My 1978 300D recently arrived and soon after left me with a dead battery. Had the battery checked at Autozoo and they said it was dead. Older MB oem battery so replaced. Drove for about a week and then the cranking slowed and the battery finally died. Multimeter says 12 volts when the car is running. Grounds are tight. The battery light typically stays on until I rev the engine at start up. Am I right in assuming that the alternator is the problem?

thanks for the help.


Kevin
1978 300D
1979 240D

i recently went through the same problem. first off never trust autozone and their machines. they said my optima was dead but i still had faith in it so i went out and bought a charger and walla it was just fine. they said my altenator was bad as well although i took it home and jumped the car and it was putting out 12+.

keep in mind a drained battery will usually not recharge efficiently or not at all from the altenator. it needs to be trickled or fast charged. i would suggest checking to see how many amps the altenator is putting out.

if your altenator is putting out the correct number of amps then i would turn off the car and put a ammeter at the battery and see if any current is being drawn while the car is off to see if you have any shorts. if you do then you can put the ammeter at each fuse to further trouble shoot your short.

__________________
83 300d euro
85 944

Thanks, I hated giving up that pretty MB battery.

Kevin
1978 300D
1979 240D

New battery dead after one week, and voltage at only 12 v? Yeah, that sounds like a failed alternater or a bad short somewhere.

Start engine and report back with voltages under these conditions:
- power drains (lights, wipers, radio, ac/blower) all turned on and engine at (a) idle, and (b) 2000 rpm
- all power drains off, engine at (a) idle, and (b) 2000 rpm.

I'll check the voltages tomorrow with power drains on but I can report that with everything off, the voltage does not change at idle or when revved (12 volts). Thanks for the reply.

Kevin
1978 300D
1979 240D

Kevin....just to be on the safe side check all your grounds. Loosen and wire brush all the grounds bolted inside the engine compartment...against the fire wall, fender wells...any that you can find. Then retorque them and loosen them a couple times to make sure they are making good metal to metal contact. I would also coat them with dialectic grease or vaseline after you tighten to keep them from coroding (sp?) again.
Just a thought

__________________
FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K

second what f18 said. also check resistance between battery ground and altenator ground.

**addition**
remember current is more important than voltage. you can have 3000000 volts (exageration) but still a very small current.

__________________
83 300d euro
85 944

Shouldn't charging voltage be more like 13.5-14.5 volts? How about voltage regulator. What is the output voltage at 1000 rpm?
Lewis

__________________
1978 300D Just awakened from a 8 year nap.
2005 Jeep Liberty Diesel Sport w/large Rectangular hole in top

Yes, it should at least 14.5 volts. I would put in a new voltage regulator.

__________________


1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

Ummm....12+ volts coming out of an alternator means that it IS bad...unless by + you mean 14 volts.

-Jim

__________________
1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.

For what it's worth, I have a 96 e300d with a scanguage attached to the obd port all the time. Immediately after starting, my voltage reads anywhere from between 13.1 to 13.7 with the lights on. If I turn on the rear defroster, it will usually stay in the 13.4 range. Hope this gives you some help.

Thanks for the help guys. Does anyone know if the voltage regulator is separate from the alternator on this model (300D 1978)

Kevin

1978 300D
1979 240D

Yes, it's most likely the brushes are worn out. Replace the module and give the brushes time to bed in. My car was starting to crank over very slowly in colder weather recently and I replaced the voltage regulator module. The brushes were worn on the old one. At first it did not appear the new regulator was working well, as my battery was not getting charged up, but after driving 50 miles or so, my battery was fully charged and stays that way.

__________________
'83 240D with 617.952 and 2.88
'01 VW Beetle TDI
'05 Jeep Liberty CRD
'89 Toyota 4x4, needs 2L-T
'78 280Z with L28ET - 12.86@110
Oil Burner Kartel #35

http://i16.photobucket.com/albums/b1...oD/bioclip.jpg

See other threads on voltage regulator replacement, especially within the last month or so. You may need an offset Phillips head ratcheting driver. At least I did. Made two swap-outs easy.

Got mine at Sears, $7.
http://www.sears.com/shc/s/search_10153_12605?vName=Tools&keyword=offset+screwdriver&sLevel=0

Good luck.

__________________
FRANKNBENZ

1985 300D "Old Blue" 265,000 (parts car )
1985 300D "Gray Ghost" 245,000
1985 300D "Silver Bullet" 160,000
1975 914 VW/Porsche "BC Car" 125,000
2010 Prius "Shocking!" 60,000
Text Message only: 301-728-1115