Very Rapid Battery Drain. C280
 

Hi,

after much work already - can anyone advise plz.
I have 1999 C280 W202 (112 engine) in excellent condition except for battery drain. I've searched posts for ideas and learned plenty but not found solution.
The battery charges fine and holds charge when disconnected. When connected I measure about 5 amps draw at the battery terminal. I've disconnected all the fuses from the three boxes very methodically. Have disconnected the alternator (both terminals), and still get the same current draw. I've been inspecting cabling and not found any problems yet. I measure only a few Ohms resistance between X3 terminal post and local chassis earth (with alternator disconnected)- so there must be a near short somewhere. I'm suspecting faulty capacitor C2 in X4 under drivers footwell carpet. Like I said - I think I've eliminated most other things.
Could you suggest where I might look, what I've overlooked, and how to test,
thanks.

I have one of these:

MASTECH MS2108A 4000 COUNTS AC DC Current Clamp Meter Backlight | eBay

It is what I would use to trace down the current since it has a DC clamp on. There are lots of clamp on current meters but most can only do AC. This one can do AC and DC. With this you could follow the current to its destination.

I don't have any bright ideas where all that current is going. You might try disconnecting the alternator to eliminate that.

Disconnect the large wire at the alternator and see if the draw goes away. ( the big one that leads to the battery) A shorted diode in the bridge will do this, my 97 C280 had same problem.

Also remove and check the small alt wire for power on the wire, it is switched power that tells the alternator to wake up. If the wiring has been hacked or a aftermarket radio installed / wires bridged, the alt can stay turned on. With the wire attached,key off, put something steel against the pulley and check for magnetism. If it sticks the alt is staying on.

A clamp on amp meter is very helpful in finding drains.

Thank you - I first replaced the small alternator cable (D+) without the large one (B+)
and did not notice any significant current draw. I then replaced the large alt cable (B+) as well, and did not notice any large current draw. I did the magnetic test and did not feel any attraction. I then inserted the ignition key and turn to position 2 (but did not attempt to start the car) to look at warnings and hear buzzing from relays etc. At this point the alt is energized (by the magnet test). I then turned off the ignition, withdrew the key, but found the alternator still energized. So this is presumably the sink for the current. I have already replaced the regulator in the alternator having suspected a charging problem when I first had flat battery. Where should I go from here?
p.s. have ordered a current probe. I inspected C2 in the footwell and did not find any issues there. Where is the alternator controller if it's not the alternator itself?
thnx

The small cable on the alternator ( D+ ) is what turns on the alternator so it starts charging. Connect a test light from D+ to ground and watch the light result. Try this with D+ attached to the alternator. Remove D+ from alternator and watch light result. If D+ stays lit when disconnected from alternator and key off / out, there is a problem in the car wiring.