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Old 04-12-2003, 08:33 AM
Join Date: Aug 2002
Posts: 134
Summer behind the door, another A/C thread

Hello everyone!

I see there are some new A/C threads (after reading which I got quite uncertain about my AC, I know I shouldn't have read them in the first place, I wouldn't be uncertain now) so I thought I ask you for your opinion on my AC.

When I bought my 300D 2.5 Turbo the AC worked only a few days. I suspect the system must have been pressurized prior to sale with either freon or just air so that the compressor would kick in and give a feeling the system worked. Anyhow when no cooling could be observed I went to a AC shop to have the system repressurized. After 10 years (that was the age of the car at the time of purchase) it may happen that the original freon disintegrated, downgraded and losed its ability. Fair enough, I thought, I would have the system refilled. Alas the shop carried no original R12, as its use had been banned years ago. They did not release the old oil (said most of it had anyhow escaped with the freon), only added R134a suitable oil and R134a, along with green tracer dye. The AC then worked but even as I was leaving the shop it worked only moderately. After a few days there was almost no coolness at all. I went to the same shop, had the system checked with UV to find out there was an evaporator leak.

My first move was to call my local MB shop to enquire about the price. As you all know their answer wasn't exactly what I wanted to hear. I went to a junkyard specialising in MBs. I took out an evaporator, the system held some pressure. So far so good I thought - still pressure = no leaks. I replaced the evaporator in a garage all by myself - a day's work. Had it R134a refilled (they also shot some more oil in the system) but the system didn't work quite right again - it was as cool as the first day when I had it refilled but still no the real coolness I had expected. Surely enough - the evaporator was a sieve, leaking on 3 spots. I took it out (it is a pain to take it out, trust me) and ordered a non-genuine new W124 evaporator.

Now what happened - I installed the evaporator but before I had a chance to fill the system a car rammed my Merc from behind. Boy was I lucky! It was the other driver's fault, as set in the local police report. Had only minor bumps on the trunk but surely enough his insurance paid for all the dents accumulated with time on my MB as well. Enough is enough I said, I didn't want to spend the rest of my life removing and re-installing the dashboard every month. I replaced the non-genuine new evaporator with the original leaky one, called MB, they replaced it with an original MB one, even issued a report the evaporator had been leaky as a result of the accident. The insurance paid everything (it was some bill really) so I was with an original new evaporator, gaskets and labour all done at MB.

I went back to the AC shop, had it filled with R134a. Found out one of the hoses to AC valve on the firewall was cracked, there was leak. Had the hose replaced (at MB, couldn't find any inner strength to spend any more time messing with AC components anymore), went back to AC shop (we know ourselves by Christian names by now), had another shot of oil and had it R134a filled.

The system cooled by all means better than any time it had before. But ... not as much as I expect it to perform. Remember one should add approx. 10% of R134a less than the R12 as R134a expands more. I thought what the heck and overcharged the system by some 20%. I listened carefully for strange noises, there were none. The system then cooled satisfactory (but not astonishingly) for about a month.

Then I came across a man who doesn't do ACs on cars but industrial refrigerating systems - open freezers in shops, freezer chambers etc. He had a supply of R12 left. He also had evacuation pump for car systems for some reason. He evacuated the system, put a red colour tracer dye in it, some of the original R12 oil and filled the system with R12. The system has worked good ever since! (Almost a year). In the meantime I found out that when idling at lights the cooling effect went down quite a bit. I wasn't happy with that but found out it was one of the auxillary AC fans stuck. Didn't want to rotate. I gave it few knocks with a wooden mallet and injected with WD40, it works perfectly since. The other thing to watch with an auxillary fan is the resistor spring located just behind the driver's light. As the fan is a 2-speed system, with a high temp (or pressure, don't know which) the fan rotates on full 12V, while when the pressure isn't as much high it rotates on a lower speed. Then the current goes through the resistor spring. The spring itself was OK but the points on which the spring connected with cables were corroded. Took out the spring off its ceramic housing and cleaned the contacts.

In other words, part of why my AC didn't cool as much and lost cooling effect while idling was that as the freon temp rose, the lower speed auxillary fan did not kick in. It only kicked in the full speed when the freon was too hot. That resulted in that the freon wasn't being cooled down progresively but there was a jump. Of course then the cooling process wasn't as effective. After having that mended I noticed immediate improvement.

