2008 ML 320 CDI. Well maintained car from Florida purchased this summer, I noted no issues with the AC this summer.

Now that winter is here, odd issue noted, when the controls are set to Auto and 72f selected, the car warms up, the fan spools up as the coolant warms and heat flows out of everywhere but the floor vents. An excessive amount of air is ducted through the two outer dash vents and not to the floor as expected. Closing them does not force the air to the floor. The air flowing out under the front seats is COLD, not good for rear seat passengers.

If I switch to manual mode, recirculate works, I can force air to the windshield, out the center ducts but not to the floor. The fan controller works, although the fan increase speed button sticks occasionally.

Using two different bi-directional scan tools, the only code is in the AAC module, 9403, M2/12 rear shut off flap positioning motor LIN bus communication fault. I removed the lower right portion of the dash, glove box and the center console and removed the actuator. There is 12v on the LIN bus wiring. The actuator is moving and the flap moves without binding when the actuator is removed and the flap moved manually. I switched the rear shut off flap actuator with the right blending flap actuator (same part number) and still the problem persists, as does the code for the LIN bus error.

If I set both temps to maximum and push the maximum windshield defrost button, hot air comes out of the drivers side windshield duct but cool air is issued onto the passengers windshield. If I leave everything set to AUTO and crank the temps to MAX, hot air flows out of everywhere evenly, except the floor. There is no variation in air temps side to side when set to auto.

I'm leaning towards an issue with the AC controller but welcome other thoughts.

Thanks, Dave in Ottawa with cold feet.

 

Follow up, today I took the controller out of the dash and back probed the connector to check for LIN voltage on terminal 16 of the controller and it is present. I also disassembled the controller to clean up the sticky heater fan button. Didn't note any terminal pin issues or anything obviously bad on the controller board.

I still have the error code for the LIN comm error on the rear shut off flap positioning motor, even with a different actuator replacing the original one.

I can observe the different actuators moving with the scan tool, from 0 to 100% and they all seem to be ok however the data for the M2/12 controller shows a maximum of 42% even when the request is for 100%. Since the problem is still there with a different actuator, and the original actuator shows 0 to 100% in its new position, I don't believe my LIN issue is with the actuator.

Guess this is a good excuse to ask Santa for a Pico Scope. I still have very little air (almost none) to the floor vents and excess to the two outer dash vents when the maximum to the floor is requested.

Any ideas?

 

Another observation today. Still have code 9403, M2/12 Rear shut off flap positioning motor LIN bus communication error. With the ML warmed up (its -1c today) I cleared the code and it returns almost immediately. No or very little airflow to the floor. I had the temps set to 72 and "Auto", the vehicle was warm. With the iCarsoft monitoring the M2/12 actuator, I shut off the car and pushed the "AC/Rest" button which will keep the blower running and the car warm for a period. Immediately heat start to waft from the floor vents. The position specified by the module was 100% and the current position of the actuator motor was now at 100% too. I've never seen it above 43%. I did note that the "actual position of motor" was at 8000 Step. This must be an error as all the other actuators position between 0 and 2800+/-.

After a few seconds of floor heat it shifts again to the dash and the actuator numbers return to a normal range.

My 14 CLS always ducts to the floor during "AC/rest" operation with the car off.

I do feel that there ACC module is causing the issues with the LIN bus but again, hopefully someone will chime in.

 

Fixed!

Many thanks to Konigstiger!

After pouring over wiring diagrams and troubleshooting flowcharts, I didn't find any obvious faults, corroded connectors, bent pins etc.

I borrowed an Autel MS909 and did a relearn for the actuator motors via the tool and voila, everything is working properly, heat on the floor, even heat both sides on defrost.

There were several historical under voltage errors stored in both front and rear SAM modules. I hypothesize that somehow the heater actuator motors somehow lost their way and needed to be "reset". Pressing two buttons on the control panel does not perform a reset with the 164 platform as it appears to on earlier models.
The battery in this car was original (car came from Florida) so the new battery I've installed with a reset using the Autel seems to have fixed everything.