AC Problem: DTC B1057, fault in CAN communication with RCP - Mercedes-Benz Forum
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post #1 of 11 (permalink) Old 07-19-2019, 02:34 PM Thread Starter
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AC Problem: DTC B1057, fault in CAN communication with RCP

2005 SL500, R230 has 129000 miles. Have owned it for 4 years, never any AC issues before. Car was in storage all winter.
Brought out 2 months ago and the Air Conditioning is only blowing hot air. AC button has red light on. Push it and it flashes twice and then stays on.

With the warm weather I started troubleshooting.
Error code B1057, fault in CAN communication with control unit RCP (Roof Control Panel). I have cleared the error twice, returns immediately but the red light is off on the AC button.

With the engine idling and the AC on,
The diagnosis shows:
N22b1 - in car sensor is at 40 degree C
N706 - in car sensor is at -128 degree C
B14 - Ambient sensor is at 46 degree C
B10/6 - Evaperator sensor is at 39 degree C
B11/4 - coolant sensor is at 97 degree C
B12 Refrigeration pressure measuring 10 bar with AC on and 8 barr with AC off.
A9 Refrigeration Compressor 70% with AC on.

I am not sure what to look for with the error B1057?
The in car sensor N706 reading -128 degree C, I think is wrong. Where can I find this sensor to replace it?
Is the refrigeration pressure of 10 bar satisfactory?

I am using the i980 carsoft reader. Thank you, Patrick
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post #2 of 11 (permalink) Old 07-19-2019, 07:20 PM
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N706 (I think it's actually N70/6) is in the overhead control panel. There is a "themistor" in there with a fan that draw air across it. This is a common failure on many MBZs. I have read where some people have purchased a new thermistor for a few dollars and soldered it in place. Here's a thread about it on a different model:

https://www.benzworld.org/forums/c20...re-sensor.html

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post #3 of 11 (permalink) Old 07-19-2019, 07:44 PM
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The in car sensor N706 reading -128 degree C, I think is wrong. Where can I find this sensor to replace it?
It is more like an open circuit, maybe a broken wire ?
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post #4 of 11 (permalink) Old 07-19-2019, 11:28 PM
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I recently fixed the in-cabin temperature thermistor by soldering in a pair of jumper wires to replace what had broken. The original wire is hair thin steel and rusts. They should have used copper.





This was a fairly quick and easy fix, too. If you don't already have some solid core wire to use, you can just pull several individual strands out of a larger gauge wire (like lamp cord) and twist them together to make a replacement. Just be sure to not mess up the actual thermistor bulb itself with too much heat while soldering.
Hope this helps.
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post #5 of 11 (permalink) Old 07-20-2019, 07:39 AM
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I too have a 2005 SL500 (149,500 miles) and have seen this fault appear at least a dozen times over the five years I have had the car. Each time I read the fault with my Carsoft i980 unit and simply clear the fault and the unit starts functioning again. Something trips the unit off once in a while but my 5-minute reset seems to always put it back on track for another extended period. Once or twice, it has taken the AC ten minutes or so to get back to really cold air, but that is a rare delay. I'll keep doing this until I ultimately have to repair some circuitry or switch at some point.
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post #6 of 11 (permalink) Old 07-21-2019, 05:47 PM Thread Starter
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AC Problem: DTC B1057, fault in CAN communication with RCP - Almost Fixed

The thermistor fix is done! Actually pretty easy. In car sensor N70/6 now reading same as first in car sensor about 40 degree's C today.

I am still blowing warm air with the AC on... No DTC's but B12 refrigeration pressure is still at 10 bar, too low? Next is to try a freon recharge.

Thanks everyone for the information, great stuff!!
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post #7 of 11 (permalink) Old 07-24-2019, 09:45 AM Thread Starter
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AC Problem: Still blowing warm air

I am ready with an air conditioning gauge set and tank of freon to do a system recharge. My i980 reader is showing that refrigeration pressure is 10 barr with the AC on and max cooling.

Does anyone know the system fill pressure?

The under the hood sticker gives a volume amount of 2 pounds. My AC gauge set will let me look a t both high and low side pressures.

Thanks,
Patrick
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post #8 of 11 (permalink) Old 07-24-2019, 08:57 PM
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Use your gauges and an R134achart to see what the high and low pressures should be given the ambient temperature.


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post #9 of 11 (permalink) Old 07-25-2019, 06:03 AM
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Originally Posted by Pat_Opsahl View Post
I am ready with an air conditioning gauge set and tank of freon to do a system recharge. My i980 reader is showing that refrigeration pressure is 10 barr with the AC on and max cooling.

Does anyone know the system fill pressure?

The under the hood sticker gives a volume amount of 2 pounds. My AC gauge set will let me look a t both high and low side pressures.

Thanks,
Patrick
Connect the gauge at both sides and measure the pressure at both sides with the engine and the a/c not running for a while (like overnight) so everything is stabilized. Then start the car and put the a/c on, check that the compressor is running first, and read high and low sides within few minutes after turning the a/c on, and after 10 minutes.

If you had 8 bars of high-side pressure with the a/c not running, and then it goes up to 10 after the a/c is running, it is more likely that the compressor has an issue.

Note: 1 bar is 14.5 psi. So to convert readings to bar, divide the gauge psi readings by 14.5.
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post #10 of 11 (permalink) Old 07-27-2019, 01:04 AM
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Originally Posted by Pat_Opsahl View Post
B14 - Ambient sensor is at 46 degree C
Ouch!
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