mihailenator

Hi guys, a few days ago the weirdest issue occurred with my AC and heating.
Basically if I set the temp for both the driver and the passenger to anything above 61 F, the fan is blowing extremely hot air. Regardless whether I set it on Auto or Manual, hot air at all vents.

If I set either side to 61 F or LO, then the fan is blowing ice cold air from the AC from all vents, again regardless whether in Auto or Manual.

When I used my icarsoft to monitor the position of the HVAC flaps, I noticed that as I manipulate the temp settings, the flaps would assume two positions only:
1. When I set either side to 61 F or LO, the icarsoft shows the flaps in position 0.12 or some value of that sort, and respectively the system blows cold air;
2. When I change both sides to anything above 61 F, the icarsoft shows the flaps moving into position 0.89 (quoting the number by memory), and respectively the system starts blowing burning hot air.
The flaps never permanently assume any other position between 0.12 and 0.89, they go through the values between these two numbers, but do not stop until they reach either end of the spectrum.

Any ideas how to troubleshoot?
I already did the button combination to start the program that resets the HVAC flaps, did that a few time , made no difference. Also disconnected the battery for 15 min to try to reset the ECU, again no change.
I was also able to get into the other AC hidden menu, where it shows 01,02,03,04 etc on the left hand side and some respective values on the right hand side, but not sure what to do there.

This is a video of the AC menu and its values as I go through the settings -

Rudeney

When you did the test/reset function, did the lights stop flashing? If so, that means that all the damper servos are report back properly. I suspect this is a problem with the sensor in the overhead control panel. See if iCarsfot can read that. Also, a quick test for that in a cabriolet is to open the roof, which disable that sensor. I am not sure, but opening the windows in the coupe may also disable it. If it is this sensor, it's actually an inexpensive fix if you can do a bit of soldering:

Here is the part you need:

https://www.jaycar.com.au/10k-epoxy-...istor/p/RN3440

Here's how to remove the OCP:

Coupe: http://benzbits.com/w209/OverheadControl%20Panel.pdf

Cabriolet: http://benzbits.com/w209/OverheadCon...lCabriolet.pdf

You'll need to remove the circuit board and access the thermistor, soldered above the small fan.

mihailenator

Hi, yes the lights stop flashing once the vents go through the motions, which I also thought indicates a successful test completion. I'll do the roof test later, but I'm hopeful this may actually be the problem as I've been having water issues in that area lately.

Will report back soon.

mihailenator

Didn't have much time to test in the rain today, but it did look like while the roof was open, the hvac system operated in a normal way. Closed it up, and it went back to the broken hot/cold mode.

Rudeney

In that case, I am fairly certain the problem is that thermistor.

mihailenator

Big Thank You, and just to confirm issue is resolved after replacing the thermistor in the overhead unit. In fact, the thermistor had completely vanished from that location under the small ventilation fan. With the element order from Australia I had simultaneously ordered a second hand overhead unit, which happened to have the exact same issue as well.

Anyway, I have a few spare thermistors now, if anybody needs one gimme a shout.

Rudeney

I'm glad to hear that fixed it!