Help, my A/C is acting crazy. Computer trouble, perhaps? - Mercedes-Benz Forum

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post #1 of 5 (permalink) Old 12-07-2009, 08:57 PM Thread Starter
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Post Help, my A/C is acting crazy. Computer trouble, perhaps?

I have recently purchased a 2001 C320. The A/C unit has not worked since the purchase. It only comes on for about 1 second and then quickly shuts down. The control unit looks like it's still functioning, but there is nothing coming out. No air flow whatsoever. It will turn on intermittently a few times during a drive, but quick shut off (after only 1 second). I was told it could have something to do with the 'Comfort Control Unit/Computer'. I was also wondering if it might just be low on refrigerant, and be shutting off because of that. Another side note: the system sometimes runs for about 30 seconds after the car has been turned off.

Any ideas? Is there any way to check the coding to see if it might be the CCU or low refridge?
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post #2 of 5 (permalink) Old 12-08-2009, 07:23 AM
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One problem area mentioned quite often is the fan speed regulator. Try a search and you'll find a lot of threads about those. If you are low on refrigerant, the comprssor will not engage as a built-in protection mechanism. Also the light on the EC button will not go off. That might also be the case but it sounds more like your regulator to me. The climate control has a self-diagnostic setting that you can use to check several other things including refrigerant pressure. Good luck and welcome to the forum.

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post #3 of 5 (permalink) Old 12-10-2009, 02:49 PM Thread Starter
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Alright, I finally got a chance to run the diagnostics. I'm not sure what to do with them now, however. The post I found them on, does not go into this detail. Any ideas? The codes are as follows:
00 78
01 73
02 66
03 50
04 50
05 55
06 1 85
07 14
08 1 05
09 14
10 50
11 0
12 0
13 0
14 0
15 30
16 0
17 0
18 32
20 (Ranges between 7-0)
21 68
22 0
23 0
24 1 38
25 E
26 0
38 1 00
39 1 00
40 1 00
41 1 00
42 0
43 0
44 1 00
45 1 00
48 0
49 0
50 75
51 47 02
52 47 02
58 0
59 0
60 0
61 0
70 0
71 0
72 0
73 0
74 0
75 0
76 1
77 0
80 0
81 0
82 0
83 0
84 1
85 0
86 0
87 0
96 1
97 13
98 13
99 77

Then they just start repeating over again. Please let me know if you can make any sense out of this, or if you have any ideas on where to get started with replacement parts. Thanks,

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post #4 of 5 (permalink) Old 12-12-2009, 06:09 AM
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Can you see the compressor clutch when the engine is running? Do you have access to a set of a/c guages? if so post pressures and what you see with the a/c on and we can help.
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post #5 of 5 (permalink) Old 12-13-2009, 03:02 PM
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Originally Posted by mycompanyname View Post
Please let me know if you can make any sense out of this, or if you have any ideas on where to get started with replacement parts. Thanks,
I do not know if this W208 a/c diagnostic info is any use to you. I saved it in case I ever need it on my W209, but it may also apply to your C320 a/c system. I hope that it helps.

The "REST" Menu Feature of the Climate Control

These are for the W208, other models may differ. After you get home from a drive (20 minutes or longer), let the car idle for 60 seconds while in park with the AC on. Press and hold the "REST" button on the climate control. After approximately 5 seconds, the display will change. You can now release the REST button. The left "auto" button will allow you to scroll through the menu.
Here are what the codes mean (W209 also has the bold options):

01 The in-car temperature sensor.
02 The bumper temperature sensor.
03 The temperature of the left side heater core.
04 The temperature of the right side heater core.
05 The temperature at the evaporator sensor.
06 The engine coolant temperature.
07 The barometric pressure of the Freon.
08 The Freon temperature.
09 Blank.
10 The voltage at the heater blower control (normal is 08-6.0, which is .8 to 6 volts).

11 Emissions sensor voltage (to detect atmospheric gases and recirculate interior air).
12 Sun sensor voltage (to shift airflow in car).

20 The current at the auxiliary fan (the electric fan on radiator, in milliamps or mA).
21 Engine speed (x 100 RPM).
22 Vehicle speed (km/h or mph depending on installation).
23 The percentage of battery voltage at terminal 58d (a percentage of total voltage, 99.0 = 99%).
24 Battery voltage (e.g. 12.8 = 12.8V).

40 The climate control software version. - V8
41 The climate control hardware version. - V5

Either turning the car off or pressing the REST button again will take the HVAC out of diagnostic mode.

Some notes.

Common problems that can throw the Electronic HVAC for a loop are bad temperature sensors.

Naturally, if the in-car temperature sensor (#1) is broken and reads low, the AC will cycle off early while in auto mode. If it’s broken and reads high, you may freeze to death while in auto mode.

The evaporator sensor (#5) should be close to the in-car temperature sensor while the A/C is off, and a lot lower while the A/C is on.

The Freon pressure sensor (#7) is a good way to determine whether the Freon is low. Low refrigerant pressure commonly causes the left side to blow warmer air than the right side. Of course, the logical question is, "What’s normal?" My Freon pressure sensor (#7) reads as follows - when the car is started, and the AC is turned on, without touching the gas, it’s as high as 15. It drops to 7 while driving, and is commonly at 11/12 while waiting at lights.

Assuming the heater is not on, the left and right heater cores should be about the same temperature. If one is higher than the other, and it’s higher than the temp in the car, the Duovalve may be sticking. Low refrigerant pressure (#7) can cause the left side to blow warmer air than the right side, but low pressure shouldn’t cause the air on one side to be warmer than the outside air.

Last edited by keyhole; 12-13-2009 at 03:04 PM.
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