I recognize this thread is a little old, but I was wondering if anyone could tell me which wire is the one to attach the diode to? Car is a 2008 R320 CDI. I know this is a 164 thread but hoping to be able to figure this out. I have attached a picture of what my harness looks like.

 

@edwardsgw , my wire was blue/green but another similar thread references a 2008 GL320 having a purple(violet)/black wire. Perhaps that is what you have, as I see a purple one in your picture. You may have to trace it coming out of the compressor.

https://mbworld.org/forums/gl-class-...-9006-a-2.html

Good luck!

 

I saw the reference to those colors; in fact the wiring diagram for my car says it should be violet with black stripes. I separated the connector at the AC compressor and did resistance readings to confirm which wire. The wire leaving the compressor connector was solid green (the wire on the compressor itself was red) and it is the solid green wire shown above. This is for a 2008 R320 CDI. This doesn’t match anything I’ve seen.

 

Thank you for the information. My electric AC compressor does not want to turn on. Dealer wants to charge me 7k to replace the entire thing. I really feel that the compressor is fine and that its just not receiving power. Would you be able to get me my wiring diagram? Thanks.

 

[QUOTE=Barium;7511166]Hello everyone - long time reader, first time poster.

I picked up a 2009 GL320 two months ago and the AC compressor failed after 3 days of ownership. Being cheap I set out to fix it myself. Diagnosis showed that the AC compressor was actually locked up and thus, at a minimum I needed to replace the compressor in order to pressurize the system to find other potential leaks etc. I bought an aftermarket (but new) compressor from Rock Auto. Install went fine but once hooked up I kept getting an error (P9006 AC compressor short circuit). The cabin fans would sort of cycle up and down as the ACC repeatedly tried to start the compressor but to no avail - the AC simply wouldn't work. I hot wired the "refrigeration/AC control valve" with an external 12 V power supply and the AC system worked like a champ (super cold). So off to the forums I went. I read a bunch but the most common story was that aftermarket compressors simply don't work and I would need to buy the Denso branded version. I also read that I needed to complete and "AC compressor adaptation" via SDS which I don't have. I wasn't satisfied with these answers as I really didn't want to empty all the R134a out of my system again and I had a hard time believing that AC compressors were really different enough to need "adaptation." Luckily, I finally considered replacing the AC control valve with the control valve in the cratered compressor I removed. In doing so I pretty much destroyed the old control valve. Here is what it looked like after removal and non-reversible disassembly:


AC control value disassembled.

What I found out via disassembly is that there is a diode in parallel with the solenoid winding. This diode effectively clamps the reverse polarity voltage present when the coil is discharging (i.e. turned off). This made me wonder if, for some odd reason, the compressor I bought didn't have a diode in the same location. A quick visit to Polar Bearings excellent "RCV" website showed that there are two visually identical parts but one has a diode and one doesn't. Why Audi, BMW, etc. don't use a diode whereas MB does is beyond me (thanks for making these cars hard to work on!) but I was sure this was my problem. I dug out the diode from the disassembled control valve:

Note: I used this crappy diode because I didn't have a better replacement handy (with Radio Shack going out of business and all electronic parts are always a few days shipping away). I am quite sure that any schottkey diode (e.g. 1N5817) should suffice.

I then soldered in the diode to the spare wiring harness I had on my cratered compressor:

And here is the final form before installation:


Final form with heat shrink
Note - the big round crimp terminal is to provide the necessary ground - the stock ground connection is inaccessible without removing the compressor which I didn't want to do.

I installed the modified wiring harness, zip tied the old wiring harness out of the way and fired it up. It has worked perfectly for the last week, no more errors, no need for "adaption", no requirement to buy a "Denso" branded compressor.
Obviously, the "right way" to fix this issue would be to install the correct "with diode" control valve - but again this requires removing all R134a and somehow getting the control valve out (I had a very hard time with my "scrap" compressor).

All this trouble is caused by the lack of a 5 cent part. It took me quite a while to figure this out so I'm hoping that this post helps some struggling with the same or similar issue. I assume this fix would apply to all GLs (the early ones at least).

Best[/FYI Diode 1N4004 worked ok in my W211 and had no codes. Also harness was not for the 211 application, i had to add ground for the control valve.