The vehicle is a 420 SEL, W126, 1986 version. I have a leak in the a/c system, still using R-12, which is getting progressively worse and I've got to start chasing the source,without knowing where to start.

In searching the archives, there are a number of stories about failed a/c evaporators and the horrors of having to remove the dash for replacement. I'm of the impression it's even a nightmare to replace an expansion valve.

I've owned numerous automobiles, both foreign and domestic, and I've never experienced a failed evaporator. It makes me wonder if somewhere along the line, Mercedes produced a poor design.

I would appreciate knowing if the evaporator failures are mostly associated with one particular model and year and how susceptible the 1986, 420 SEL might be to this problem, whatever is causing it.

William

A/C evaporator leakage is rare thru-out the industry with 1-2% failure rate. However the W140 chassis has been around 8-9%. ON your car, W126 chassis in general the rate is probably around the 2% rate. Remember that domestic cars from that era aren't even on the roads any more & if they are most aren't worth fixing the A/C system!

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MERCEDES Benz Master Guild Technician (6 TIMES)
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Mercedes Benz Star Technician (2 times)
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27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
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Retired Moderator

Your response was very helpful, even a relief, and I thank you.

Could you please answer a couple of other questions at your convenience?:

1. Can you confirm that the capacity of the a/c system on the 420 SEL, '86 model, is 1.1 Kg = 2.425 pounds? In research, I also came across another source quoting the capacity at 2.9 pounds. I'm confused.

2. Could you please tell me what High & Low side pressure gauge readings I should see when the a/c system is topped off with R-12, at idle speed and 1,500 rpm?

Thank you for your valuable help.
William

Filling capacity of R12 was 1.3 Kg. If converting to R134a use only 70% of that figure.

Pressure Readings are temperature dependant! will vary all day long. Similar to most any other car at equal temps.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

I've not heard of any W126 evaporator failures. However, I'm 50% on the W124s I own (one of two), and W124 evaporator failures are common enough that very shop who works on them know about them.

My friend Hans has had four or five in the last years, only one of which he actually did, he usually shops them out, too much work for a one man shop. This rate has been fairly steady over the last couple years.

The one I had that failed leaked at the cold (inlet) side where the copper inlet lines are crimped to the aluiminum heat exchanger. I personally see this as a sure bet failure -- the copper will expand at a different rate than the aluminum, and stress corrosion/work hardening/fatigue is an absolute certainty. The replacement (and the one in the 300D, I believe) was all aluminum instead of having copper inlet/outlet lines.

I would characterize this as a design failure, but I'm not surprised it was not changed during the design life of the chassis. I suspect the failure mode wasn't discovered until production was ended, it takes a long time to occur.

W124 evaporator failures are MUCH more common than any other chassis from Benz, unless more of the newer ones are similar.

As to leaks, I would get a good leak tester and start sniffing, starting with the compressor manifold seals and proceding to the compressor body seals, high side, expansion valve (inside, under the dash above teh driver's foot, I believe on the W126), then the low side. Don't neglect the check the actuall hoses, too, they can fail with time. Oily residue around connections is a hint of a leak.

An evaporator leak is best found with UV dye -- the dye will show up in the condensate drains if the evaporator leaks.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Peter,

You certainly made it crystal clear why the a/c evaporators in the 124 Models are prone to failure and I haven't heard that kind of talk since I took a course in metallurgy. To better understand how the copper and aluminum connections occur in the 124 evaporator, I searched out a photograph and it quickly became evident.

I also found a photograph of the replacement evaporator you mentioned, that comes "all aluminum". I found both versions being sold under the same part number, with the "copper/aluminum" version less expensive than the "all aluminum" and an uninformed DIY buyer might be inclined to make the mistake of opting for the less expensive version. Hopefully, others reading this thread will make a mental note of this information and check it out.

In further searching, I found a photograph of the model 126 a/c evaporator and it is entirely different in design and is "all aluminum".

Many thanks for educating me on how to go about searching for a refrigerant leak.

Based on the response I received from M.B. Doc it appears that I need to chase a set of R-12 gas tables that will provide information on what the "Hi & Lo" gauge readings should be for different ambient temperatures. This gets a little more complicated considering one of the threads I read where the engine revs have to be factored into the equation (either idle speed or 1500 rpm). Perhaps someone can identify where I could find a chart that would tie all of this together.

William