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.

[J.HIDALGO]

I hope you changed all your vacuum actuators while you were there. It will save you having to take the dash out again.
I also did mine and I am NOT a mechanic. Just a simply human DIYer that likes to work on MB's.
It is a hell of a job but "doable". I also converted to R134 and it is not bad for the Florida weather.
Congratulations on the AC job!

Nope, I didn't know about vacuum actuator failure on Benzes. The second time doing a job is always much easier.

Jag Guy

[John Plut]

Excellent post. It answered my question on how the evaporators fail.

I happen to have the same opinion that if you are careful and patient when doing repairs, the "difficult" ones are less troublesome. It also helps to have an extra car so you are not without transportation for three days.

- John

[Cap'n Carageous]

A freind of mine owns a shop. He is replacing an evap in a Jeep Cherokee. He said the total bill would be $500. Parts and labor!! I'll bet he won't do mine that cheap!!

[stevebfl]

I can tell you that a Grand Cherokee is almost as difficult as the 124 evap.

After maybe a thousand conversions, probably at least a hundred on 124 cars, I don't have the momentary luxury of glorious results. I'm happy that John's system is working well, but from vast experience I can say that yes, the reason the new system appears good is the poor performance of the old system - not the 134.

I would also say that ester oil is not the way to go. I agonized over this for a long time as ester was the original mechanism for conversion, the "Holy Grail" so to speak. My worries came because as we started using only MB rebuilt compressors we became worried that failures wouldn't be covered under parts warrantee if the proper procedures weren't followed. MB often does failure analysis and kicks back stuff they won't warrantee. (warrantee is not a gimmick at the wholesale level).

What I found over time and investigation is that the viscoscity of standard ester oil was 100. PAG oils come in 46, 100, and 150. The Nippondenso compressors used on MBs use PAG 46. Experience has shown better cooling and longer lives of compressors with the lower viscoscity oil. My suspicions are that many of the brutal failures of the early 124 conversions were the viscoscity and over oiling.

[2phast]

Anyone know why MB designed a sub panel in the 201 chassis (for access to blower motor/evaporator) and not in the 124? Seems ridiclous that you need to pull the dash on the 124 for this job. On my 1992 190E, the evaporator replacement was simple and only took a little under two hours, given the fact this was the first time I have done this procedure, next time it should only take me half the time.

[Cap'n Carageous]

I have a brand spanking new Robinair Cooltech sucking on my system even as I type this. Going to the auto parts store soon. Do I use PAG 46 and how much? System has 2 year old compressor (Nippondenso) but has not been operated for over a year. The compressor has never seen anything but 134 and was originally charged with ester.

[stevebfl]

One of the worse thimgs you can do with A/C is to use too much oil. I would not switch from ester to PAG without flushing the system. The story about most conversions is that they worked (for better or worse) because the mineral oil just sat around in the system and didn't mix so one could forget about it for the oil calculation.

When the system has too much oil the oil impedes flow and causes a strain on what is a gas compressor. Too much liquid and its toast.

The problem is that both ester and PAG will flow with the 134 so you can't just add PAG. This is my speculation, I have read nothing on the subject.

We used ester successfully in the bulk of our conversions. I wouldn't change without doing it right. And if I was doing it right on my own car I would be changing it back to mineral oil and R12.

[400E]

I have read that most folks recommend an ALL COPPER evaporator for replacement since it appears that the only evaporators which went lame were the aluminum ones. My tech used an aftermarket copper evaporator when mine went out.

Just thought that needed saying for others who are considering replacing theirs. Hopefully "Jag Guy's" aluminum evaporator will have a long, healthy life ...

Are all model years of the W124 subject to this failure or did M-B improve things as time when on? I'm more worried about somebody tearing my car apart to replace the evaporator than I am about the cost.

TIA,
Bud

1991 300E, 53K (most of it in Minnesota but now in the tough conditions of Arizona)

The subject vehicle (original post)is a 1991 model. From what I could see of the construction of the old evaporator, I believe the leaking problem is related to Behr's use of copper tubing which had to be mechanically sealed to the aluminum core. It looks like the dissimilar metal interface caused corrosion and eventual leakage of the mechanical seal. The replacement part is all aluminum with welded construction.

As far as using aluminum vs. copper for the overall construction is concerned, most manufacturers today use all aluminum evaporators for R134a systems. One does have to be careful, though, not to introduce moisture into the system during charging as the moisture forms an acid that will oxidize the aluminum core from the inside out.

I learned these things from reading everything I could find about R134a conversions, including a manufacturer's conversion instructions (Sanden), before attempting the job.

[400E]

I don't want to start an evaporator war, but I think it's an important question, when one is considering a very time-consuming and expensive job.

Here's what Steve B. said in 2000:

"We use a copper tubed evaporator that we sell for around $300. We do charge about 15 hours labor to remove and replace the evpaporator, five vacuum servos, retrofit and charge. "



Steve:
Do you still recommend copper evaporators?

[stevebfl]

Yes, I do. They do an excellent job of heat transfer (the best actually). They also are built like crap and we no longer use them. It was always a compromise getting them in the case.

We are currently using the all aluminum (I think) ACM (I think) evaps that come with the expansion valve and entrance seals. They fit like a glove and I am really not worried that they would fail for Metallurgical reasons. Even the originals lasted ten years. My job is a good repair. The fit and sealing of the case is atleast as important to me as the ability to outlast the the original.

The results of poor fitting copper evaps come in all manner, from airflow restrictions to leaking cases after butchering. I agree that the bimetallic aspect of the original evaps was as much their long term problem as anything. I also submit that the problem is worse in big cities either due to evironmental issues or a general tendency to blame evaps when leaks can't be precisely identified. Wish I knew trhe answer to that.

[400E]

Interesting! Seems like this issue is evolving over the last couple of years. Will be interesting to see what the longevity of the new all aluminum units will be.