1978 450SEL 6.9 Battery Draining
 

After driving 1400 miles round trip out to South Dakota I have my first gasser! I've been working though the neglect and doing all the basic maintenance stuff. One issue I'm stuck on is the battery drains when the car is off. Yesterday I put a multi meter between the battery negative and the negative cable and found that 0.37 Amps is constantly being drawn when the car is off. I pulled all the fuses and it was still drawing 0.37 Amps! I don't really know where to look next. Are there any fuse boxes other than the one in the engine bay? Are there any circuits on the W116 chassis that are prone to draining the battery?

Do-it-yourself replacement radio wiring and add-on electronic gizmos are frequently not installed properly and are usually the cause of these problems.

Look for non-factory wiring added onto the positive of the battery or spliced into positive cables prior to the fuses.

my first thought on a 116 drain is the climate servo. good luck, chuck.

The original Becker Mexico radio is still in the car. Oddly enough, it magically started working again today as I was driving home from work. I just checked the current draw and it's still 0.37 amps. The climate control doesn't work. The heat is on all the time. The blower motor works properly though. Is there a way to test the climate control servo for power draw without tearing into the dash? Also, does the W116 have a strip fuse for the blower motor like the W126 has?

With the engine off and key off, place a steel screwdriver tip at the back of the alternator and feel for magnetism.

Also, put your meter between the large wire on the alternator and the stud it came from.

I've seen diodes fail in the conductive state ( I'd say shorted but too many people miss use the term to describe an open circuit. ) causing a drain.

97 SL320 - a bit more info please. What are you looking for when you test for magnetism? What does it tell you?

That tells you that the alternator is draining power. Diodes only allow electricity to flow one way, but they can fail. Sometimes they fail conductive allowing the coils in the alternator to draw power from the battery. The fix is to replace the alternator or replace the diode, if it's serviceable. Thanks for reminding me 97 SL320, I'll check later today.

Today I fixed the odometer on the 6.9 and while I was in there I disconnected the clock. The hands moved, but it didn't keep constant time anyway. That was the problem, no more battery draining!

So did the disconnected the clock really fix the battery drain???

if not i have an idea...

Your clock should only be a 20 MA draw, at most. May be just a coincidence. I would keep an eye on it. I'm with Porkface on the heater servo....