Okay, my headline is a little simplified but it's basically true. Would appreciate any insight, recommendations, or guesses! This is a 2015 E350 4Matic wagon. Had it for a couple of years, the issue has persisted the whole time.

In the summer, no issues -- always blows ice cold.

In other seasons, especially noticeable in spring or fall, a typical scenario: Outside temp is 62 F. Auto climate is set to 68. As soon as the engine is up to temp, it starts dumping heat into the cabin and doesn't stop. I have to turn the temp setting down to 62, 61, 60, sometimes all the way to LO. Between 62 and 67 it doesn't really get any less hot. It always works setting to LO however: at that point, I can hear a flap adjusting somewhere inside, and I get cool air. Eventually, it's too cold, so I up the temp to 68. It continues to blow cool air but the fan speed decreases, so it feels okay. I suspect the air is probably colder than it should be for 68, but I live with it. It will stay this way for the rest of the drive, but if I turn the car off, the whole scenario repeats when I turn it back on. On the other hand if I bump it up from a now-cool 68 to 69, 70, 71 ... not much changes, maybe fan speed decreases a little. When I hit 72, 73, or 74 somewhere in there, it switches back to dumping very hot air into the cabin and lowering it back to 68 it will continue blowing hot air. I again have to lower it to the low 60s or LO and we repeat the cycle.

No error codes detected by two mechanics I brought the car to (for other issues, but they didn't see any codes about the ACC having been stored).

The fact that it can put out intense cold or heat tells me those systems are at least working. (I did replace the compressor recently due to faulty magnetic clutch, but it changed nothing about this issue.) The fact that it doesn't happen in summer tells me the temperature sensors can't be totally broken. So I'm left thinking it's either:

• a temperature sensor is maybe reading a little bit off, but not so much to interfere with cooling properly in the summer (maybe it thinks it's always colder than it really is?)
• whatever servo that controls the hot/cold air mix has a hard time with middle settings - it wants to stick open one way or the other
• some general corruption in the climate control computer

I scanned the forums and have seen some similar issues reported, but no apparent common solution. I'm going to get a scanner to read the temperature sensor data (and that has bi-directional capability in case there is some testing I can do of blend actuator doors or something). But would really love some advice of other things to try short of taking it to the dealer, where I'm afraid they'll just say I need to rip and replace the entire system.

 

perhaps its the ambient temp sensor. make sure you did not put any phone holder in the ac vents in the central. The sensor is located nearby the center vent. I was having similar issue, why only blowing cold air when it set to max cold (especially if used on long drive). After relocating the phone holder I can set the ac to 24 C auto and it will blow cold air (I live in hot climate)

 

Thanks for the reply. So my scanner is telling me I have two cabin air temp sensors, one by the ignition switch/key, and the other in the overhead console. Plus vent temp sensors, external temp sensors, evaporator temp sensors, and left and right sunlight sensors. My scanner shows that all sensors are reporting normal looking values except the overhead console one, which is labeled n70b1. It is completely stuck on -50 C, never fluctuating. Some people report the climate control computer averages the two interior ones. Regardless, that reading would explain it pumping out hot air.

Others have reported the thermistor on top of the fan (which draws air to the thermistor) in the overhead console gets fried, or is rattled loose (broken lead). The thermistor by the way is a resistor that changes resistance based on temperature, so the resistance can be read and mapped to specific temperatures. At least in my W212 however, the thermistor is kind of built into the fan housing, which is probably an improvement over the old design as it's definitely not going to rattle loose. I tested it though and it outputs expected ohms and changes appropriately with the temperature. The fan can be pulled from the circuit board as it mounts with pins into a socket, and I tested all the pins are connecting. So at this point, I think it must either be the circuitry on the overhead panel itself, or something deeply unfixable in the climate control computer. I've ordered a used overhead control panel off eBay (only $40) and see if it gives me the same issue.

In the meantime I see in my scanner there is an option to mark the n70b1 as "not installed" so I'm going to try that while waiting for the part -- maybe that will force the ACC to consider only the temperature reported by the single cabin sensor near the steering wheel, which is probably good enough for me anyhow.

