A buddy of mine runs an alternator/starter shop. Per him, about 99% of recurring alternator problems can be attributed to a bad ground, either engine or chassis. The bad ground may or may not cause any other electrical problems, but will blow the diodes in the alternator very quickly. Even running an extra ground circuit is very cheap insurance.

SteveM.

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'93 190E/D 2.5 Turbodiesel 5-speed (daily driver)
'87 190D 2.5 Turbo rustbucket - parts car
'84 Dodge Rampage diesel - Land Speed Record Holder
'13 Ram 2500 Diesel
'05 Toyota 4Runner

Once you know you have a good one, make SURE it is actually connected when installed. Corroded connections in the three wire plug or a missing output wire (or misplaced) so you never connect to the positive battery post will result in a "dead" alternator because it has not field current.

Hi ho, back up the train here Charlie. My alternator (77 240d) has a 3 wire plug thingy that goes into the alternator. And that's it! I always assumed the main alternator output wire was one of those three in the connector. Am I missing something important here. The alternator has the regulator that screws to the back, and it has a large capacitor across the back, but it has NO BIG OUTPUT WIRE like every other alternator in existance. Please clarify. Thanks, this could be the ticket.

Kip Amore

light one. The two heavy ones are tied together at the starter solenoid where the battery connects and, according to the diagram(Haynes manual) of the alternator, they are also tied together inside the alternator. The small wire continues thru the firewall and maybe goes to/comes from the charge light. Is this light gauge wire supplying the alternator field current? Someone clear this up for me please.

Jim

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Jim

'49 170?(donated to church in Darmstadt '58)
'58 220S(crusher, after '73 fire[San Antonio])
'72 280SE 4.5
'77 240D
'81 300SD
'83 240D parts car

Looking at the wiring diagram again, looks like one side of the charge light goes to the fuse block and the other side goes to the diodes in the alternator, not the ones that charge the battery. I believe the field is controlled by the regulator & I can't find a schematic of the regulator in the Haynes manual so I don't know where the sensing voltage for the regulator comes from....Help, please.

Jim again _ still

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Jim

'49 170?(donated to church in Darmstadt '58)
'58 220S(crusher, after '73 fire[San Antonio])
'72 280SE 4.5
'77 240D
'81 300SD
'83 240D parts car

Sensing voltage is the battery line, power to energize the field coil comes from the ignition switch through the regulator.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I have a 1974 240D with a similar problem, although I haven't replaced multiple alternators yet (luckily).
I charge the battery up completely overnight, drive to work, and by the end of the day the battery is completely dead. Replaced the alternator, and the problem still occurs. I thought it might be a trunk light on, so I checked that - nope, switch works fine. No other indication whatsoever of current draw. I haven't checked (or even found, for that matter) the glow plugs relay, so I'll check that tonight.
How do I measure the voltage across the glow plugs while the car is running (or off, since a bad relay would presumably cause this large current draw while the car is sitting when I'm at work)?
Could the problem be in the starter circuit in the dash? A grounded short in the switch mechanism, perhaps?
Or could it be in the ignition switch wire connections?
Again, no lights are on when the car is off, no apparent short (no obvious arcing, etc.), but the battery (brand new) is completely dead within 6 or 8 hours every time.
HELP!!!!!

A simple test light will tell all.
With the car warmed up to roughly 60C - 90C.
Check for power at the glow plugs, with car running.
Power is bad in this case = bad relay.

Ed,
Leaving a dome light or a trunk light on should take a LONG time to drain a battery, especially one as big as would go in the MB diesel. I couldn't say for sure, I would suspect a dome light would probably run for WEEKS. Lights and wipers are a different story, as is a dead short. There is one thing to NOT rule out - you got a bad battery brand new. It happened to me once, and it can happen to you. That would certainly explain it. As would a high draw item like glow plugs.

Kip Amore
-still walking-

Well, last night I tested for power at the glow plugs with key and starter knob off - no power (good there). Tested again after car was started and warmed up - no power (good there). Still haven't found the glow plug relay, but I DID find a blown 16 amp fuse on the little fuse box that sits on top of the inner fender in the engine compartment (not the main fuse box, but just forward of it). There are two 16 amp fuses in it, but I don't know what they connect to. It sounds like I'm probably looking for a direct short somewhere.
Any advice as to the quickest and easiest way to track down the source?

Also, I tried two different brand new batteries, both of which became quickly discharged, and I'm currently using the other new battery in my 1986 Ford F250 diesel pickup, so I know its not a bad battery.
Thanks,
Ed

The only way to narrow down where the power is being drawn is to pull the fuses one by one and test. If you have a handheld multimeter, you can set it to check amperage draw, and put it across where the fuse used to be. Some circuits, like the clock, will always draw something. I couldn't say, but I'd suspect the clock circuit draws 50 milliamps. But since your battery is draining fast you'll see 1 amp draw at least. Definitly the best place to start are the two blown fuses.

Once you find the circuit that's drawing the power, you need to find out why. That's going to be a whole 'nuther problem. Others on this sig will have more specific info than I will when you find the problem area. It's going to end up being a broken wire or something simple though.

Kip Amore

a schematic drawing of the Bosch regulator on my '77 alternator? I'm still having trouble making sense of how the charge light iluminates when the alternator is not charging but does not lite when it is charging.... One side of the bulb is grounded and the other goes to the D- connector on the alternator, which is also one input/output of the regulator.

I know - if it works, don't worry about it - I just wanna know HOW it works.

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Jim

'49 170?(donated to church in Darmstadt '58)
'58 220S(crusher, after '73 fire[San Antonio])
'72 280SE 4.5
'77 240D
'81 300SD
'83 240D parts car

Actually, it doesn't work. I've had alternators fail in 3 different MB chassis, and none of them illuminated the charge light when they failed. You know the alternator isn't charging when the lights go dim. That's about it.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

darn.. i didnt know about this... and i've usually done this to my car before.. when it wouldnt start... i would grab the battery from our van.. and use that.. and start it.. then while engine running.. remove the battery.. and replace it with the weak one.. thinking that the alternator would charge the weak battery!..

no wonder my alternator went south after that!...tsk..tsk..tsk...

i guess we learn new things everyday!..

tnx!

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Mike

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1975 200 - Sold (no pix);
1978 200 - Sold - http://www.pbase.com/hboy/redbaron
1979 300TD - Sold
http://www.pbase.com/hboy/greenwagon
http://www.geocities.com/hboy726/300TD.html
1985 230E - now my daily driver...

OK so when does the light come on???? Mine comes on when I start the engine but I have had a recent dead battery and it never came on.

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Tom Engel
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83 240 230K Manual (8/2003)
83 300D 265K (3/2004)
Just a Driver!

Check grounds by going from centre of negative terminal on battery to clean area on block with your meter.I would prefer right to alternator case but too dangerous on this design with engine running. You should read virtually no voltage drop. Have friend crank engine and do same test again while cranking. Normally you may see a small voltage drop but I would start to check out cables and grounds if above about 1/2 a volt. The cables and grounds etc are quite old on most of our cars. Guess our babys are senior citizens of the car world now. Also would not hurt to check for any voltage drop between engine block and clean area of body shell. again with the engine running. Quick, fast, painless checks as you already own a meter.