Check the bottom of you A/C compressor and see if it has refrigerant oil on it. Mine did, so I cleaned it with brake cleaner and will monitor it closely to see if I can tell where the leak originates. I suspect it’s the upper control valve which is actually on the bottom of the compressor. See the photo below.

The valve has 3 o-rings on it and is held in by a snap ring which you can see in the photo. It seems like a likely place for a leak to develop. I think it can be removed with the compressor installed in the vehicle; of course you will need to evacuate the system first.

I noticed the oil on the compressor when I was inspecting the underside of the vehicle when I changed the oil. My A/C works fine but it must have lost a little R134a and I want to fix this problem so my compressor will last as long as possible.

I happened to check the price of a compressor and was told no remanufactured compressors are available for the one in the ML which is a Denso 7SB16C. A new one is about $1000. Hopefully a remanufactured unit could be found somewhere.

Worst case would have the A/C compressor seize and have that wonderful serpentine belt will travel over the pulley on the A/C clutch which will not be moving. That may very well equal shredded belt. Which means your stranded.

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The valve has 3 o-rings on it and is held in by a snap ring which you can see in the photo. It seems like a likely place for a leak to develop. I think it can be removed with the compressor installed in the vehicle; of course you will need to evacuate the system first.

I noticed the oil on the compressor when I was inspecting the underside of the vehicle when I changed the oil. My A/C works fine but it must have lost a little R134a and I want to fix this problem so my compressor will last as long as possible.

I happened to check the price of a compressor and was told no remanufactured compressors are available for the one in the ML which is a Denso 7SB16C. A new one is about $1000. Hopefully a remanufactured unit could be found somewhere.

Worst case would have the A/C compressor seize and have that wonderful serpentine belt will travel over the pulley on the A/C clutch which will not be moving. That may very well equal shredded belt. Which means your stranded.>>

Ron,

I believe you will find the oil leak is originating above the AC compressor. On my 98 ML320, I found that small cover with the allen socket screw in the center, was leaking engine oil. This is on the front of the engine, driver's side. The O-ring that seals it to the block was hard and ineffective.

An interesting aside, this cover only leaks when the engine is running. When the engine is shut off, I believe that this cover leaked air into the engine block, allowing the engine oil to drain back to the pan. Some of the startup clatter that many of us hear is due to lack of engine oil in the lifter passages.

I believe my engine got noticably quieter, on a cold start, after I fixed this leak. Just my opinion, YMMV.

As to your comment about the AC compressor siezing and frying the belt, Mercedes has had a speed monitor on their AC compressor since about '87. The compressor clutch is immediately released if the ECU notices the compressor running at a speed less than normal. Some who have washed their engines have experienced AC compressor lockout due to wet belts. Turning the key off resets the lockout, asuuming the compressor has stopped slipping.

Regards,

Jim in Phoenix

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Jim,

That's a good lead I'll check that area carefully. Right now everything is clean so I can not say for sure where the oil that collected on bottom of the compressor orginated. It did look like compressor oil, as it was quite clear looking and uncontaminated. Maybe it was power steering oil, but I checked the level and it was right where it should be.

As far as the Mercedes putting that speed monitor on their AC compressors, that good news. For me those serpentine belts are nothing but trouble. I prefer to change and adjust individual belts myself. And should one break you change can it on the side of the road yourself with minimal effort and very few tools.