[W126 Coupe] Back to the battery drain issue

[Vance Rowley]

Hi Dick,

 Yes it sounds a bit over the top under the circumstances re the PLC, but thanks for the offer anyway. But some things in your comments have given me some ideas. Mainly your comment regarding the dome light. One thing which has never worked on my car is the interior dome light has never come on when you open either of the doors. It works fine if you were to switch it on manually with the switch on the dome light itself. This has never bothered me and have never thought much of it. I have checked the obvious culprits in the past such as the door switches themselves. But your comment has really got me thinking if the dome light is operated by a relay via the door switches, then maybe the contacts on this relay are the dome light problem, but maybe the relay or control box is what is taking the power. Just thinking out aloud. I will need to check the circuits on the manuals I have recently purchased. Does any of this make any sense?

Cheers,
Vance.

----- Original Message ----- 
From: "Dick Spellman" <spell.yy at verizon.net>
To: "Vance Rowley" <vance.rowley at tiscali.co.uk>; "Mercedes Coupes Mailing Lists" <mbcoupes at mbcoupes.com>
Sent: Saturday, February 04, 2006 11:34 PM
Subject: Re: [W126 Coupe] Back to the battery drain issue


> Hi Vance:
> A PLC (programmable logic controller) programmed to count the number and 
> duration of input state changes would do the trick.  You could download 
> the data collected to your laptop or run with the laptop connected and 
> it will announce each time it occurs.  A bit pricey for just observing 
> the problem without drilling down to the circuit causing the drain.
> 
> I'd pull the seat heater power or just pop up the two switches and 
> remove for a bit, the switches trigger the under seat relay packs.  
> Other possibilities: A dead short could easily pull 6 amps without 
> blowing a low-end 8 amp fuse (white) and maybe the interval is not 
> enough to smoke the insulation on the wire yet, but, I'd bet the day is 
> coming!  I've experienced a dead short on the rear door switches that 
> run the dome lights on one MB.  Lights went dim, could smell burning 
> insulation on the wires and only happened when both rear doors were open 
> at the same time.   I've heard of the wire lead that runs along the 
> trunk hinge (not sure what it runs, license plate lights?) and along the 
> hood hinge (washer outlet power) that have caused fires on these cars.  
> MB recommends you check them.  I've heard of the factory fix in 87 for 
> early MB's where the heater fuse that used to heat up in the fuse box 
> would cause a melt down on the external temp sensor circuit under the 
> fuse holder and cause a dead short to the hot on the external temp 
> sensor and guage in the instrument cluster.  This ignited the wire 
> harness behind the IC. (I had this in the 86 300SE Euro and repaired 
> same.)  MB fix was to externally mount a foil 30 amp fuse in a 
> horizontal fuse holder on the engine fire wall.  One last thought, what 
> about the clock, this has 2 capacitors that pull then store power for 
> operation?  Seems like a lot of juice for such a small consumer.
> 
> If you think a PLC would be of help, send me your mail address 
> off-line.  You'll need a PC with serial port to get the results.  I'll 
> program one up and send your way as a loaner.
> 
> Regards,
> 
> Dick
> 
> Vance Rowley wrote:
> 
>> If I could come up with some kind of counter to read how many 6amp 
>> peaks there are on average per 12 hour period (while unattended), then 
>> this in combination with your suggestion would be perfect. This way I 
>> could have the meter (and counter system) connected overnight with one 
>> fuse at a time pulled until I come across the next morning where the 
>> counter is zero with a particular fuse pulled. This way it could be 
>> done overnight and completely unattended.
> 
>