secondchance550

Need help from another R230 owner with SDS or icarsoft MB II.
I have a 2007 SL550 with AC issue I have been struggling with for 2 years. I went to 2 indy shops. First one gave up after replacing the compressor and 2nd one after trying to pull codes gave up and referred me to a dealer. Dealer hooked up the car to SDS for 2 hours (and charged me of course) and tells me no code.

Symptom: AC works fine when cold. After being driven for a while, evaporator temperature goes up from 41 - 48 (AC works great at this temp) to 50, 57, 64, 71 and settles around 73. In the meantime, B31 multifunction sensor reads reasonable relative humidity and reasonable dew point temp 30% and 45 to 50 degrees first thing in the morning. Then, as the car heats up (20 minutes or so) dew point temp drops until reading -4, -5 degrees! At this point, I have 71-74 degree evap temp.

Help needed: I found OCP (overhead console), under air temp, provides 2 data. 350 N70m1 (inner blower motor temperature sensor) and says "active" and 340 N70b1 (inner air temperature sensor). N70b1 temperature I am getting is 32.45 degrees even when the cabin temperature is 80 plus degrees! 32.45 degree reading stays constant no matter what and is inconsistent with N70b1 temperature reading I get from AC control data reading.

Request: Can someone with SDS or icarsoft read OCP/air temp/N70b1 temperature reading? I have 2 overhead consoles and I get 32.45 degree reading from both.
Are there 2 temperature sensors in overhead console? I love this car and this AC issue/puzzle is driving me nuts!

I tried replacement driver side SAM, passenger side SAM and overhead console with same part numbers from Ebay (same part number negates need to have them programmed I found out) and I still cannot track down what is causing this.

Your help is truly appreciated!

Rudeney

If I get a chance in the next few days, i'll take a look. But if you suspect the sensor in the OCP is bad, try opening the roof. With the roof open, that sensor is ignored. If that makes the air cool, then it's the culprit and the good news is that it's a cheap fix - you can buy a new "thermistor" for a few dollars and solder it in place of the bad one.

bobterry99

On no account should the evaporator reach this temperature regardless of any sensor inputs.

I'd use my diagnostic tool to see the on/off output control signal to the A/C compressor (expressed as a percentage). I'd also look at the refrigerant pressure either with my diagnostic tool or preferably with a set of gauges.

Rudeney

Bob, if the ACC is not calling for cooling (due to incorrect temperature sensor readings), wouldn't it open the compressor's swash plate so that it creates less pressure in the refrigerant circuit?

secondchance550

I will try dropping the top and see if that has any effect. Now that temperature is falling in N. VA, it's beginning to diagnose AC issue difficult.
Reason I am looking for OCP air temp data from a healthy car is to confirm 32.45 degree I am seeing points to either failed temp sensor in the overhead console or circuit or component failure resulting in 32.45 degree reading.
What is baffling is AC control reading for N70b1 (overhead console) reads correct temperature. One possibility is that overhead console having 2 temp sensors. I have a few thermistors on hand if necessary to replace.

secondchance550

Typically, with AC on, compressor percentage is always at 70%. I think I have seen evap temp as high as 71.xx degrees with compressor percentage at 70% and refrigerant pressure at 7-8 bar. If I turn off AC for a while and turn it back on, I see pressure reading go up to 10 bar or so and evap temp drop to 67 degrees or so briefly and temp increase back to 71-73.
Another interesting event is, on a long drive, after about 30-40 minutes AC light would go off, triggering B1256-000 malfunction in refrigerant circuit. One time I turned off AC and kept driving. I noticed B31 sensor dew point temp go from .4 degrees or so to around 13 degrees. At this point, turning AC back on made the AC work with evap temp around 46 to 48 degrees.

bobterry99

I believe the compressor is governed to maintain an evaporator temperature of a few degrees Celsius regardless of whether the interior of the vehicle is being heated or cooled.

Your refrigerant pressure is entirely too low. Around 16 bar or higher would indicate a properly-charged system.

secondchance550

I think evap temp is maintained at or around 48 degrees when AC switch is on until AC system determines cooling is not needed. When compressor is turned off, I saw evap temp reflect ambient air temp.

AC system was charged by a professional MB mechanic using air temp/lbs table.

secondchance550

Rudney,

When you get a chance, can you note ambient air temp, OCP air temp for N70b1, B31 dew point temp, AC control N70b1 temp, AC compressor percentage and refrigerant pressure?
This is the first car I have owned with variable displacement compressor and finding it very confusing.

bobterry99

On a warm day properly-working A/C will produce air measuring 35-40 degrees Fahrenheit at the vents. Obviously the evaporator must get much colder than 48 degrees to achieve that.

Still, you could have low pressure as a consequence of a leak, a faulty compressor, or perhaps a faulty driver-side SAM.

secondchance550

I had the system recharged 3 times since May 2017 and readings have been consistent. With traditional R134 AC compressor, 240-260 psi is what I would expect. But with Mercedes variable displacement compressor with taking in sensor inputs from multiple sources and determining chilled air output, I don't know if one should expect 40-45 degree drop relative to ambient air temp at all times. Evap temp of 48 on 95-96 degree day represents about 45 degree drop. I noticed 37 degree evap temp yesterday when the ambient air was around 60.

bobterry99

That would be at a temperature in the mid-80s, and you should expect the same pressure from a variable-displacement compressor at maximum output.

The documentation states the Mercedes compressor operates with a control signal ranging from a 2-3% to 100%. But in my limited experience in practice the maximum control signal is 70%. So, even if you reject my assertion that the compressor is governed to maintain an evaporator temperature in single digits, you should recognize a fundamental problem with your cooling performance.

secondchance550

Sorry for being dormant. I was in S. Korea due to my father's passing.
I am totally exhausted - mentally and physically from jet lag.
I'll be back to continue this discussion once I recover.

Thanks all!

secondchance550

Checking low and high side pressure should be simple enough. I will either buy or borrow a AC recharge tool/gauge and take readings.
In the meantime, OCP N70b1 reading of 32.45 degrees is still really puzzling especially when ACC N70b1 reading reads realistic based on ambient temperature.

Rudney,
When you get a chance, please take OCP N70b1 reading and let me know.

Rudeney

I still intend to do this! I just have not had time. I was barely able to squeeze in the time to replace my pulsation dampener.

secondchance550

No worries! I have a whole winter to sort this out...