One for the AC experts.
 

The AC compressor on one of my 420's is begining to make a little noise. Neither of my other two cars make any noise from the compressor. They all have the Nippondenso 10P17C compressor. The noise is not bad but I can hear it when it turns. I never was able to do that before. I am assuming that the compressor is getting noisey because it is near the end of its life.

So here is the question;

Should I replace the the compressor before it fails or wait till it fails? The fact that it is begining to make noise an indication that failure is iminent?

My thinking is that if you replaced the compressor before catastrophic failure occurs, then the system will not be subjected to having all the debris that the compressors shed when they do finally fail. So it seems like it would be better to change it before it does fail and save yourself all the hassle of flushing the entire system. On the other hand, it might not fail for a very long time in which case you have wasted a lot of time and money. The biggest job in flushing these systems is removing the expansion valve so the evaporator coil can be flushed. The rest of the system is very easy to flush. If I wait till the compressor fails, then a complete and thorough flush is mandatory.

Anybody have any experience or opions on this subject?

You can try adding lubricant to the system to see if doing so quiets down the compressor. It is possible for these things to lose oil out the compressor front seal while (pretty much) retaining the refrigerant charge. I lost a 10P15c this way...

A good shop can add oil without discharging the system. They will have a special tool which holds an ounce or two of oil. Fresh refrigerant is then used to push the oil into the system. If this quiets things down, you've found a new lease on life... Have them inspect the system for signs of oil loss. A front seal leak will dribble oil out the bottom of the compressor pulley and back along the body of the compressor.

Be certain the correct oil is added. If you are still running R12, then mineral oil is correct. If converted to R134a, then you have either PAG or POE. These two don't play nice together, so you need to determine which you have and add only the same.

I am personally far from an expert on AC issues but I did post a similar question on the Ritter/Easley list. Nippondenso compressors can and do self destruct and cause "black death" from Teflon particles pushed throughout the system. The wisdom of the other list was that when a Nippondenso starts to make noise, time to consider replacing. They tend to go out after 150,000 miles.

The problem with black death is that you need to replace, not just flush, virtually everything forward of the firewall -- condensor, manifold, etc, so not a trivial expense.

Apparently MB is now selling rebuilt Nippondensos. Were this my car I'd save my pennies and replace the compressor, and not use it in the meanwhile.

Here is a post that Stevebfl wrote, where he talked about Nippondenso compressors and black death:

http://www.peachparts.com/shopforum/showthread.php?threadid=40718&highlight=black+death

If the compressor goes, it might be impossible to flush out the system.....