View Full Version : [OLD THREAD] Battery drain on 1993 500

08-12-2011, 05:17 PM
12-01-2008 07:24 AM #1
Mike Spicer (http://500eboard.com/forums2/member.php?12-Mike-Spicer)
Join DateDec 2008Posts2

I have a 93 500e with 157k on it, and for the last year my mechanic and i have been chasing an electrical draw that randomly drains the battery. sometimes it will go for a month or 2 without issue.

I have tried disconnecting:
rear trunk light
rear antenna
vacuum system
dome light

new alt
new wiring harness
several batteries

this car is the best car i have every owned but i cant trust it and i cant use it. Please send me any ideas on what to try next as i want to keep the car and get it fixed.
12-01-2008 07:57 AM #2
imported_gerryvz (http://500eboard.com/forums2/member.php?2-imported_gerryvz)
Join DateNov 2008LocationThe Woodlands, TXPosts24

I believe it could be corrosion between two of the pins in the mushroom plug connector (the 38-pin diagnostic connector) under the hood that is grounding them. I have seen this very recently (through MBI) on a couple of W140s and W124s and it is a total stumper for the mechanics, until they figured it out. Jim at MBI is the mechanic who has successfully diagnosed and treated this. If it's not any of the typical items that you mentioned that provide a constant draw, then it is resident within the "live" portion of the electrical system. That 38-pin connector is always live at some pins even when the car is off.

Have Mike Burback call Jim at MBI and talk it over with him, or bring the car into MBI and have Jim look at it. It's a weird thing that's difficult to believe, but it happened on several vehicles now that MBI has seen and they're now alerted to it and on top of it.

In any case this will be a simple and inexpensive fix once it's diagnosed.

12-01-2008 06:15 PM #3
Mike Spicer (http://500eboard.com/forums2/member.php?12-Mike-Spicer)
Join DateDec 2008Posts2

thanks Gerry i will follow up with Burback and MBI, hopfully I will get back on the road fast.

02-15-2012, 09:05 PM
I have 92 500E with 103k and I too have been chasing an electrical issue that is draining the battery over 3-4 days and driving me crazy. I disconnected the negative terminal at the battery and hooked in a test light between the disconnected negative cable and the negative battery post. It was glowing brightly..! I pulled the fuses one by one, and on pulling fuse C the light went out. Fuse D also triggered the light but very faintly...maybe the clock..? Anyway, refering to the ETM I disconnected all the same above utilities - , radio, amps, antenna, rear trunk and dome lights, vacum supply pump etc... The test light still glows bright.

Opened up the magic box on the passenger side of engine bay to check on the above suggestion regarding the 38 pin diagnostic. I unscrewed the plug and examined it top and bottom but it appears clean although it did have an acid like film on it....but no corrosion/green stuff was evident. I cleaned it anyway and still the test light shines bright. By the way when I ran a meter across all the pins...one test lead in one pin and then the other test lead into the other pins inturn, I got continuity signals. Is this correct...? It seems strange that the pins are mostly all connected. Kind of plays into the short findings by MBI.....but the plug was clean. I don't where the wires all go but could they be shorting somewhere else...?

One other thing, there are 2 other fuses next to the main box in a separate holder. One is 8amp and the 25 amp. I understand these are for the phone and heated seats. When I pull the blue 25amp fuse, with the test light connected as above and FUSE C in place the test light pulsates dim and bright. Does anybody know what caused this....?

Anyway, I don't think this has anything to do with battery drain issue as with these fuses in place, but FUSE C out, the battery does not drain.

I would appreciate any other ideas.

02-16-2012, 07:34 AM
There shouldn't be a 25A fuse for the phone or heated seats.... those are 16A max, if not 8A (I forget which should be in there).

What items are fed by Fuse "C"? The pulsating could be related to the seat belt warning lamp in the sun visor trim panel, this flashes when the key is turned on.


03-08-2012, 11:20 AM
Thanks gsxr..! You are right...the pulsing is the seat belt warning lamp....never thought of that.

Fuse C feeds (30 circuit - hot all times) rear dome light, trunk light, antenna, left seat memory, ATA control unit, left and right front door and trunk lock actuators, vacum supply pump, left and right exit lights, CD changer, audio power amps and radio.

Any ideas..?

03-08-2012, 11:53 AM
Hmmm. From that list, first things I'd try is unplugging the radio/amps/CD/antenna and see if that eliminates the drain, followed by a close inspection of the wires near the trunk lid hinge that go from body to deck lid (common breakage area). Ideally you'd use a DC ammeter but the light bulb method should give you a rough idea...


03-08-2012, 12:14 PM
Thanks gsxr. Yeah I unplugged every thing powered by the C fuse including radio, antenna, amps and CD but the light stays on. Agreed...ammeter is best.....but I know that the drain is on this circuit as when I pull the C fuse, light goes out and battery does not drain. I will check the trunk hinge area...maybe something is shorting.