Any of you can do a quick check on the spring - just rotate one of the fans fast by hand - the other should also rotate slightly. (Caused by induction, one of the fan acting as a dynamo). After I cleaned the corroded contacts on the spring my fans rotate both when I rotate one. They must be interconnected at that spring spot. (Though I am not sure about that). Sure is though that when the spot is corroded they won't rotate when rotated by hand and also will likely not rotate properly on low speed.

After I fixed the aux fan and the resistor spring the AC now FINALLY works just great!

With all the background youi now have here are my questions on you :

1. The oil. I know one should clean and flush the system in such cases but please don't limit your knowledge to that only. There must have been some R12 oil from the manufacturer. The system had a leak, the evaporator was leaky and greasy as well. Some of the oil must have stayed in the compressor I know. There has R134a oil been added with R134a. Another leak, been filled again with 134a AND another shot of oil. Then, after vacuuming the system (20 minutes only), original R12 was put in, with yet another shot of R12 oil.

1a. Is it safe to always add some oil when refilling the system? (The mechanic said it was better to have more oil in the compressor than to have none). Won't it be too much of oil accumulated in the compressor? Will the evacuating process suck some oil or does ALL the oil stay in the system? (Apart from the oil that leaks out, naturally). Or, is it safe to presume that it is OK to always add some oil to the system, as vacuuming will also take some oil out?

2. What do you think of the mixture? I now have R12 oil but certainly with parts of 134a oil. Can they be mixed or is this an imperative no because damage WILL occur? (As opposed to MIGHT occur). I think I might have heard the mechanic say that the 134a oil would not bond with R134 and therefore R134 oil must be used with 134a freon but it could be mixed with the old R12 oil without damage but I can't exactly remember whether he said this.

3. One of the valves (through which the system gets filled) leaked. I could see bits of tracer dye and small bubbles inside the valve. I had the valve replaced with a a new valve I had ordered from MB and screwed back on really tight. It still bubbles. I am confused - is this because there is low pressure in the system so the valve doesn't close properly tight? It is new, mind you, the only other things that comes to my mind is the valve seat in the hose is bad. Any other ideas? Fortunately enough there is a plastic cap that goes over the top of the valve with a rubber seal, this holds the pressure.

To conclude I must say that the AC worked great the past 2 months of last summer, there is still pressure now, I turned it on on a relatively warm day 2 weeks ago, seemed to work as it should.

Thanks for all of your replies in advance.

Last edited by Jassper; 04-12-2003 at 08:44 AM.
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Old 04-12-2003, 10:11 AM
Registered User
Join Date: Sep 2001
Location: central Texas
Posts: 17,281
First.... you have more 'inner strength ' than most people. Two... you have more Good Luck and more Bad Luck than most people....and sometimes combined in one incident....
This is a serious and complicated set of questions you have asked... and there is no doubt about to be alot of opinions running the gamut thrown at you.... my condolences on that....
I suggest you print out your post and mail it to the Real Pros at Lyle AC in San Antonio... give them time to read and study the question, in other words... not a phone call ......
Sure, let the guys on this forum have their fun.... but believe the guys at Lyle... ( shades of " SNL " Arnold look alikes saying" hear me now and believe me later"....)
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Old 04-13-2003, 03:13 PM
Join Date: Aug 2002
Posts: 134
Hello Greg,

thank for your reply. Your suggestion to write a letter to SA sounds interesting but as for I not being in the US I don't think I would ever be paying them a visit and why take their time when I would be of no benefit to them.

It was another relatively hot day today - the cooling action was really good. I suppose there is nothing I can do but wait, try and see what happens. I mean, the AC will either work or it will not. If not I am not willing to spend any more $$$ on it. Having a window open reminds me of a SL(K) which was my dream anyhow However, as for now it seems the system works fine.

One more question though - there have been suggestions that with time the freon collects more and more humidity from the outside air, forming HCl that harms the system from within.

How can an overpressurised system collect humidity from low pressure outside air and form HCl that harms the system? A leak from high to low is an obvious option, but how can something (like humidity) transfer from low to high pressure area? It would make sense to expect that nothing can enter an overpressurised system, would it not?
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Old 04-13-2003, 03:47 PM
Registered User
Join Date: Sep 2001
Location: central Texas
Posts: 17,281
They are only referring to a system which has been opened up... When closed, new reciever dryer installed, and properly evacuated you won't be getting any extra humidity in the system.....

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