[W126 Coupe] Long term Battery drain problem

[Steve Nervig]

On Mar 11, 2006, at 9:56 PM, Vance Rowley wrote:

> Hi Guys,
>
>   I hope that some of you guys have had some similar experience to  
> the one I have had regarding this intermittent battery drain  
> problem (and solved it). This has been going on now for years. As a  
> reminder for those not seen my previous posts, I am getting an  
> intermittent current draw from somewhere that will drain the  
> battery enough not to start the car after 2 or 3 days standing  
> parked. I have had months of testing and circuit testing, It has  
> come down to a proven fact without any doubt something  being fed  
> by fuse #2. All the usual lights  (trunk light, battery,  
> alternator, starter motor etc) have been ruled out, plus the fact,  
> this is not a constant current draw, this is intermittent. The head  
> unit for the stereo system is also on this fuse, but I have done  
> considerable testing of the head unit which for memory takes no  
> more than 70ma with ignition off which is fine with a 100Amp/Hour  
> Battery (which has been recently been drop tested again). Other  
> items on this fuse are the hazard warning system, (which can take  
> up to 6.5Amps while active) but I have never seen them come on, by  
> themselves. I am currently in the process of trying relay "B in the  
> fuse box which I have removed. I have ruled out the stereo head  
> unit and the stereo amplifier unit (as I had the amp unit  
> disconnected completely from the system for over a week). Fuse #2  
> in the fuse box  feeds several circuits. These being 1. Diagnostic  
> terminal #6, 2. Hazard warning system, 3. clock, 4. Front roof  
> light, 5. Electronic Radio (this is now the memory feed to the  
> aftermarket stereo head unit which takes no more than 70ma in off  
> mode), 6. Vanity mirror light (which both work as they should). The  
> only things I don't quite understand are the last two circuits by  
> there description. They are 7. Relay comfort circuitry term 86, and  
> 8.electr illum control.
> Relay "B" is a combination Relay for Turn signal, Hazard lamps,  
> Wiper / Washer system. I have currently replaced fuse #2 and  
> removed Relay "B".
> BTW just to clarify, with fuse #2 removed, the battery does not  
> drain off. It is most definitely one (or more) of the above which  
> is the problem.
>
> So, if there is anyone out there that has had any similar  
> experience with one of these circuits fed by fuse #2, please let me  
> know.
>
> All the best, (and banging my head against the wall)
> Vance

Well, Vance, I can relate to your frustration, because I've had a  
similar problem with my W124 coupe for about 5 months. Battery drains  
after about three days of sitting. We may have found it just this  
week, and your mention of the comfort circuit caught my attention.

On the 124, there is a comfort module, N52, sometimes called the  
convenience module. It's a relay that ties into a multitude of  
different systems, including seats, belt extenders, overhead lights,  
etc. On my car it is located underneath the left rear seat bottom. It  
has five different connectors going to it.

We disconnected it and reconnected it, and it seems to have stopped  
the drain, although I am not positive. My indy's theory is that some  
of these electronic modules will sometimes reset if you disconnect  
them. Don't know if that is a valid theory, but I know of one other  
case (also a 124) where the same type of battery drain was happening,  
and disconnecting and reconnecting the comfort module fixed it.

You didn't mention what year and car you have.

But in looking in the 126 manual I didn't find a similar module/ 
relay, so I don't know if this scenario applies to your car or not.  
But I thought I'd mention it.

Regards,
Steve Nervig
1992 300CE Sportline

--
If you don't know where you're going,
any road will do.