Yes you can unplug it (the smaller plug) after you start up the engine. I have seen this just a couple of times. It is more common for the relay not to work at and usually it is just the flat fuse that is blown. But what you found is definitely more common! Checking for a battery draw (drain) is the normal next step after checking the alternator and the battery....

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