[befco]

Can someone please tell me what these two relays(?) under this black box are for? I have a drain on my battery, and I traced to this, cut a wire and battery drain has stopped. Car runs and seems fine. It's a 1978 300D, picture attached.
Any info would be greatly appreciated, thanks.

 

[mike-81-240d]

Aux cooling fan, and horn relays.

 

[befco]

Aw, excellent I'm glad I didn't drive it to far.
Thank You!

 

[MBeige]

Aux cooling fan, and horn relays.

The horns have no relays.

That box contains the AC compressor and auxiliary fan relays.

 

[befco]

OK, so the AC compressor, but is the cooling fan for the radiator?

Secondly, if I replace the relay is that the likely battery drain culprit?
Thanks.

 

[MBeige]

OK, so the AC compressor, but is the cooling fan for the radiator?

Secondly, if I replace the relay is that the likely battery drain culprit?
Thanks.

The auxiliary fan is for the condenser (electric fan in front of condenser), not the main engine fan behind the radiator.

If the relay is bad it could be stuck on but it should not cause a drain. Use a test light on the fuses to check which component/s causes the drain.

 

[befco]

OK thanks on the Fan question.

On the power drain I put a test light between the Neg. post and battery clamp.
Then pulled each fuse 1 at a time. All no change on the light going out.
I pulled the cover from the 2 relays and when I bumped a red wire & the test light went off. That's how I stumbled on the 2 relays under the black box.
Is there maybe something wrong with the AC Switch in the car?

 

[MBeige]

AFAIK there should be no red wire in that location. Those are supposed to be factory wiring. Do you have a pic of this red wire? It could be aftermarket, and you'll have to trace the wiring down.

What wire did you cut?

 

[befco]

Oh, ok, that makes sense, the wire is black until the splice, then red to the inline wire linked fuse to the relay, that, is the suspect, not stock.
Here's the pix

 

[mike-81-240d]

Most definitely has been tampered with.

 

[befco]

Gud dung it, OK, ok, I see I have my work cut out for me.
If I leave the red wire unconnected worst case is no AC, and or AUX fan on the AC condenser, correct?
WTF is that new wire for any ways, an upgraded AC pump perhaps?

 

[MBeige]

Actually I've been trying to search for a thread I found a few months back regarding a wire similar to what you have. It may very well be a factory wire because the red wiring goes all the way into the insulation. No way would aftermarket wiring go that deep.

That has to do with the compressor fuse (black connector probably has a fuse inside). It's possible something is giving power to your fuse. I've been searching for 30 mins but couldn't find the thread, but I do remember that's a factory unit. I was thrown off by the photo you posted because that's an '85 300D, this wire was probably on the older models.

It's possible you could have a stuck relay, loose connection or a short somewhere, providing power to the compressor unnecessarily.

What happens if you swap the ice cube relays and you put the key in position 2? If the auxiliary fan turns on the relay for the AC compressor is stuck closed.

 

[mike-81-240d]

It may very well be a factory wire because the red wiring goes all the way into the insulation. No way would aftermarket wiring go that deep.

Last I checked Mercedes Benz does not use cheesy fuses, electrical tape, and crimp butt connectors.

 

[81240D]

how bout a engine block heater ?

 

[befco]

I switched the "Cubes" but it made no difference.
I'll have to check the block heater idea tonight.
It just seems strange the black wire going into the wire cluster is drawing power full time,
Maybe it's a bad A/C switch?

 

[MBeige]

Does the AC work? Does the compressor run even if the climate control says it should be OFF?

 

[befco]

I'm thinking that is the whole problem. When I bought the car there were 2 problems. 1-battery drain. 2- AC was not working .
Know I have found the battery drain is in the AC relay, I clearly see the AC is the problem.
Not so clear is the solution...
I'll have to check the wires on the AC pump 1st off.

 

[MBeige]

I believe fuse #8 is for the AC compressor (double check), if this fuse tests fine (no drain detected) then the problem is probably downstream.

As a test, you can connect the test light and pull switches or connectors related to the AC one by one until the test confirms the drain is gone.

The compressor should also have its own lead. It is activated by the pressure switch on the AC receiver/drier (the other switch has a pigtail connector, this is for the auxiliary fan).

 

[befco]

OK, I'll check all that out, many thanks for all the tips!

 

[andresq007]

has anyone found the solution for this? I have the exact problem on my benz.