I recently purchased a 76 300D that turned out to not be charging. Checked dash bulb (OK) but found the alternator itself was unplugged under the hood.
I plugged it back in, but the dash warning light would remain lit with ignition off, and go out when ignition switched on. Tried starting the engine, and the alt began to smoke (To be fair, an oil leak had fouled it a bit.)
Pulled plug back out, removed voltage regulator and found very short brushes. Any clue if a bad reg/brushes would cause such symptoms? Thanks!

It's odd that the light stays on with the car off. Any evidence of "custom" wiring? If it were me, I'd spring for a new regulator and see if that fixed it. If it didn't the reg would go on the shelf (they fit a lot of different alternators) and the alternator either to the rebuild shop or trade it in on a rebuilt. As you can see I sometimes throw parts at problems till it fixes it... Maybe someone else will chime in with a better idea.

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Muleears
'07 E320 Bluetec 133K my DD
'04 Jaguar XJ8 VDP, 34K
'10 Hyundai Accent 60K Grocery Getter
'02 VW Golf soon to be on the road again
'97 E300 Diesel Son's DD
'61 VERY tolerant wife

Hampton Roads, VA USA

Gone but not forgotten:
'67 250S 95K
'86 300SDL
'87 300D Turbo, 364K! R.I.P.
'98 E300 Turbodiesel, 213K
'02 S420, 164K
'01 Prius 138K

Thanks...new reg ordered and should be here early next week. Do you (or anyone else) have an idea on how the female plug for the alt can be tested? As in, what pin should get what?
There is no obvious wiring issues, but I'll keep poking around.

New voltage reg installed. Plug in alternator wiring, and am met with a few sparks (indicating voltage draw, I guess)
Same result: alt light on with ignition off, goes out when switched on. Did not attempt to start. Is there a resource to tell me the proper pin-out of the chassis plug to the alternator?

If someone put the wrong battery cables on the wrong poles it would pop the diodes in the alternator. This can be easy to do if both the cables are black or if someone replaced the negative cable with a red one.

Replacing the diodes is a simple job, or it is on most alternators.

You could also pull the alternator, take it to an AutoZone or someplace that can test it, and they could confirm if this is the problem.

The light being on when the ignition is off is the usual giveaway that the diodes have popped.

When the charging light stays on, it usually is an indication of bad diodes in the alternator.

This article is bike related, but explains why the light comes on on Bosch systems:

Bosch Charging systems

Alternators are relatively cheap to buy or rebuild. Sounds like you may need one.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Thank you for the input. Guess it's time to pull the alt and have it tested...being an A/C car, it won't be quite as easy as I may like, but I'll get it done (hopefully soon) and report my findings.

I'm not familiar with 115, but based on a general understanding of how alternators work, my initial thought is that the L (Indicator) and B+ wiring is reversed. Test the alternator for sure, but also check the wiring. The B+ wire should carry 12v at all times, and on many Benz applications, there are two B+ wires. The L wire will have something less-say 9v, but only when the key is on. If one of the B wires is swapped with the L, the Lamp may light when the car is off, the other side finding ground through some convolution of the dashpod circuitry. When the key is on, there will be 12v on both sides, lamp goes out. But only one B wire to carry power, which could cause some smoke.

Mx:

With reference to the diagram supplied by Graham, which matches the charging circuits of the 114/115 cars, the D+ terminal of the alternator is indeed connected to the indicator (L). With a diode(s) defective in the shorted condition rather than open, B+ is fed to the D+ terminal, and then, as you describe, to the indicator. Also as you describe, there are numerous connections that become grounds when not powered, and as a consequence, the indicator will be lit.
All this will occur without any fault in, or reversal of wiring.

Make sure you get it tested by a proper auto electric shop familiar with Bosch systems. Auotozone and similar shops will test it, but .... I know they told me I needed a new alternator for my E320 about 6 years ago and it's still working fine!

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

So, I finally have more spare time to tinker with the Mercedes. I would like to move the A/C compressor (on its bracket) out of the way so that I can access the alternator for removal. I have removed one nut from near the intake, two by the engine mount, and one bolt from the belt tensioner area, yet the bracket will not budge.

Am I overlooking another attachment somewhere?