W245 2009 Air Conditioning Temperature haywire

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New Member
Dec 19, 2021
B 200 CDI Sport W245.208
Hi all

I'm not sure if this really belongs in Servicing, Interior or whatever.
Feel free to lambast me if I've put it in the wrong place.

On my W245, the air conditioning seems to pick hot days to blow hot, cold days to blow cold etc.
If I set the temp to min, it always blows cold, and if I turn it right up, it will blow hot.
But if I set it to a specific temperature, it doesn't seem to want to hold that, it just has a mind of its own.
It's as if it's not getting a valid temperature reading from the cabin air temperature sensor.

Anybody know where the cabin air temp sensor is located, and how easy it is to change?

I've had an hour or so on WIS/ASTA and found it mentioned at N22/7b1 but not much to suggest where this is, or what to do about it.

The sensor is NOT in the dome light assembly.
There is a small grill next to the temperature control knob, I suspect it may be behind that.
But looking at the pics of the control knob assembly on ebay, I don't see it as a changeable unit.

That said, before I try disassembling anything I'd like to try an AC reset, but the videos on the subject refer to buttons that aren't on my unit.
I only have AC, Recirc and Rear Demist buttons, while the videos all say hold Defrost and Recirc.
AC and Recirc together for a while start the flaps moving (I can hear them), but the LEDs don't flash to indicate reset.

Any ideas?
In the unlikely event that anybody is interested.......

I tried out iCarsoft on this one lunchtime, monitoring the internal temp sensor.
The temperature sensor kept climbing, even though the internal temperature itself was stable.
In recirc, the temperature sensor reading was more sensible, and stayed pretty static.

Looking a lot like a duff internal temperature sensor, as I originally thought.

So I got hold of a replacement temperature control panel from ebay - it had a couple of scratches and chips, so I took the board out of it and put it into my housing and refitted it.
Temperature is now staying decidedly more sane in both recirc and fresh air mode.

Using iCarSoft, I could also see that when in recirc mode, the temperature always reads a degree or so higher than when in non-recirc mode.
So it looks like there are 2 sensors, one used when in recirc, the other when in normal mode.

The sensor in the control panel is a tiny SMD device, I'm thinking that there probably is a similar one in the dome light after all, I just missed it because it's so tiny.
Swapping it out for a standard bead thermistor looks like it might be a possibility, but replacement S/H parts on ebay are cheap enough that there's no point.

Tip for anybody else contemplating this fix : the control panel lifts up to remove, you need to take the stereo out and then the control panel just lifts up and out.

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