[W126 Coupe] Long Term Battery Drain Problem Solved

[Vance Rowley]

I would just like to say thanks to all that gave me ideas what to look for and where for the long term battery drain problem I have been having literally for years (although you wouldn't know if you used the car on a daily basis). For those that did take an interest (I know there was a few that didn't and voiced their opinions abruptly) I thought it might be useful for the group to let you know of my findings, as it is the most unusual cause, and who knows, if it can happen to mine, it could happen to another one of these cars somewhere.

As you may remember from my previous posts on this subject, I was getting an intermittent current draw (around 6Amps) which was draining the battery in 1 or 2 days parked (3 days max). I traced this down to fuse #2 which is hot at all times and is an 8Amp fuse. This however feeds a number of circuits and took some time to locate the problem as I would need to wait at least 3 days for the results after disconnecting one of these circuits.

To cut a long story short, I managed to get it down to component level. It turns out that the delay circuit on the front roof light had decided to energise itself whenever it felt like it. Not only that but as can be seen in the photo, one of the resisters is badly burned. From looking at others in circuit that are in tact, it looks like this should be at least 1k Ohm possibly higher. Even though this resister was badly burned, it was still in circuit and just happened to measure only 2 Ohms. As far as I can tell, this was being put under load with the wonky delay circuit which of course using Ohms law just so happens to come to 6Amps.

I cut the resister out of circuit and all seemed well. I then decided to get a complete new unit which I managed to get at a good price and since being fitted I not only haven't had the battery drain problem anymore, but for the first time since I purchased the car have an interior light that actually comes on when I open one of the doors. Who would have thought such a thing could cause such a problem.
I hope nobody is cursing me for placing the attachments, but I did compress them considerably and as they say "a picture is worth a thousand words".

Thanks again and I hope this info might be of use to someone else someday.
Cheers,

Vance
1991 Euro 560 SEC.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Delay Circuit.JPG
Type: image/jpeg
Size: 23749 bytes
Desc: not available
Url : http://www.pairlist.net/pipermail/mbcoupes/attachments/20060507/03d5c617/DelayCircuit.jpe
-------------- next part --------------
A non-text attachment was scrubbed...
Name: roof light.JPG
Type: image/jpeg
Size: 8396 bytes
Desc: not available
Url : http://www.pairlist.net/pipermail/mbcoupes/attachments/20060507/03d5c617/rooflight.jpe