 

Depending on model year and car options, n70b1 can be via LIN or CAN BUS.


WITHOUT CAN BUS



I used to have erratic reading from N70/3-b1 <<<<---- take note I wrote N70/3 and not N70.
Tested the thermistor, it was OK.
Clean all the connector into N70/3 modules, and all then normal.
So bad contact I suppose.
Read here on N70/xx , post 52 : https://mbworld.org/forums/e-class-w...ml#post8411067



What scanner are u using ?

This is if using Xentry and XTool and my N70/3 is a NON CAN BUS module. This is when it was erratic.


.







This is using Xtool. When no more glitch.



Measuring the sensor/thermistor

 

More info on N70/b1 : Post #16 onwards
https://mbworld.org/forums/e-class-w...ml#post8764892

 

Thanks! Great information you shared. My scanner is called the LAUNCH Elite ( ).

I'm not sure if I have a CAN bus or LIN bus connection to the OCP. I did try cleaning the pins to the fan and reseating it. The whole board looks very clean and undamaged, so I'm imagining it's a fault in one of the tiny chips near the fan. If it's a bad trace, it must be a totally invisible fault. Regardless of cause, or CAN vs LIN, it seems like there must be something wrong with the board, so I'm hopeful the new OCP I have coming will clear up the problem.

 

For any future reader stuck with this issue, I solved it by getting a new (well, used) OCP off of eBay for about $40. At first it also was reading a straight -50 C as well. But thanks to comments made by S-Prihadi in this thread and elsewhere, I had it in my mind maybe there is something to this sensor being on the CAN Bus in some cars and the LIN Bus on others. I found a setting in my scanner to switch from CAN to LIN, and the sensor started reporting correct values. Problem was totally solved! It confused me at first, but then I did think about the fact that the part that came out of my car (the OCP) was 212-900-10-23-9051 but actually the replacement part I got was part number 212-906-46-01-9051, which is discontinued. The part one I had in my car originally is still available from MB, so I assume it is a newer version that uses CAN bus, and this older version I got uses LIN bus.

As I mentioned above, there is also an option in my scanner to mark the OCP temp sensor as "not installed." I did this for a while before the replacement OCP arrived, and it actually worked pretty well. The only disadvantage is that it then relies solely on the sensor by the steering column, which is sort of tucked into the dashboard where I assume it's better insulated and hotter. It's temp was always reading hotter than the air in the cabin actually is, so on cold days the climate control was blowing a bit too cool and I had to increase the temp. But worth a try, certainly an improvement over the prior situation. Now with both sensors it seems to average them or have some other algorithm that is tuned pretty well.

If you have a LAUNCH Elite scanner, here are the menu paths to change the sensor installation value, and the CAN/LIN bus setting for the sensor:

Enable/disable OCP temp sensor
Control Units > Air Conditioning > AC - Air Conditioning(N22/7) > Special Function > Configuration-Manual Settings > Workshop Coding > N70b1(Interior Temperature Sensor With Integrated Fan)
Select INSTALLED or NOT INSTALLED

CAN/LIN bus toggle
Control Units > Air Conditioning > AC - Air Conditioning(N22/7) > Special Function > Configuration-Manual Settings > Workshop Coding > The Value Of The Interior Temperature Sensor In The Overhead Control Panel Is Transmitted Via The CAN Bus
Choose YES for CAN bus and NO for LIN bus

 

The original OCP reporting should not have to be recoded to work well over whatever CAN/LIN bus W212 is using.
A used OCP unit: yes perhaps settings were mismatched to your chassis and added extra issue.

I dealt with this (scanner) issue that was only poor OCP support from AUTEL.... absolutely ok with a LAUNCH scanner!

A good working scanner should read all temps in the central AAC Module. That is where temps are acted upon with flap actuators.

W212 has no coolant valve to control heater core: always 200F engine coolant inside dashboard.

Double check AAC Module temp data are nearly accurate... nothing like 0° or -50°.
Then suspect flap actuators.