electrical short
 

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.

X2

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"...

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.

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.

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

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.