4 amps sounds like a lot of current to flow through a diesel-soaked wire laying against something grounded. Based upon your prior investigations with pulling all the fuses, it would have to be in an unprotected circuit as well.

Do you have any aftermarket electronics that may have been installed with a direct connection to the positive battery terminal?

Do you have the wiring diagrams for this car? Are there circuits that have fuses or breakers that you haven't yet eliminated?

Just looked at those pictures in an earlier post... I'd suspect the condensor (no. 38), or the starter. Have you disconnected the starter motor?

I have disconnected the starter.

No aftermarket electronics.

I have not found that stinking condensor or connector #29.

It is up to 5.5 amps now.

Battery is unhooked and I am driving a back up car (99 Maxima).

-Jim

Man, what a frustrating problem...
Keep trying, it'll be that much more gratifying when you get it licked.

Four or five amps at 12 volts is 50+ watts. That will cause some heat. Do you know anyone with a thermography scope? You can rent those in some cities.

Hmmm.... Glow plug controller? It's not fused at the fuse box....

I have not tried that yet. It would have to be on the wire leading up to the controller and/or the hot side of the controller itself. That could be a winner. Even with the battery fully charged and the charge booster attached to the battery it is hard to start when the engine is cold.

Almost like the glow plugs are not working.....;)

Starts great once the engine is warm.

-Jim

Found it! Aux Fan Relay!!!

Back on December 19th I came out of the hotel to go take a test and found the battery dead on the car. When I jumped it off to get it started the aux fan started running as soon as the cables were hooked on. By the time I got home that night the fan was not doing that any more.

This spring the ground came in, or at least I noticed it finally.

Once the weather warmed up I noticed that I was having temperature problems.

In April I bought a truck and started driving it every day. I forgot about the car for a while.

Yesterday I was filling out some paperwork and had to look up the day that I took a test, the day the fan ran all night and killed the battery. It all came rushing back...fan running on its own...fan fixes itself...temperature problems...ground... A coil on the Aux Fan Relay shorted itself to ground!!!

.01 amp draw with the engine off and the Aux Fan Relay removed! Life is good!!!

-Jim

Yeah!
 

'Nice save!


EDIT:
I've gone Off the Planet again /somehow converting your W140 s350 chassis to
a 1987 300D W124.

Corrections to make below also!

I'm not sure which one it is. It is the one pictured when you look up the Aux Fan Relay at Fastlane. MB part number 001 542 82 19.

12Vdc 40 Amps. Decent sized sucker. Circled in Red in the photo.

-Jim

Yes,it's a "Big Ole Honker"
 

You'll notice it has 8 Poles
(Quantum leap here...That is your Single Auxiliary Fan Relay...it does both Low
and High Speed relay duties.)

'90-91 OM 602
 

Gets two Auxiliary Fan Relays:

(on the '90-91 [OM602?] K9 [the High Speed Fan Relay]"Load" side is "Straight" wired to X4/10 with No Fuse In Line.)

The original Relays were not fused,
and at some point the part number superseded to the current Relays both of
which are fused.

1.001 542 96 19 (K9 High Speed Auxiliary Fan Relay) [With 30 Amp Fuse]
[This fuse is the "Load" side of the K9 which is wired directly to X4/10
Which makes this circuit "HOT" whenever the Battery is connected]
2.002 542 01 19 (K10 Low Speed Auxiliary Fan Relay) [with 15 Amp Fuse]
[MB says you may upgrade the 15 Amp Fuse to a 25 Amp Fuse]


If you take the time to follow the developments of these relays circuitry
and fusing onward you can visualize the areas that
caused dangers with: Dead Shorts , Etc.

Wow! I never went back and updated what happened. It has been 3+ years now since I had this problem. I finally identified which relay was the right one. My foggy memory says that I had to remove the fuse tray to get to it. I bought a new one on eBay from some guy in the northeast and put it in. It has worked great with no issues. Easy fix once I figured out which one it was. The problem is that I don't know what to put in here to help the next guy out! (I've forgotten pretty much all of the details.)

Jim