A/C recharge question

[Chayro]

I purchased a 98 ML320 about 2 months ago and I don't think I really tested out the A/C until this week. At first it didn't get that cold and I thought the system was shot, but then, after some use, it got cold, but not as cold as I would have expected. I assume that, because it gets cold, the compressor is OK, and that it may just need a charge. Of course, I have no idea of what it may need. Can MB units just be charged up, or am I looking at a nightmare here? Thanks for your help.

[JerryWagner]

If the A/C compressor clutch engages then you have pressure in the system, if it has been sealed it should not need a charge, I would run it for a few days to lubricate and use the system, would not recommend re charging it yourself, if you hook up to the high pressure hose instead of the low pressure hose you could do serious bodily injury, take it to a professional and let them charge and test for leaks

[Chayro]

Thanks for the info. The only thing I do myself is put gas in it. My main question was whether MB units can be charged like other units, or is some special procedure required?

[Colin Luck]

Any AC unit can be regassed without a problem even the old R12 gas can be replaced with one of the newer ones that is more friendly to the atmosphere.

But you main question was the AC not getting cold and then a few days latter it got better. Most likely some gas has escaped from the system this normally happens through the compressor seal if it isn't used all that much or left not running for long periods of time that is the AC not the car. Now provided that everything is OK all that should be required is a top up of the gas and just about any repair shop will be capable of doing this without a problem. But and here is the big one if the AC has not been serviced for a long time the receiver dryer should be replaced to stop junk getting circulated through the system and destroying the compressor which is expensive compared to the receiver dryer. If you are told that the receiver dryer needs replacing don't skimp on this as it is the cheapest insurance possible to protect the compressor.

What they should do is remove the gas and oil from the system and filter it so any contaminates are removed then crack the lines and replace the receiver dryer {the round tank shaped thing generally black with a clear round opening on the top and sometimes with a pressure switch screwed into the side.} They should also replace any seals that may be there and then evacuate the system and put in a small charge of oil/gas and test for leaks the current crop of oil used fluoresces under the right light and you can see a green stain where any leaks are or they run around the system with a sniffer not as reliable but still effective.

Generally most gas leaks out the front seal of the compressor after long periods of no use as the seal drys out and can no longer keep the pressure in, but from your description there doesn't seem to be much of a problem and a top up should keep it getting cold very quickly.

Col

[CLK320MIA]

With the ignition on, press and hold the res button on your climate control unit. After a few seconds, some data will show up. scroll to the value for #7 by pressing either auto buttons. This is your refrigerant pressure in bar. Now, if it is low you have a problem. From my understanding, MB units are sealed and should never lose refrigerant. If it is ok, then you can look for other values that may be off, here's the list....

01 = In-car temp sensor
02 = outside temp sensor
03 = left heat exchanger (heater core) temp sensor
04 = right heat exchanger sensor
05 = evaporator sensor
06 = coolant temp
07 = refrig. pressure in bar
08 = refrig temp sensor (23*9 = 23.9deg C)
09 = nothing
10 = Blower control voltage (great for evaluating blower controller problems)
11 = emissions sensor (B31) e.g. 3.1 = 31.v
12 = sun sensor 4.2 = 4.2v
20 = aux fan control 6.1 = 6.1ma (max 10ma)
21 = engine speed
22 = vehicle speed
23 = Circuit 58d e.g. 99 = 99% of battery voltage
24 = BATTERY VOLTAGE e.g. 12.5 = 12.5v
30 = left rear air conditioning heat exchanger temp sensor (not all vehicles; obviously)
31 = right side
32 = rear evap temp sensor
33 = blower control voltage rear 0.8(MIN) - 6.0(MAX)
34 = left rear temp selector
35 = right side
38 = software status e.g. 37 rear air conditioning
40 = control module version software status (?)
41 = control module version menu hardware status
42 = control module variant code 1st number
43 = variant code second number.

[BamaMB]

Is the value shown the compressor discharge pressure or compressor inlet pressure?