I just bought a 83 240d. The problem I am having is when you turn the key off the battery light comes on. If you do not disconnect the battery it will drain it. The car does run I just drove it 300 hundred miles the bat. light is not on when running I just disconnect the bat. when I park it. Any ideas?

Thanks

Blown diode in the alternator.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

X2

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

x3

x4, but for future reference it's called a parasitic draw. A short is current taking an unintended path to where it's not to supposed to go. If you get a short you'll commonly blow fuses. Just a heads up.

Well, technically when the diodes fail, they fail "shorted"...

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I do agree it's a shorted as far as the alternator goes but I would think it's a draw still because of an unexpected drain but that's what I've been learning as I go along.

__________________
Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

Doesn’t sound right to me. Sure, a leaky diode can cause a drain after shutdown. But why is the light on? The light requires the switch to be in the run position. I think the switch is the problem.

Check your alternator for a bad diode. If you have a multi-tester, put it into the continuity mode. Disconnect the positive battery cable from the alternator post. Put one lead of your multi-tester on the alternator positive battery post, and touch the other lead to the alternator casing. If you get continuity, reverse the multi-tester leads, and if you still get continuity, one or more of the diodes are bad.

__________________
96 E300d

Once again, a bad diode doesn't explain why the light is lit.

I have to agree that it's a diode. I did an experiment on my car, simply cross wiring D+ and B+ while the car was off. Surprisingly, the light comes on. There must be some sort of switchover ground that doesn't appear in the wiring diagram. Something to investigate next time I have the ignition switch out of the dash.

One side of the diodes (Cathode) connects direct to the battery, that's why a shorted diode can drain the battery. Ign switch is not in the circuit.

https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/ETM/82_300.pdf

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Of course a bad diode can drain the battery, that's obvious. But think through the whole problem. The battery light on the dash is lit when the key is off. Your diagram actually solves that puzzle: when the key is off, the dash light grounds through the glow plug relay.

Attached Thumbnails