I have a battery drain that kills the battery almost overnight.
I have isolated it to the large relay in the fuse box (left rear)
Relay 125 542 01 32
I can unplug the relay( leaving the harness plugs attached on both ends) and the drain goes away.
Does anyone know what this relay controls? I don't have a wiring diagram.

Thanks and Merry Christmas!

ill look it up for you

did you do a amp draw test on your battery?

It looks the largest relay you have controls these items-turn signal,heated rear window,wiper motor,time relay heated window

one of those devices is activating the relay when its not supposed. I would disconnect the items listed one by one until the drain goes away

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

With an amp meter installed between the battery's terminal and the battery cable remove fuses one at a time until the load goes to 0. You can use a test light too: The light should not light much, if at all if you put it between B+ and the battery cable. But if there is a draw it will glow. Just lift the fuse contacts until the light goes out.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

I confirmed the drain with a DC meter in series with the neg. post and battery lead. The drain was .015A with the relay pluged in and 0 with it unpluged and it threw a good arc when the lead terminal was brought to the battery post.
Tomorrow, I will futher investigate the systems that this relay feeds.

Thanks for the replies!

the test light method does really do anything for a parastic battery draw. The only and real test is an ammeter hooked in series between the postive terimnal and the postive cable and read the read out. A normal reading should be 30 miliamps for the SDL.

Pulling the fuses one by one works good. When you pull the fuse that makes the amp draw go back down to 30 Miliamps then you found the problematic circuit then its a matter of hunting down whats causing the drain.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

If its killing the battery overnight i'd think the light would glow. Worked great when I was troubleshooting a W123 with a failed power antenna But if the light doesn't glow when you connect it up then you know it won't work to troubleshoot that particular draw.

-Jason

All the light tells you if something is getting electricty or not. A device could be acting normal and something else could draining the battery but with the test light you wont be able to different between the 2.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

If you put the test light between the battery and the battery cable, like an amp meter, it will light up when there is a moderate current draw. Remove the draw and the light goes out.

true but it doesnt tell how much its drawing which is why the dealership does the ammeter test and not the test light test. Some devices on cars are drawing a small current on the battery such as the clock, radio and the test light wont tell you the difference between these and the actually circuit(s) draining the battery. You will be wasting time chasing down circuits you dont have to chase

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

You don't care exactly how much is being drawn. There is probably one system that is stuck "on", making a high draw, and will make the test light illuminate. Simply going down the fuses till the light goes out is easier then looking at a meter IMHO. If something is drawing enough power to kill the battery overnight it's going to make the test light illuminate.

The clock, radio memory, etc won't make the light illuminate once they are over their inrush draw.

Can you trace the draw to a fuse? Might help you isolate it further.

Sixto
87 300D

Actually you do care exactly how much is being drawn. Mercedes specifically states...

central locking system pump- max 15ma
Dome lamp- max 10ma
electronic radio(becker)-max 20ma
safety belt extender system- max 5ma
Antitheft system-max 10ma
Antenna-max 2ma
Electronic time clock-max 3ma
electronic seat adjustment(perseat)-max 1ma

Your mentality of not caring how much each draws suits you well. Dont work @ dealership or on other peoples car's.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Nevermind then

I love numbers and data as much as any other German car enthusiast. I prefer to hear them coming from one's mouth as opposed to other orfices. It is for this reason i intorduce to you a textual state diagram in the form of a few simple questions and statements.

Statement #1: The the normal draw as specified by Mercedes is not enough to light a test light.

Statement #2: The normal draw as specified by Mercedes is nowhere near enough to drain a battery overnight.

Problem: Battery draining overnight.

Logic step #1: Because of statement #2 we know that there must be more then the normal draw on the battery.

Test #1: Grab test lightplace between one of the battery leads and the car. If it does not light go to Logic step #3, otherwise proceed onto logic step #2.

Logic step #2: Your light has lit, this means that based on Statement #1 there must be a draw more then the normal draw as specified by Mercedes that is also enough to light the test light. Continue to action step #1!

Logic step #3: We know from statement #2 that there must be a draw but it is not enough ato light the test light. Get your amp meter, and go to Action step #1.

Action step #1: Remove fuses till the light goes out or the amp meter drops.


If you have gotten thsi far you now understand that a test light, while not a replacement for a good amp meter is an extremely convenient way to diagnose electrical draw!

had a huge drain on my 85SD like this. Found the rear seat lamp housing shorted, and only when the switch for the rear sea was on.

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83 300SD Dark Silver Dark brown inside