I put a new battery in my 240D and it died the other day. I recharged the battery, put it in and started the car. I removed the cables and checked them for power. Nothing. I did the same thing on my 300D and I got 20 volts at the cables. Is there something else maybe that wouldn't let power get back to the battery? Voltage reg perhaps?

Just check the voltage with the battery installed and the car running, are you getting 13.5 volts, or so? It is pretty common for the voltage regulator/brushes to wear out on these. The voltage regulator is installed within the alternator, but it is easy to replace (two screws):

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41W20IZG9G&year=1981&make=MB&model=240-D-002&category=All&part=Voltage+Regulator

Edit: This shows an external voltage regulator on some cars.

Take it to Autozone and have them check it for free.
No you don't have to take it out of the car.

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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If you are getting 20 volts on your 300D and your electrical system hasn't self destructed than I would take a look at your VOM. Maybe it too is at fault as well as your charging system on your 240D.

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Doug

1987 300TD x 3
2005 E320CDI

i went to autozone for a alternator/battery test (car wouldnt start, this was three weeks after getting it)and they told me they were only pulling 14 volts (which i thought was right) and that my alternator was going bad. went to an alternator rebuild shop and they told me they didnt know what they were talking about and that my alternator was good, and i just needed a new battery. well 3 months later its still running!!!

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76 300D 108,110 miles
99 Saturn sl1 80,000 miles - moms
91 Grand Marquis 45,000 miles - Grandparents
97 Ford F150 - 16,000 miles - Grandparents
73 Ford Monarch (parked 65,000 miles - Grandparents

I have never really gotten useful results from one of those "free" tests. I've found that measuring the voltage with the battery connected and the engine running is as good a test as any. If the battery is charged (about 12v with the engine off), you should seen between 13v and 14v with the engine running.

I'm hoping it's just the voltage reg. This car is quickly becoming a money pit. It will be practcally new by the time I'm done though.

An easy test is to put the max. load on the battery/charging system and measure the results. Start the beast and trun on everything .. high beams, AC, radio, interior lights, trun signals and measure the voltage across the battery poles. Now do the same without the engine running. Measure the battery voltage. Now if possible, start the car, trun everything on and measure the voltage. Depending on condition of the charging system and if someone altered the system, you should have 13 - 14 or more BUT less than 18 volts while the battery is charging from a large drawdown.
Voltage regulator (at least on the MB's I've worked on) replacement has been a simple 5 minute fix after the car is on the ramps. It is simple enough that I carry a spare because I can change this on a roadside if needed.

Never, never, never, never, never, ever take the battery cable(s) off a running car. The voltage regulator needs the battery as a voltage source/reference to keep voltage stable.

Without that reference, the regulator can produce very high voltages (Over 120V DC on modern alternators). You can guess what that can do to electronics.

I would be very surprised if your radio/cruise control/glowplug relay work properly after that.

Doooohh. Having problem with cruise and radio now. Do you think maybe just the regulator could be bad?? I tested with cables on and everything running with a charged battery. The power started at 12.5 and slowly dropped off. I'm pretty sure nothing at all is coming back to the battery. Question is, would a faulty voltage reg. produce these results or should I just fess up the $100+ for a new alternator w/ regulator?

Yes, either a bad voltage regulator or an alternator can do that. It's cheaper and easier to replace the voltage regulator, so that's the best place to start. If it turns out to be a bad alternator, you end up with a spare voltage regulator.

I went to autozone to have an alternator and battery checked. The goofball started to pull off my battery cables while my car was running. I stopped him before he got them loose and explained to him why you can not do that on these cars. He said it would tell him if the alternator was bad or not if the car died once he pulled the cables from the battery. I tried to explain to him that it is a diesel and that will not work on a diesel. All he wanted to do was argue so I packed up and left without giving him the $70 for the battery. I ended up going to another parts store and just buying a new battery. I installed it and everything works fine now. I dont trust anyone to touch my car but me.

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2005 Accord Hybrid (Wifes)
1995 Subaru Impreza L AWD (Snow car)
1984 GMC Sierra 1500 (Mine)
1983 300CD Best $ I ever spent. (Mine)
1984 190D (sold and glad I did)
1983 300D (sold and wished I hadn't)

Thats why he works at autozone!