Hey there everybody-
The 96 E300D had a series of symptoms that were finally rooted to a failing OEM battery. I will describe it all here to hopefully save some other poor soul some time.

Please don't flame me, but I have to point out that yes, it's true, the dealer-sourced OEM battery that was 3 years old and came with the car crapped out. And NO, I'm NOT forking over another 3 bills for another one of those....

First the symptoms...
There has been no problem with the battery, no idiot lights, no slow starts, no issues at all- (read no fore warning!) So one morning, it is 25 degrees out, but she glowed just fine and started as usual. The wife ran her in the garage to warm it up a bit and headed off to the truck stop for a fill up. She parked the car, filled it up, which took several minutes- and when she got back into the car- all the idiot lights lit as usual, she glowed the plugs, and as she attempted to crank the starter- it moaned, the dash lights went out and it would not crank. There was no battery light on the dash, and nothing to indicate what was failing to function.

Oddly the red light on the rear view blinked during the start attempts, but the green light didn't light up. (I searched our forum for hours for this and never found anything about it. I eventually gave up and decided the voltage drop when she attempted cranking was just freaking out the anti-theft circuit)

After several minutes she tried again- with the same result- the dome light came on as usual- and was not dim, the blower came on when she put the key in "on" position to glow the plugs. The glow plugs cycled and after the lamp went out she tried to crank the starter and it either moaned slowly, or just clicked, the red mirror light blinking away.

After more than one hysterical phone call, and remarks made by people at the filling station (the diesel pump is between two "regular pumps" so the dead car must was blocking the diesel and a regular pump- which must have been the most desirable of all 6 pumps at this station or something...) I checked the battery with a multimeter- 11.? volts, so I tried it myself- same thing. I then tried charging/jumping it with the 190. No luck. By now we were freezing, tired, frustrated and she was an hour late for work.

So, I drove the wife home, got her in another vehicle and returned to the dead 300d. This time, I was armed with another battery- but it had sat in a truck that had not been driven in over three weeks. I tried jumping- no luck, same symptoms, and then gave up and replaced the battery under the seat with the alternate battery- and the symptoms were replicated again, and again, and again. So- the towing service was called, the e300 went to the shop and I trolled on the forum for hours until I had to give up and go make a living. I called back to the shop and told the manager about weird battery symptoms, starter solenoid probs, the K40 crap relay that has plagued these cars and caused all sorts of weird glow plug, fuel and starting problems, an issue due to broken battery leads and etc. He said he'd follow up.

I have now heard back from the mechanic. He trickle charged both batteries overnight and then load tested them. The big Mercedes $300 job read "ok" initially, but as soon as a load was put on it- it dropped to just over 7 volts. Not nearly enough to crank a high compression diesel motor in a car with too dang many little electrical things. Next, he load tested the suburban's battery- and saw that it too had a big drop- but not as bad as the Merc battery. He told me he dropped a new known good battery in it, and it drove fine and started without problems 6 times. So he told me to bring him another battery and he'd drop it in. He said he could get me another battery from the local MB stealer- if I "really wanted him to do that," but I told him I'd like to shop around.

I made some calls and it seems my choices for off the shelf batteries are pretty limited in my area. Interstate makes a battery that would work- but it's $150+ and a "special order" that takes "days" to get here. That model is also "probably" made by Johnson Controls and differs little from other JC batteries. It also seems (not absolutely verified- but pretty sure) Johnson Controls makes the H8 batteries for both Autozone and Advance- the difference between them being the decal that is slapped on. (Even the warranties, CCA, dimensions, and cases are virtually identical.) Exide is apparently still making batteries for walmart (according to the guy I talked to there)- and I've had mixed results with those. Optima is another option- but the report I got on those is that they haven't been any more reliable beyond 2 years than other batteries- which hardly justifies the cost, and again would be a special order....

So, I went online and found Advance had a coupon for online purchases- that knocked the price on their "gold" 36/84 month battery down from $138 to $113 with a $12 core fee. Supposedly I'll get another $50 coupon in an email later on too... we'll see. If I can replace the battery for $100 that might help pay the bill at the shop for diagnosing the issue...

I plan on replacing the k40 relay in the future to avoid future problems, (much like the insurance of replacing a "main relay" in a 90's model honda) but I am keeping my fingers crossed this does "fix" the issue. If not- I'll be sure to follow up here. Cheers- J

Update-
Picked the car up yesterday and drove it all day. She glowed and started without fail all day long. It seems the battery was providing enough juice to do everything except provide enough amperage to crank the diesel's starter. As an aside- the mechanic indicated a number of problems on "newer" cars whenever there is a problem with the battery, alternator or some aspect of the "charging system." He said that Dodge minivans, and Mercedes SLK models, among others might fail to shift properly because the transmission sensors don't see proper voltage to upshift as a result of the charging system "issues."

He told me about the BMWs and Mercs with "backup batteries" these days- in order to continually provide electricity to the computers in them. And told me some horror stories about computer replacement following relay or sensor failure, and reprogramming issues, etc. I honestly think we may be entering a time when our vehicles are really more "technologically advanced" for their own good.

I went to the Chicago auto show yesterday- and saw all sorts of new bells and whistles on 2012 and 2013 models- and kept thinking "I wonder how much electricity that must draw?" and "Geez, is that really necessary?" and "Good grief- that battery is tiny! I wounder if you could get 4 years out of it..."