I have no ground at my AC compressor on my 300 SEL. The cable doesn't work.
Would you run a new cable or try and find the problem of the old cable?

The compressor doesn't have one. the clutch should only have one wire. Pull the clutch apart and see if had two wires. You may need to get a new coil for the clutch if one wire has broken off just in case it has broken off. There is a special puller required to get the clutch off too.
Normally they only have a plug to connect the compressor to the circuit.

Hi Sir,

my compressor plug has two leads, I guess one ground and one +12V.
In addition the compressor body has an extra ground cable for whatever reason.
Both ground cables are not working. There is no continuity from ground cable to Battery -. My take is that I need to replace the ground connection.
Any thoughts ?

How do I know? I have +12V at the Plus lead when I turn on the AC fan with temperature knob at low...but no ground at the second lead.

Then it's simple,you need a new clutch coil. The compressor is normally just grounded through it's body to the engine. if you have no ground,check the braided earth strap on the left rear of the engine block ,but it's unlikely to be a problem as the engine will be difficult to start without a circuit there.

Thanks Mercmad

Just to be clear. I have no compressor installed yet as I am going to put a new one in (hopefully with a good clutch). I am just checking the cable connection.

You are saying that ground is existing anyhow through the bracket/connection to the frame...true... but why is there a separate ground cable for the clutch if this is not needed?

Martin,
I have 4 of those compressors and clutches. if you need.

I am not an electrical engineer, and I have noticed the extra (over-the-edge) grounding the MBZ engineers do. Such as the fuel injector wiring, a separate ground from each injector to the chassis near the firewall and under the battery. Aircraft are done in a similar manner.

Better to have more grounds than less.
I do pay attention to clean all grounds as they complete the circuit.

The motor mounting schemes and many rubber mounts are in the paths of the grounds hence the extra straps and the extra wiring.

The extra ground is to help prevent radio noise and prevent a ground loop being formed which would transmit static to the cars radio.

Try replacing the grounding strap from the engine to the body. They are cheap and break down inside so there is no fixing them.

Also clean to bare metal the engine and the body where the strap connects. You don't have to take a wire brush to the entire area, just clean it up a little.

The worst that could take place is you will be out a few bucks and a little time, but this is something that needs doing every 30 years or so anyway.

Werminghausen, the double lead connector you have addressed does indeed contain a ground connection; and in some versions made for MB it also contains a diode!! That diode can have failed, or if you are checking for continuity with your meter connected in only one polarity arrangement you will observe a (false) lack of continuity. Try changing polarity.
Frank

Edit: The ground circuit goes to terminal 85 of the A/C system relay, and then to the grounding tab of the blower switch.

Hi Frank,
what I did was connecting my test light to the 2 leads of the compressor plug (I don't see a diode there in the small plug)
No light in the beginning with the engine running, AC fan on and Temp know down. Then I tested +12V at one lead and indeed there was +12V.
But ground was not working...(so no light ). I connected the ground cable for the test light at the body and sure enough there was light!

I was thinking to replace the ground cable of the compressor plug. Not sure where the cable is starting. But does it matter? All I need is ground...correct?

If I understand you correctly, your test light contains its own internal power source (battery). If so, it is a continuity tester. However, if it is not self powered, then it is a voltage tester.

If it is a continuity tester, then connecting one lead to the body and the other to the ground lead of the compressor plug should show continuity (light on). Is this what you were describing in the second part of your test sequence? If you are not able to establish continuity to ground through the plug, you may intercept that wire (brown insulation) at any convenient point and run a new wire to ground.

Will you be using a Frigidaire compressor? Those had two connections emerging from the clutch coil, hence the two wire connector.

Hi Frank
Yes I am using the firgidaire compressor with the 2 connectors
My test light has no internal power source. It is self made: 2 cables and a light bulb with a fuse.
So I connected the 2 cables of test light at the clutch plug connector (2 leads) and nothing happened (no light).
This test simulates if there is power to the compressor clutch.Test was negative.
Then I tested the 2 leads of the plug separately: I found +12V is working fine but there was no continuity to ground.
When I connected the minus cable of test lamp at the frame (with the +12V cable still connected to the plug) the test light went on.
It means I have power at the clutch if I replace or repair the ground cable to the plug.

I hope this clarifies the test.
Martin

Clearly you are on your way to the solution. At the risk of being redundant here is the edit line of my post of 14 JUN:

"Edit: The ground circuit goes to terminal 85 of the A/C system relay, and then to the grounding tab of the blower switch."

If the ground circuit at the relay is open (between terminal 85 and ground)
the whole A/C system may not work. Your previous posts, however, suggest that you have power at other points (clutch control, blower), which would indicate that 85 is indeed grounded.
Frank

Update :
I was installing the new compressor today and now I saw that the 2 prong connector (Plus and Minus) to the compressor is picking up ground (Minus)directly from the steel frame of the compressor with a separate cable. The Minus port of the 2 prong plug has a cable screwed to the compressor case....silly I didn't see this before.
The compressor clutch is seeing 12V when I turn the AC fan on....the compressor clutch should work. I am happy now.

Martin