secondchance550

Hi, Guys,

I picked up a 2007 SL550 about 3 weeks ago and had two issues.

First was inability to turn off red battery warning light. Despite both battery replacement and disconnect and reconnect of front and rear battery in prper sequence. Second issue was sporadic AC operation.

I figured I could use all the diagnostic help I can get so I took the plunge and bought a iCarsoft MBII i980 unit. This is a reasonably priced, easy to use tool and I am impressed!

Within 5 minutes of hooking up to the car I was able to locate and erase battery warning light. Evidently, at least for 2007 (and perhaps up) battery ground disconnect and reconnect sequentially does not work. It took the scanner to erase the warning.

Secondly, I was able to pull DTC for AC - "B1256-000 Malfunction in refrigerant circuit". Anyone knowledgeable out there help me understand what above DTC means?

Below are values I got from the scanner - 1st column when the AC is working, second column is when the compressor is shut down due to malfunction detection:

N22b1 (in car temp sensor) - 27 deg C - 28 deg C
N70b1 (in car temp sensor) - 26 deg C - 27 deg C
B14 (ambient temp sensor) - 31 deg C - 31 deg C
B10/6 (evap temp sensor) - 15 deg C - 24 deg C
B11/4 coolant temp - 62 deg C - 96 deg C
B12 (refrigerant pressure sensor) - 13 bar - 10 bar
A9 (refrigerant compressor) - 70% - 0%
Fan capacity requested by AC - 26% - 0%
Compressor request - on - off
Feedback of AC compressor actuation - on - off

An example of scanner display

Plutoe

Although, I can see that if you start from 0 and go to 5 you would be impressed, however the tool lacks much serious diagnostics--there are many other sensors in the system that the tool missed, for example along with pressure refrig temp is also important, plus it seems that you can't click on the fault and get an understanding of what to do next----------all this aside all you have to go on is the ability of the compressor to make pressure and if not why. In your case 13 bar is low it should be 17 bar----the other issue is the evap temp--that should be 5C or 41F


My WAG is hopefully you are low on refrigerant and not a faulty compressor--what you need to do next is return the tool and visit a shop that can put a set of AC gages on the car and tell you what to do next!!

secondchance550

Even 3 is impressive relative to "zero". I am scheduled to go to a shop this afternoon. I am just doing some homework before so that I don't get bull s**tted by the mechanic.

Rudeney

Looking this up in SDS, the diagnostic stepss for B1256 are:

Based on your readings, I'd say refrigerant is low.

secondchance550

I came to the same conclusion. Time to top off the refrigerant.