124 AC Evaporator Replacement
My 300TE and I suffered from a leaking AC evaporator, as many Mercedes seem to do. About a month ago, I changed the evaporator and converted the system to R134a. This note is to dispel some of the discouraging information that I have read here at the Shop Forum and at other bulletin boards concerning the difficulty and results of this particular repair job.
Let me start by saying that I have been a full time auto technician in my lifetime, but that was over a decade ago so it would be fair to say that I am somewhat "out of practice and out of shape" when it comes to auto repair. I have read in other posts that the job of removing and replacing the evaporator on the 124 Benz is best left to a professional. Of course the judgement is subjective, but I found that the repair took just an average amount of mechanical skill; the trick is to be organized and patient. You will need the factory repair manual. Lots of tie-on labels with notes for reassembly make up for a failing memory. The job will take a fair amount of time, I took three and half days with alot of lemonade breaks and trips to the parts store.
The old evaporator was, indeed, leaking, but oh so slightly. It would typically take about a month before I would notice the system beginning to lose its charge. I had added R12 with dye to try and find the leak before taking the system apart, but never could locate the source; once I got the evaporator out, I could see why I never would be able to find it. The juncture between one of the small pressure bleed tubes and the evaporator core had the slightest tinge of dye; there is a copper to aluminum interface at this joint on the original evaporator. Fortunately, it looks like the new evaporator is improved by using all aluminum construction.
The R134a conversion was a breeze. I used an OEM all aluminum part because it has a superior heat transfer rate, and that is probably needed for the R134a. In addition to the evaporator, I changed the receiver/drier and all of the orings to R134a friendly pieces. While I had the lines disconnected, I flushed the condenser and pump with AC flushing solvent that is sold in an aerosol can.
For reassembly, I decided to use ester oil instead of PAG because it is less fussy about contamination with mineral oil that might remain in the lines. Mercedes says to use 36 oz. of R134a refrigerant and 4 1/4 oz. of lubricant, so that's what I did. I borrowed a portable vacuum pump to pull a 28"+ vacuum on the system for about an hour prior to recharging it.
The results are fabulous, the AC duct temperature is colder than it ever was with R12, and even colder than my much newer Saab. I attribute the improvement not to the R134a, but to the fact that the original evaporator fins were coated with deteriorated foam from the recirculation flap, which prevented good air flow across the coils.
The total cost of the job was $320 and one lo-o-ng weekend. For what its worth, I consider it time and money well spent, especially during this brutally hot summer.