I have a 1991 400E, and it has developed a problem with the battery being drained overnight.

It is sitting outside with a freshly charged battery. The battery is only a couple of months old, replaced when I put in a new alternator. I diconnected the negative cable, and I am measuring 4.25 amps draw with every single fuse pulled. I disconnected the alternator too, and the draw didn't change.

There is nothing still on in the car... No lights, nothing.

Where else can I look? I am a little stumped...

The wiring diagram for this car is a nightmare... It's like 45 pages long or so...

Check your radio amplifiers in the passenger side of trunk. See if one of them is warm. Thats it. Thats what I had and traced it down.

I replaced the radio in this car back in 1996 with a Sony, and all the stock power amplifiers are gone. It has two Sony amps mounted under the rear deck (One for the speakers and one for the Subwoofer) along with a ten CD changer. (The sub is mounted under the cover for the original medical kit...)

Both amps are wired to the battery, the radio turns them on... But they are not pulling anything. I pulled the fuses first thing... No change. Then I started pulling fuses from the fuse box... I was surprised when the current draw didn't change with every single fuse in the car out... Then I thought it must be a bad alternator... I just replaced it a couple of months ago, but I thought that it could have a bad diode... When the draw didn't change with the alternator disconnected... That's when I was seriously confused at where the draw might be coming from...

I figure it might be a relay somewhere that's on one of the "always hot" circuits... But damn, this car has a lot of circuits and relays! The wiring diagram is kind of an unholy mess... i am not even sure how many pages it is, but it's somewhere between 45 and 60 pages...

Thanks for any help anyone can give me on this...

pull the fuses one by one until the drain goes away. One you have isolated the circuit then start looking at all possible consumers on that circuit. do the ampdraw test all doors closed, key out and must be done in series.

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

I pulled every single fuse out of the car, and the drain didn't go away. So it's something that is always hot, and unfused...

do you have an immbolizer installed?

next suggestions is to look at the power distribution wiring diagrams and see whats hot all the time.

Download 1030 - Testing no-load current consumption Model 124.pdf from FileFactory.com

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

While I was at it, I pulled every single relay out of the component box behind the fuse box... No change in the draw...

The wiring diagrams for this car are not very user friendly... I used them when I installed an aftermarket alarm... Mostly I used the factory alarm wiring because it was so convenient. I put in a code alarm, since I am very familiar with them, and back in the day they made a great alarm... (They got bought by Audiovox, and I wouldn't put one in a Yugo now...

So with every relay pulled out, every fuse pulled out, the aftermarket crap all disconnected by pulling their fuses... Still get 4.25 amps going somewhere...

I am wondering if I can find someone to borrow a heat sensor from, one of those non-contact infrared detectors... This is 50 watts going somewhere... I could find it with that!

Sigh...

You can get a really good infrared thermometer for about $50-60 online.

Could the problem be the starter solenoid? Starter switch? Seems like those are some of the only things still connected.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I would use a digital readout detector. With it I would start checking draw at each line because on that car there are lines that are fused not in the fusebox. For example the power antenna could stay on when it comes down even though it appears off cause it is controlled by the radio. Yet the power antenna has its own fuse in line at the back. So check there. And check any line that is not supposed to be live with the digital readout. One help is to check the battery when you turn off the engine and then each hour to see how much power is being consumed which may give you a lead as to what is discharging. A big short or a small short. Maybe you think those new amplifiers are turned off by the radio, but maybe they are not. Feeling them for warmth will tell. Good luck and keep at it.

go through the document I attached and report back with your results. You have narrowed the areas to look at since you have pulled all fuses and relays and no change so that means you have to look at switches, control modules, grounds, corrison etc

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

I did try to follow the no-load current consumption doc you gave the link for... Didn't really illuminate anything... The current draw is too high, since it should be in the 20ma range. (I'd be happy if I could just get it down to 100ma... The car has a heck of a battery, if the draw was a tenth of an amp it would never have a problem!)

The tests in the doc were the same as what I had already done, for the most part. At least the parts regarding protected components. The section about determining the current consumers that are unprotected didn't tell me what to do, so I am not sure how to test those. These include the EGR system, airbag system, knock control system, components of electronic ignition, Base Module, anti-theft systems (E.G. immobilizer) components of idle speed control, generator, starter ignition switch... Basically, I think I need some help with figuring out what the heck to test... And then, how the heck do I test it?

The factory anti-theft system was removed when I installed the alarm. My wife wanted remote keyless entry, and I also hooked it up so she could roll the windows up and down with the alarm remote... I initially was going to install a remote start system, but changed my mind when the alarm turned out to be such a pain in the butt to install. (Seriously... It took me less than 3 hours to install the same basic alarm in my Toyota, and the Mercedes was two entire days...

I am a mechanical engineer who worked his way through college as a mechanic... For most things electrical I am pretty proficient, but this car is a little out of my league sometimes...

The more I think about this, the more I am leaning towards an IR heat sensor... With this much draw, something has to be getting warm somewhere ...

The idle speed control components raised a flag, because this Mercedes has had some of the disintegrating electrical wiring problems... I replaced the engine wiring harness, and the mass air flow meter... It does have a small problem with the electronic throttle actuator, but I have not replaced it because it's a $1300 part, and the car still runs quite well... It stumbles at certain RPM ranges sometimes... It's not too bad, so I have just let it go for now. It starts up and idles perfectly, and if you drive just a tad aggresively it performs perfectly.... It just has a slight stumble at very light throttle sometimes...

Do you have any recommendations on an IR heat sensor? What would be "A really good one for $50~$60 dollars online"?

Sigh... This car is like a high maintenance girlfriend... You can love it and hate it at the same time... (I love it all the time, but it aggravates me when something doesn't work like it should, since the thing is so overly complex at times...

I forgot about that one. I will pull that fuse tonight. I remember there were a couple fuses somewhere else, too. I can't remember whether they were somewhere behind the dash or the drivers side kick panel... But I know there are at least a few other fuses at random places in this car.

(My wife tells me I have C.R.S.S. (Can't remember Sh_t Syndrome...) I think she exaggerates, but there is no doubt some truth to it... I installed that alarm in 1996, so it's not like it's fresh in my mind... And I have installed a lot of alarms over the years, so they kind of run together in my brain... (Every car I have owned since my 280ZX got stolen from in front of my house... All my kids cars, several friends cars... I usually like electrical problems/work, since I can be out tinkering on the car and not get too greasy...

This one has turned that corner from being interesting/fun, though...

Does everything work they way it should? I would do a function test of everything on the car to be sure everything is working right according to wire diagrams, key postions.

for example: all ancillary components connected to terminal 15 R like a radio can be turned on without the key in ignition

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

Actually, that's one of the things I am most fond of with this car. Absolutely everything works... It has almost 200,000 miles on it, and everything works perfectly. I have had to replace a few things over the years (1 rear window regulator broke, wiper transmission ring gear broke, blower motor quit, starter a few years ago and the alternator just a couple months ago)

It has been an almost perfectly trouble free car. The engine wiring harness was the only thing (And the electronic throttle actuator, but I haven't replaced that yet) are the only things that I would consider defects.

Everything else has just been maintenance and replacing things that wore out...

200,000 miles and it drives just like it did when I bought it... Not a single squeak or rattle to be heard...

I've only had to replace a couple of light bulbs in this thing... Until now, it's been an awesome car...