I just can't figure out why my AC doesn't work - Page 2 - PeachParts Mercedes-Benz Forum

dannym · #16 05-27-2008, 10:58 AM

I went out to work on the AC and saw oil on the expansion valve. Not Good. So I go and get the gauges and there's only around 40 or 50 pounds of charge left. Usually sits around 80 or so at rest.
Charged it up and had cold air!
I couldn't believe it! I drove around and saw as low as 30 deg on an infrared thermometer. SOB, if it wasn't leaking I'd be good to go. I think I might take it to a shop and get the leak fixed once and for all.

I wonder why it wasn't working before? I'm thinking air in the system, too much oil or a combination of both.

Danny

vstech · #17 05-27-2008, 11:11 AM

I am going to say this again and I will keep saying it untill all you jackseed a/c techs get it through your head... test for leaks UNDER PRESSURE, DO NOT TEST FOR A LEAK IN A VACUUM, AND DO NOT VACUUM A SYSTEM UNTIL YOU ARE SURE THERE ARE NO LEAKS!!!
reclaim the original charge. or if it's already empty... YOU KNOW THERE IS A LEAK! pressurize the system with nitrogen and VERIFY IT DOES NOT LEAK OUT. if it does find the leak before you try and evacuate. look for oil stains, look for bubbles, but LOOK!
evacuating a unknown tighness system is just asking for the lines to acid fail. moisture from the air mixed with oil and refrigerant will turn into an acid and eat your seals and lines from the inside out.

Cr from Texas · #18 05-27-2008, 11:11 AM

No one mentioned the heater control valve. If it doesn't get 12V, it sends hot water to the heater core. If there is a bad diaphram in the valve, hot water also goes to the heater core. Blend this with the cold air from the evaporator and it seems like the AC is not working.

I'd agree there is a good chance the evaporator core is plugged with the same junk that's in the vents.

Samuel M. Ross · #19 05-27-2008, 10:46 PM

Dannym,
I'm in the midst of converting a 1980 model ACC [with the dreaded SERVO] that's in my Son's 300D model MBZ... converting it to function manually as many of the early 240D model... as my 1980 240D does. So the first thing we invested in was a complete manual AC suitcase [MCC] from under the dash of a 1978 240D. I have the complete W123 manuals that describe in detail both the MCC and ACC systems early right on up to the end of the W123 models... and I have [ although not studied too much in detail] the manuals for the W126 models as well. We have used the "suitcase" to salvage the manual control components so that we could and did completely do away with the SERVO system and now have manual controls where there was previousely a fully automatic ACC system. In the process, I have become very familiar with the "innards" of these systems and although I canNOT say that I have looked into the inside of an ACC system's suitcase as new as your 1984 model, I think my comments below will NOT prove to be too much in error. Our latest side project is to figure out how to perform "minimal invasive" surgery on the '80 model's suitcase such that we can clean the air side of the evaporator without removing it from the car.

So given my experience/background above let me say the following:
[1] The is NO way you can cause the air to bypass the evaporator... for 100% of the air output of the Blower Fan in these cars [as I believe in just about all A/C systems work]... 100% of the air passes through the evaporator coil! I'll wager on this and be shocked IF I loose such a bet!
[2] Q1 - How did you measure the temperature drop across the condenser?
[3] As I sit here looking at an expansion vavle that I removed from our salvage suitcase carcass, I'm thinking that IF yours was the original valve, and it suffered a catastrophic internal failure... this could result input liquid expanding and essentially immediately bypassing the expansion valve and returning to the compressor. As I look at these valves, to some degree they are normally regulated by the temperature of the gas exiting from the output side of the evaporator which then in turn by way of mechanical linkage and/or rod is connected to the input side and regulates how much liquid is allowed to enter into the in evaporator's input side [to expand to gas].

I hope this helps... and I'll add this THREAD to my watch list and maybe can help later IF not already!
Regards,
Sam

dannym · #20 05-28-2008, 08:28 AM

Thanks for the info.
I measured the temp drop with an infrared thermometer.

Everything is working fine now, except for the leak. I'm not even sure if it is anymore. There was up to 350 lbss on the high side before. Now it more like 150.

The temp dial isn't working now. Does any of your manuals tell how to troubleshoot that?

BTW if I had nitrogen I would certainly have used it!

Danny

vstech · #21 05-28-2008, 09:08 AM

350 is extremely high. indicative of a plugged or dirty condenser, or a fan problem on the condenser, or a heat source feeding the evaporator.
150 is quite low unless it's very cold in the car and outside.
you've got problems, and they are all over the map. start with tracing the leak, get it FIXED then evacuate the system and totally verify that there is no leak with pressure. put in 134 as a gas only up to approx 90 psi with the system not running, and let it sit overnight. recheck with the temps the same as they were when you put in the 90 psi, and see what your pressures are. anything less than 90, and you still have a leak. find it before you vacuum the system.

Samuel M. Ross · #22 05-29-2008, 11:22 AM

Quote:

Originally Posted by DannyM

Thanks for the info.
I measured the temp drop with an infrared thermometer.
Everything is working fine now, except for the leak. I'm not even sure if it is anymore. There was up to 350 lbss on the high side before. Now it more like 150.
The temp dial isn't working now.
Does any of your manuals tell how to troubleshoot that?
BTW if I had nitrogen I would certainly have used it!
Danny

DannyM,
Regarding the MBz CD manuals... The ACC systems manuals are actually a series of some 24 "jpeg" files that vary in size from a few hundred "Kb" in size to several "Mb". Would you contact me via direct email [via my FORUM profile] and I'll see IF I can sort out something that might help you.

The link at: http://germanstar.net/126h.htm tells me that your 1984 300SD model's ~$230 "temperature dial" is part and parcial of the "Push Button Assy." so that's the tact I will take in trying to spot trouble-shooting information for you!

These CD manuals leave much to be desired for many of the diagrams are pitifully inadequate and very difficult, IF NOT IMPOSSIBLE, to read! This is but one reason I finally decided to abandon restoration of any fully automatic ACC system IF I knew the cost of parts was going to make it impracticable. My logic is that you shouldn't scrimp on any of the refrigerant containing/controlling components and these alone can be expensive... so when you see OEM control components that cost $200...$300... $500+... well the notion of using a small number of more reliable salvaged manual system components becomes more and more alluring to me!

Like I think I said before... I'm entering phase III in the complete conversion of a 1980 ACC [SERVO] car... conversion to a 1990(s)-1980(s) MCC from 240D cars... but the entire refrigerant side... compressor, condenser [parallel], R/D, expansion valve, and hoses [4 instead of the OEM 6 hoses]... all of these will be new... and for the EVAP, I will soon be flushing it both internally and cleaning it externally, all without removing it from under the dash!
Regards,