Hi folks:

New here. Quick question for you AC guys. I just got this car this week. AC not blowing cold and everything checks out, not throwing any AC codes. Fuses are fine. BUT…I’m not getting any power to the compressor and the B/12/2 pressure/temp sensor is showing 7 BAR on pressure and -150F on the temp side…Any idea why I would be getting -150F on the temp side? Could this be a “short to ground?” I’m certain -150F is not within the correct specifications. It is one 3 wire sensor, and I am getting pressure. Getting 70% signal to compressor, but no power to the compressor at all (supposed to be 700 Ma or so.) I am assuming that this sensor (or something else) is not allowing the compressor to engage. It’s a clutchless one. No error lights on AC and no CAN errors in any module. I’m stumped…Thanks so much for any help you guys may be able to provide…I’m looking forward to participating on this forum as I was on the other one for years helping folks with their 300D’s…

 

Sensor is showing -150F because it's not reading temperature, it's just a pressure sensor. It would need to be 4 wire sensor to read the temp, too.
I was on this last year, and also thought the issue was the temp reading.
Also learned the hard way that if you have power switched on and pull the connector from the sensor, it will lock the A/C and you'll need a Star to unlock it.

 

Hummm. Digging a little deeper, I’m getting an N70b1 “In-car temp sensor OCP (Overhead Control Panel)” valve of 207 F….

 

Hello Jacks Barnes,

I have exactly the same problem, my diagnostic tool is also showing an interieur temperature of -127 degrees. The temp sensor (n70b1) as i found on the internet is in the roof module. I manage to take it out and measured the ntc thermistor with 7,9K ohm on normal room temperature, when i was heating it with a heatgun it is dropping to 3,5K ohm. So it looks like it is working, but did you solve your problem already? friendly regards Adidas1977

 

I am getting the proper reading at the compressor on the OCP interior temp sensor, but not at the sensor itself. That could be my issue. However, I believe that the B/12/2 AC temp and pressure sensor is most likely the culprit, as I am getting 7 BAR on the pressure side but -150F on the temperature side. So, I’m getting a implausible value on the temp side, but zero codes thrown. A new sensor was ordered and I will test the resistance on the new one and compare to the old. Also, the low side diagnostic reading says 7 BAR, but only 5 BAR on the manifold gauge…I could be wrong, but I am thinking that the sensor will only throw a code IF the sensor shows an open circuit and may not if it’s out of specifications. I’m going to fool with it over the weekend and I’ll post my findings

 

Ok thanks for your reaction, i am currious what will be the outcome.

 

I replied on some other thread about that B12/2 sensor. If it's 3 pin version, it's only for pressure, and for temperature scanner shows -99°C/-147°F because there's nothing to read. Sensor that shows also temperature, is either 4, or 5 pin.
Caution, if you go and pull the connector from the B12/2 and ignition is on the run position, AC will lock, and you'll need a Star to unlock it.

 

I’m confused. The FSM specifically says B/12/2 refigerant pressure and temperature sensor. Wired to the Drivers side SAM, then I assume to the AC CAN BUS. It is a 3 wire sensor. I could be incorrect, but it is my understanding that one pin is power, one pin is temp signal, one pin is pressure signal and the sensor is grounded on the AC low line. I’m going to run some tests over the weekend and I’ll post my findings

 

Fantastic. I finally got a code. Code 9064. B/12/2 Refigerant Temperature and Refrigerant Pressure Sensor is faulty
Or Open Circuit. What I suspected….Now to evac and replace sensor. 👍

 

Sensors don't ground through the housing. Three wires; voltage, ground, signal. Even though I knew this, I thought that maybe this is different. Well, because I was thinking that there would be temperature sensor, too. But no.
I remembered today, that I bought a new sensor last summer. Of course I had to try it as soon as I got it, but still no temp reading. Earlier I said that if you pull the plug when ignition is on the run position, it will lock the A/C. Well, not quite. When I got that new sensor, I just plugged it in to see would there be an actual temperature. And on that very moment I introduced no pressure situation, which locked the A/C.

I've tried to find the site where it was stated that only 4-zone A/C uses temperature, and that sensor would have four pins, but no luck so far.

 

Well, not sure about that 4-zone thing, but I found a document GF83.40-P-2171T, that states that B12/2 was used up to 31.5 2005, and from 1.6.2005 there is a B12 sensor that monitors only the pressure.
I use iCarsoft MB V2.0, and it shows the sensor as B12/2, which seems to be misleading.

