I killed the battery in my 240D a few weeks ago by leaving the interior light on when the car was parked for a month. Battery wouldn't hold a charge, so I replaced it with a Bosch group 49 unit.

Now I noticed (during a trip back from Boston!) that the alternator is charging a bit, but not enough -- the battery will discharge with the headlights and heater blower on. Max charge voltage seems to be about 12.5-12.6V. Also, the lights flicker quite a bit, and the voltage actually DROPS at higher RPMS (lights get dimmer). Turning off the heater blower and running the engine seems to allow the battery to re-charge enough to make the car able to start.

Do I need a new alternator or regulator? Could a defective battery be causing this symptom? All connections at the battery and ground-strap are tight; I checked. Charge warning light is only on with the engine off, as normal.

Thanks-
-s.

I think that would be the regulator going bad. Super easy to change on a 240D, you might even be able to do it from the top.

Be sure to get a genuine Bosch regulator, not aftermarket. Or you will be doing it again soon.

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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

You need a new voltage regulator......the charge light/battery light should not be on when the engine is off.....you may need a whole new alternator....

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Also check the connector at the alternator, and make sure it is not corroded. Disconnect the battery first, it has unfused battery voltage on it.

__________________
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

If key is on and engine hasn't yet been started, the battery light should be on. Or at least it's been that way in every single car I've owned so far

Yes explained that way it is, the way you made it sound.....it was on after the car was turned off and key removed....

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Yeah, that would be strange.

But not impossible if you had a shorted diode pack 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

12.50-12.75 Volts is the Battery Voltage when the Battery is fully charged and ambient temp of about 75 degrees F. Meaning if there was any charging it was only keeping pace with the little bit of drain on the Battery after starting.

The Alternator will not work right if the Cables, Grounds and Wires are not in good shape and making good Contact.

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84 300D, 82 Volvo 244Gl Diesel

+1 what he said

If you are getting 12.5 V you are just seeing the battery voltage only, the alternator is not charging. You need to be in the mid 13s to low 14s, but not higher than the mid 14s or you will burn stuff up.

__________________
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

It is charging a bit. After a few hours of heater fan, the battery couldn't even power the glow plugs without the lights going totally dim (tested this when rolling and restarted the engine with the clutch and gears) -- after a few more hours of NO heater fan, the battery was full enough to restart the car with the starter successfully.

+1 on this too.

It might be slowly charging but its likely not adequate in the long run.

If I were you, I would try changing the voltage regulator. Maybe your brushes are worn down.

You could also swing by one of the auto stores, most will test your charging system and give you info.

Ideally, I would upgrade the alternator to something like the 115A unit like I've got in my 420SEL. I believe it is physically mountable in your 240D. Might take some work though but well worth it. MB really did not do a good job of sizing the alternators on these cars.

Even on my dad's 300SD prolonged periods of running the aux fan, blower motor and headlights will result in noticably harder starters and lower batter voltage. Its only saved by the simple fact that he doesn't always do all three at the same time so he can get some periods of decent charge. I might throw the 80A unit from the 420SEL in there at some point. 80A as to be better than the 60A that's in there now.

I've got a 143A in the 190E and a 150A in the 300D. My normal charging voltage is 13.4 to 14.1 on both cars.

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82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

How tight is the belt?

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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D

Disable or check your glow plug circuit for remaining on. The symptoms would be similar as well I think. May even just be intermittently on remember so best to disable it and check charging voltage.

Looks like it was the belt -- I had put on a new belt last month, and it looks like it had stretched a bit initially. Tightened it, drove quite a bit at night over the last week, and the problem hasn't repeated. Battery/charging voltage seems to be now between 12 and 13V. Not great, but not awful either.

Thanks.