I have had occasional drain of my battery over the last 7 years of owning this car. If the car sits for 2-4 days, the battery will be down to 9-10 volts and unable to turn the engine over (even with a new battery).

To monitor the situation I have a voltmeter installed in the car. If left overnight the battery voltage is usually around 11 - 11.5v when starting the next morning (key on after fuel pump shutoff).

The charging voltage is 14.2-14.3v, so it appears the alternator is working. I had the alternator tested several years ago and they said it was OK. One issue with the alternator is that sometimes when starting the car (usually when hot) the alternator isn't immediately working (reading is 11-12v) When the engine is revved up the alternator kicks in and voltage stays up when going back to idle.

Occasionally the radio will stop working (head unit works but no amplification) and more recently both the radio and door locks quit working.

Today I am trying again (I think for the 4th or 5th time) to figure out where the drain is coming from. I have reduced it to something on either fuse C or fuse 9 and I have eliminated some components.

I noticed a relay click when I removed fuse C. The draw at fuse C is 12ma. Probably this is the current needed to keep the relay closed? Not enough to drain the battery so I leave fuse out and move on.

With fuse C removed, I notice another relay click when fuse 9 is removed. The draw at fuse 9 is 300ma!? Also the dome lamp will turn on (with doors closed) but will not turn off.

I would think that there should be no relays clicking when any fuse is removed?

The relays A and F are the ones involved. If I remove both of them, the draw goes away at fuse C. If I leave one of them in, the draw persists. I can now replace fuse C and fuse 9 and most everything works except for power seats, windows, and sunroof.

Now here is the weird part. If I put it all back together (relays, fuses) and check draw at the battery I read 25ma, an acceptable draw! The radio and door locks work too. So somehow the fiddling I did (removal and reinstall of a few fuses and relays) caused a change.

Since the dome light comes on, it seems like something "thinks" the doors are open? (with fuse C removed, otherwise it works). So I disconnected the front door switches, that did not help.

I also disconnected the warning control unit and anti-theft unit and no change.

So based on all this, I think I have a short that comes and goes in the door wiring. Since the door wiring consists of power seat and power window stuff (which is what relay A and F are supposed to be for), it fits.

Am I on the right track here? Any comments much appreciated.

Try disconnecting the plug/wire that connects to and powers the auto antenna. I did and my battery stopped draining and my courtesy door/puddle lights started to work properly.

I have no idea where the glitch actually is, but this worked for me.

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87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)

Convenience relay. Pull it overnight and check battery in the morning.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg

12 ma won't pull a good battery down to 9 V in 4 days. Use a clamp on amp meter and probe around. Check the alternator to battery wire for current flow, you might have a positive diode starting to fail. I chased a similar problem on my Ranger of interment battery drain, changing the alt fixed it ( changing the regulator didn't )

Also, I've had electrolytic capacitors leak causing electronic circuits to turn on.

I have had the power antenna disconnected for a while, problem was it randomly stopped going up/down. Seemed like a loose connection or something so I disconnected it and forced it to stay up.

The convenience relay in the manual corresponds to relay F I mentioned in my post. Perhaps you are referring to the Convenience Module under the back seat on the left side?

I agree 12ma (or 25ma) shouldn't be enough to drain the battery. I have a clamp on but it is tough to test every wire without ripping everything apart (like the door wiring harness for example). Any advice on that aspect?

I think it must be related to something other than alternator since it is causing interior equipment to malfunction (dome lamp won't go out, radio amplifier won't turn on, door locks don't work). Then after I pulled the fuses and replace them, magically these things start working again.

Again, the really odd behavior here is that there is some interaction between fuse 9 and fuse C. With fuse C removed, fuse 9 pulls 300ma, and only quits after I pull both relay A (power seat relay) and F (convenience relay). Then somehow it goes back to normal after putting the relays and fuses back? Does this sound like a bad relay or a short somewhere else?

Try these. ( I haven't had a chance to look these over. )

http://diakom.ru/CAR/Mercedes-Benz/1994/E420/SYSTEM%20WIRING%20DIAGRAMS/


http://diakom.ru/CAR/Mercedes-Benz/1994/E420/SYSTEM%20WIRING%20DIAGRAMS/4995.pdf

yes, forgive me...convenience module.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg

Thanks everyone for suggestions. Today I have tried disconnecting the convenience control module (CCM) (under the back seat), the radio amplifiers, and the vacuum motor for door locks.

Before I started, the door locks and radio did not work. Also I found that the rear door switch and trunk switch would activate the dome lamp as well as the front doors, but the rear dome lamp would not activate. I also found that the trunk switch and hood switch would cause a 200ma drain to occur (with the trunk lamp disconnected and dome lamp switched off). After closing both switches the 200ma drain goes away and I'm back to the 25-30ma drain.

After removing each connection to the CCM I checked the meter and 200ma drain remained. Then I removed the vacuum pump connections, no change. Then I removed each of the two amplifiers, no change. (note the antenna has always been removed).

Next, I removed fuse C and fuse 9, then put them back in. Voila, 200ma drain goes away, door locks work (after plugging in pump). Power seats, sunroof, dome lamp work. Power windows do not work, should they work with CCM disconnected?

So the question remains is what could pull 200ma? Not a bulb, has to be a controller right? Could it be the anti-theft unit? I had it unplugged last weekend and there was no change.

For now I will leave CCM disconnected for a while to see if the weird condition comes back (door locks stops working).