 

Ok. Now that makes sense. I tested the new sensor today, exactly as you did and alas, the same temperature reading of -150F AND zero pressure. The CEL went off immediately after I connected the new sensor. I reconnected to the (faulty) sensor and pressure reading back to 7 BAR, and the same temperature reading of -150F, so I don’t think my AC system locked, as I shut everything down and removed the key prior to connecting and reconnecting the sensor for the test. I did get another “low refrigerant code” after I tested the new sensor and reconnected to the original sensor and the AC CAN switch is now blinking red, a signal of low refrigerant or a faulty sensor I assume, even though I show over 5 BAR on the manifold gauge. The code that originally threw finally after weeks of not throwing any codes at all was B/12/2 AC pressure and temp sensor faulty or open circuit (on the pressure side.) I had thought the sensor might be grounded at the SAM and perhaps even at the actual low pressure line, but I wasn’t sure how it was grounded as I was sure (apparently incorrectly) that one pin was for the temp signal…as the FSM stated B/12/2 Temp and Pressure sensor and not just B/12, which is only the pressure sensor. I kept looking for the correct B/12/2 sensor part and via my VIN, MB keeps saying I have the correct part, which is a 3 wire pressure sensor. So, in fact, it does not have a temperature signal at that sensor, only pressure. BUT, if you look at the schematics, it says that B/12 sensor on that 211 Bluetec is installed somewhere underneath the compressor and NOT on the AC low line under the coolant reservoir. So that threw me off. Thanks so much for your help. I was about ready to evac the system and pull the sensor and try to get the actual part number off of it to be absolutely sure.

 

Yes. I’m using Creader Elite MB and it says B/12/2, not B/12…

 

Replaced the B/12/2 pressure sensor. Code eliminated. Compressor still will not engage. Time to test compressor. No codes at the moment.

 

Commanding compressor to activate at 70%, 0 milliamps at compressor. Bad solenoid I assume. I’ll test for resistance and voltage at the wiring and solenoid.

 

Compressor solenoid reading 11.2 OHMS. I’m getting 2 different specifications regarding resistance. One spec says 1-5 OHMS and the solenoid is bad, the other spec says 8-16 OHMS and the solenoid is good. Proper compressor voltage from AC CAN. Any help as to wether or not this solenoid is good would be certainly appreciated! Thanks all.

 

When I rebuilt the AC system in my 2003 W211, the first AC Compressor control valve module (solenoid) I received didn't have a diode. Instead of the old solenoid on/off clutch that you can see and hear turning or stopped, the W211 AC compressor uses a variable control valve solenoid. The compressor always turns; the control valve module varies the compression. When you energize a solenoid (electromagnet), current flows in. When you de-energize the solenoid, the field collapses and current wants to flow backwards. This impulse going back upstream can sause arcing at the solenoid switch contacts and other places. A diode in the control valve circuit prevents that current backflow. You will also find diodes in the solenoid circuit for pinball machine flippers, for the same reasons. The W211 AC control module is inserted into the AC compressor and can be removed and replaced without removing the compressor. BE CAREFUL!
Be sure to depressurize that AC system before working the AC compressor conrtol valve loose from its tight o-rings fit. Because if you don't, it will shoot outta there like a 20mm cannon shell, ricochet off your concrete floor and embed itself in the wall on the other side of your garage. And when you buy the compressor control valve, be sure it has the dot on it and the little diode symbol on the end of it, indicating it's got the diode in it. A new control valve just might clear up your AC issues.

 

Got it! Thanks so much for your reply.

 

And of course, after you've opened the AC system you MUST hook it up to a good vacuum pump and vac all the freon and oil and moisture out of the system. Don't skip doing this or your system will corrode and fail from the little bit of moisture. Any AC place can do it. Or for about the same cost you can buy a good vac pump, a digital scale, a guage set, put in 120 ml PAG 46 oil and 950g of 134a and do it perfectly yourself.

 

Yup. Already done. System evacuated already as I replaced the B/12 pressure sensor that was throwing a code. Took about 1/2 oz. of oil out. Already have gauges and vac pump as this isn’t the first time I’ve had to recharge a system. Thanks for the information and I’ll post after I replace the solenoid 👍👍👍. Hopefully that is the issue as all the sensors check out and there aren’t any codes throwing at the moment.

 

Please post how this turned out. Is it still good?

thanks,
Peter

 

The problem was a ZGW DRACO coding error. After a low-voltage incident, the air conditioning DRACO parameters (and a few others) were showing NOT FITTED when they should have been FITTED. Seems the low voltage incident caused the ZGW to revert back to its factory level configuration. Tough one to diagnose, but I finally got it after testing everything from top to bottom. I went through each and every module, checking and comparing all the variant coding, that’s how I located the error.

 

Yikes! I don’t know if I could ever solve that one. Wow. Good on you