[W126 Coupe] Back to the battery drain issue

[Dick Spellman]

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.