Battery drain 300E
 

Looking for some help in finding the cause of the battery drain. Here is what I have done.

Overnight ended up with 8.5 volts of battery, not enough for a start. Disconnected the -ve battery, set up parallel on negative cable to negative post.

test light on bright.

Next disconnected one fuse at a time to find a marked difference when disconnecting fuse in "C" position which includes
rear roof light checked OK
rear locking system not sure how to test
seat back arrestor central locking system not sure how to test
door lights, check ok
reading lights, check OK
auto antena disconnected plug, no change top light
contol for seat adjustment. functions OK

I am not havuing luck finding. Maybe the locking system where do I start or continue?

Trunk light check out ok
reading lamps OK

Grab a multimeter and test across each fuse to see which one is drawing current while the car is off. That will narrow down the possibilities. The dash draws a little current for the clock but it is overall negligible (same with radio and memory seats).

current draw
 

if i remember right in my 1987 420sel i had the same problem and it was fuse 15....for the trunk light and all that.....but i agree that your fuse box is easiest place to start.

lyle in marfa, tx

Yep. As I mentioned in my post, it is fuse C that is the fuse that powers all the things I mention. So I have isolated the fuse of the drain. I just need to better understand what the procedure is for testing those in the circuit. I agree, that the fuse that powers the radio memory and clock had a little drain.

Knowing the amount of current flowing through would assist in knowing which item is causing it to drain - why I recommend a multimeter. Anyways, listen for the vacuum pump underneath the rear seat of the 300E. It is possible that it is not shutting off after the specified time and therefore draining the battery by staying on. Kind of a long shot but certainly possible.

300E battery drain
 

OK here is what I have found. With the -ve battery cable disconected and test light hooked up to negative cable and -ve terminal, two fuses are drawing as follows and when these fused were removed the test light went out
fuses were:
C position fuse .27 amps draw powering the items I mentioned in my previous email.
9 Position fuse .12 amps draw powering diagnostic term, hazard warning system, clock,radio, front roof light,vanity mirror light, relay comfort circuitry terminal 86.
Tests wereperformed was with the multimeter set on the 10 amp setting so I assume this was not 2.7 and 1.2 respectively. Pardon my electrical ignorance.

I have tried disconnecting all C fuse circuits in my test but I can not seem to locate rear door locking system, central locking system and control unit seat adjustment to see whether any of these are affected. I tried disconnecting the simple components of the C circuit and these seemed to check out OK. Any thoughts anyone?