Hello,
I have a 2002 CLK320 with 140,000 miles on it. Also, I live in Phoenix AZ so it gets pretty hot here in the summer.

Recently, the AC started blowing hot air. It had been fine and the car has not had any issues of late. In an effort to save some money, I'm trying to fix the problem myself. Here's what I've done so far and what I've found.

DIAGNOSTIC STEPS

1. FREON TEST: Checked the Freon by using one of those do it yourself AC recharge kits. I made sure the compressor clutch was engaged and I hooked up the pressure gauge from the kit. It showed a reading of 90 psi. I did not add any Freon as the kit says not to.

2. SENSORS: I used the climate controls to get the DTC sensor readings. The car had just been driven for 20 minutes, outside air temp was 105, so it was well warmed up. Here are the readings:
01 = 108
02 = 100
03 = 154
04 = 116
05 = 110
06 = 192
07 = 10
08 = 107
09 = 0
10 = 6.0
20 = 0
21 = 0
22 = 0
23 = 0
24 = 12.1
40 = 8
41 = 5
42 = 248
43 = 14
Not sure why sensors 03 and 04, the left/right heat exchangers, are so far off. Seems like they should be pretty close? Do I need to clean the Duovalves? Would this cause both sides to blow hot?

3. ERROR CODES: When I pulled the DTC codes I had one error. It was code B1416, Auxiliary Circulation Pump. I replaced the circulation pump with a Bosch OEM pump, cleared the code and the error has not come back. I was hoping that was the problem.

4. MISC: The thermostat was replaced about 4 months ago. Also, when I installed the new aux pump I put in new coolant. The car does not overheat.

RESULTS: The AC is still blowing hot. The error code has not returned.

I’m just a backyard mechanic so please excuse any stupidity on my part. I’ve scoured the web looking for diagnostic info and this is as far as I’ve gone. Please feel free to offer any suggestions as to what else I can check, and maybe how to check it too.

Thank you for your time and assistance.

 

First seems like a leak at the duovalve - hot coolant is running into the cabin. It shouldn't. Even if that gets corrected your ac pressure seems a bit low.. Number 7 should read at least 14 when running and can go as high as 18 if its properly charged. When engine is off it could be anything depending on outside temps. A leak is also possible esp with the age of these things.

Couple of things can be going wrong - compressor could be weak or damaged and if the gas is properly filled the number 7 would never get as high as spec.
You really should get it properly checked out at a heating/cooling shop that has done MB's. It worth the 69-99 for a couple of reasons. They have the proper gauges (not the cheap gauge on a DIY fill bottle) and they will evac the system and fill it by WEIGHT NOT PRESSURE. Those two steps are crucial because the first step draws out all the air and the second gives you a proper charge. They will look for leaks (esp between the condenser and the drier) and check the operation of the compressor. If you fill it improperly and there is air in the system the thin wall aluminum pipes will corrode very quickly and you WILL get pinhole leaks. Think EVAPORATOR replacement (VERY EXPENSIVE TO HAVE DONE - DASH PULL OUT) or at least many hours to pull the dash. AC Pipes are not cheap and the compressor isn't that bad DENSO about 215 (AMAZON). If you do the compressor I suggest a new DRIER and EXPANSION VALVE to get the job done properly.

 

Thanks for taking the time to give me some great insight on what could be going on.
I'm glad you mentioned the duo-valves. I also suspected the duo-valves because of the variance in temps between the two sides. I took them apart this morning to clean them but they looked great. Of course, after I put them back together I started getting the B1417, duo-valve error. Lucky me.

So, it's off to my MB mechanic tomorrow as I'm tired of chasing ghosts. Like you said, he's got the real gear to check things out and I'd be thrilled to just pay $100 to have the system fully evacuated and charged.

I appreciate your time. Thanks so much.