Proceedure to change evaporator 1981 300sd

I don't know why I like this car so much but I do, it's a love hate relationship. I have been beating the problems out of this car one at a time and now when I think I am at the end of any thing else that
could go wrong I find that I have no AC. The R12 is all gone suddenly last week. I am in the HVAC business for a living and work with air-conditioning every day but not on cars. I have determined that I have a leak in the evaporator. There is a strong detection of refrigerant coming from inside of the evaporator box. There are no leaks about the expansion valve, I am getting a strong detection from the openings of the evaporator housing and the detector will go off when sampling the air from the vents when the blower is first turned on. I am quite surprised that a copper evaporator could leak but it seems mine is leaking. Not wanting to put more R12 into this car at this time, I have blown out as much mineral oil as possible and replaced it with ester oil then charged with 134a for now the car cools OK but I am sweetening the charge every 3 days to stay cooling. My plan is to replace the evaporator and re-service the system with R12 when the system is leak free. Now to the part I need help with, this looks like a hell of a job, as best I can see the entire insides of the front of the car will have to be removed in order to get at this evaporator. I have looked at my Benz Shop Manuel CD and can find no instruction on this procedure. I am hoping that someone has in print the step by step procedure for this job or If any one has actually changed an evaporator in a 126 body before could they tell me what they can tell me about this job?
Any help will be seriously put to good use, Bruce Buchanan
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BEFORE you tear into the evaporator .....
Take a look at the very top of the receiver/dryer. There lives a brass plug that has a small "blow out plug" soldered into its center - which "blew out" on my '80, not too long after its R-12 was recharged. It was resoldered and the R-12 refilled and hasn't acted up since.
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I am looking for any way out of this that I can find but the fact remains that refrigerant is only detected at openings to the evaporator area. I have gone over the whole system with an H10 type electronic detector. It raises no indication of refrigerant leakage at any point except the evaporator. My detector yelps and squeals as it is poked into the duct next to the drivers well. I have am leaking about 6 ounces a day witch would be awfully hard to over look that size of leak in an open area such as around the receiver. I plan as a final test before jumping into the evaporator to isolate it alone with fittings and do a pressure drop test with dry nitrogen. It is frustrating to not be able to get at the evaporator. I work on air conditioning equipment daily but am able to see and reach all parts of the systems we work on. R&R time on a typical residential systems evap coil is about 45 minutes plus about another couple of hours for proper evacuation and servicing and adjusting the system properly. It looks like my car will take many days for the same.
I do want to make sure of this diagnosis and will recheck the fuse plug at the receiver before going off the deep end but for now I am trying to get a scope of what is involved with changing this evaporator because for now that's what it looks like it's going to be. By the way I discovered this lack of air conditioning on the test drive I took last week just after putting in my second tranny from the junk yard. Good grief! The good news is the second tranny goes right into 4th ok. Some day I will get every thing to work at once. Bruce
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I suggest you fill the system with UV dye and use UV light to detect leakage. A leak sniffer is nice, but even running engine exhaust will set it off.
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Thanks for the post, I seem to have car exhaust coming only out of my evaporator box, LOL. I am sure that it is refrigerant that I am detecting and short of finding any other leak after rigorous searching, I seam to get a strong detection at all entrances to the evaporator. I am using a very high quality sniffer that only alerts on a narrow selection of refrigerants. I am not trying to be a know-it-all but I have been looking for and finding leaks for 40 years. What has me shut down is I have no experience with this particular leak situation so I am looking for help with any idea that will explain what I am finding. Keep in mind that I plan to do an isolation test of just the evaporator before I would invest the labor of changing it. I need to find the fittings soon, I probably will modify an old expansion valve so I can pressure up the evaporator with nitrogen, if it drops then that will be my final diagnosis. In the mean time I believe I have a bad evaporator and am looking for the grand procedure of how to chang it. With so much stuff to remove and replace it would help if one would know the fastest order to proceed in. Thanks again for the post as any comments are greatly appreciated. Bruce
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Welcome to Mercedes dirty little secret: HVAC. The older ones barely work at all and the newer ones, while pretty good seem to have the car built around them. That's because they do.
Replacing a 108 heater fan or any halfway modern heater core is a long and tedious job. I have a receipt for replacing the heter fan in my 108 from the PO he had done in 78. 40 hours labour.
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Richard Sexton | Mercedes stuff: http://mbz.org
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Richard Sexton wrote: car will take many days for the same.

In all fairness to MB, is it any easiery to replace a heater core or evaporator on most other cars? Because of where they are located, it isn't ever going to be easy.
As for the blower, I'm not familiar with the 108, but if it takes 40 hours, I agree that sounds like a bad design. On my 116, it's maybe a 2 hour job. You have to take off the right hand lower access panel and remove the glove compartment. But then it's right there in your face, held in by 3 easy to get at screws. That design covers a lot of cars cars, the 450SEL, SE, 6.9, 280SE, 300SD, from 1973 to 1980.
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No, not really.

The hack 108 approach is to cut the cowl and replace the fan with one from a Ford Courier. It works I hear, and well.
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Richard Sexton | Mercedes stuff: http://mbz.org
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Hey Bruce. How'd you ever make out?
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