Replaced the battery in my 300D and went to get in the car one morning car was dead. Took back to store where I had purchased battery they gave me a replacement. Two mornings later car was dead. I replaced the alternator with a Bosch rebuilt, drove car around to a few errands at my last stop car was dead. I got a jump went home and checked the charging i was getting 12.6 volts. I again got another new alternator and installed it at that time I was getting 14.1volts I thought problem was solved. That same day had about 5 errands to run and low and behold on my last stop it wouldn't start completely dead. I know glow plug relays are known to drain the battery's on these W123 chassis but I just figured that was overnight not while the car was being driven. I have checked my ground and it is good, I have checked my chassis ground and that is good, my positive cable looks fine and I have made sure the plug on the back of alternator is secure. Any ideas on what could be going on here? One strange thing each time while I am driving my A/C stops working and usually after this happens within the next few times I stop the car it will not start. I have charged battery and I am getting 14.1 volts under load, at idle i am getting around 13.5 and with the A/C running at idle I am getting around 12.8........very irritating issue.....

Put a voltmeter on the battery and monitor it as the drive around to see if you can detect a problem. One of the voltmeters you stick in the cigarette lighter should work. A stuck glow plug relay will draw a lot of amps from the battery. You should also disconnect the positive post with the engine off and the key out and put an ammeter between the post and the cable to see if you are drawing current when the car is sitting.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

X2
You could try unplugging the GP relay once the motor is running & see if the problem is not there then.
Does the interior light go dimmer when you turn to glow & go brighter when the GP relay clicks off?

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Don't overlook the possibility of a bad connection somewhere between the alternator and battery. The crimps around the wire become loose over time.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

+1 on what Kerry said except lift the negative post and insert your ammeter there. The current does not care whether you are checking the + or - post.

Pulling the positive post while the negative is still connected is dangerous.

One slip of the wrench while in contact with the + is not good.

And of course do not pull the battery while the engine is running, this will damage your regulator.

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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

Haven't had much time to check out the car. Today stuck my meter on there under load was getting 14.3 volts, when I turned the a/c on it dropped to 13 volts. Does this sound right or could the compressor be pulling to much?