By the way, I just checked the ETM and it calls for a 25A fuse for the heated seats in the aux fuse holder F22/1....next to main fuse box.

03-08-2012, 12:37 PM
You're right! The ETM shows a 16A fuse up to model year 1990, and a 25A fuse from model year 1991. Interesting... I'll have to compare what the fuse box info card says. Thanks for the info!

For the drain: Have someone open & close the trunk lid while you watch the light bulb. If no change, have someone wiggle the wire bundle around (gently) and see what that does. I've had to repair the wiring at the trunk hinge on a couple of cars, it's a semi-common issue, although the typical fault is a broken wire that doesn't result in a battery drain. Next up is getting a cheap multimeter (even the $5 Harbor Freight units will suffice) so you can measure the actual current draw. A very small amount is acceptable.

A less likely possibility is the battery has a problem. May not hurt to charge it up and have it load-tested, most of the local McParts/FLAPS will do this free. I don't think this is your issue, but it's something to keep in mind.


03-08-2012, 12:49 PM
Thanks again...let me go and do as you suggest. Wil get back to you on my results.


03-09-2012, 10:50 AM
I found it….!!!

I tried the trunk wire thing as you suggested but to no avail.

However, something you said about the flashing test light being an indication of the seat belt warning light got me thinking again about this occurrence. So I ran the test again.
To recap….with both fuse C and the phone fuse (8amp in F22/1) in place, and a test light connected between the disconnected negative battery lead and negative terminal, the test light was a very bright steady red. When I pull the phone fuse, the test lamp pulses on and off.

So this time I put long leads on the test lamp and taped it to the outside left rear fender so I could see it while working in the front and the trunk lid closed. I closed all doors, taped the hood ATA switch closed and put keys in pocket.

Again, test lamp flashed as soon as I pulled the phone fuse and went out altogether with fuse C out. Put fuse C back in and the light starts flashing. How can this be and what is causing it…??!!! #*%?/@%!!

I went round the back of the car, opened the trunk and just stood there watching the test lamp flash away as if I expected it to tell me something…right..??. Well, in a moment of silence it did…!!! In that silence I heard a very faint ticking noise in sync with the pulsing test light, coming from somewhere inside the trunk…!! What the hell ??!!
It was coming from the multi CD unit in the trunk…!! I pulled the trunk lining (again..!), found the power plug connector and pulled it all apart. The noise stopped and the test light went out…!! I’ll be darned..!

I realize now, that in my haste to disconnect everything the first time round to test, I had only unplugged the amp/tuner from the multi 3 pole power connector leaving the power still connected to the CD unit as this is an uninterrupted wire straight through to the CD. The amp/tuner plug takes a split off it. I thought I had isolated both and hence why I was still getting a fault the first time I checked.

It turns out, that something is mechanically crook inside the Becker CD unit which is either not turning off, or, is activating a small motor relay constantly and thus draining the battery.
Why the flashing with the phone fuse out..?? Well, all I can guess for now is that the stray fault current is looking for an easy path to ground and so for whatever reason chose the phone circuit to drain as the phone and audio are interconnected in some complicated way. As soon as I pull the phone fuse, there is either now enough power to pulse the little relay (the flashing test lamp) or not enough power to keep it permanently closed but make it open and close. I don’t know….just thoughts.

Anyway, with the CD unit out, everything is just great….no battery drain…:)

Now who repairs Becker CD units…??

Thanks for the input GSXR…!! The dialog really helped the evaluation process and you did say check the audio system again…!!

03-09-2012, 11:46 AM
Now who repairs Becker CD units…??Where are you located? In Europe or the US? It's pretty hard to get the units repaired, often you can just turn your unit in as a core and get a refurbished one, or a used one outright. There may be someone here on the forum who will sell a used one. If you are in the US talk to Becker North America in New Jersey ... their contact info is at www.beckerautosound.com. If you are in Europe, I have a contact in Germany (Munich) who specializes in repair who can very likely help you.

Let me know

03-09-2012, 02:10 PM
Thanks Gerry...I am in southern California. I will try Becker USA.

03-11-2012, 03:17 AM
Can anybody help out with the CD magazine for an original Euro MB player. The years I have had mine I dont know if it works as it never came with the car and MB dealers in UK say it is not available, and as the car is not an original dealer import to the UK they are not interested.

It does drain the battery over time


03-11-2012, 04:14 AM
Phil, your best bet is try these guys in New Jersey http://www.beckerautosound.com/ as Gerry suggested. I've used them before myself and they are very helpful..Shipping time is around 5days and no taxes paid if it's a recond/used item. Hard to find these in breakers yard as they generally just get pulled out quickly with the cds when the car is wrecked..

If you want I can check with my guy here at MB Brentford . Never have problems ordering parts here and the guy I deal with is very knowledgeable. My car is a swiss import!