ML320 ML430 Blowing Hot Air Only Problem W163: Solution

Last updated: October 24, 2026

Is your ML blowing hot air even though you have the AC in a cold setting? A common problem with the ML320/ML430 is that the AC gets stuck on a Hot or Cold setting. The usual cause is the blend motor flap getting stuck and no longer moving when you rotate the AC knob between Hot and Cold, though it is worth confirming that before buying anything, and there is a free test for it below. If you remove the glove box, you can manually open or close the blend motor flap. By manually moving the blend motor flap, you may be able to free it up, and your AC may start to operate normally again. Otherwise, you need to replace the following part. Click on the link below to check the price on Amazon.

Defective Part

Mercedes w163 A/C heater control Flap Actuator Motor ml-class heating airflow Part Number: 1638200108 163 820 01 08

A common symptom of this part failing is the fact that only hot air blows from the vents. The air coming in may be cold, your AC setting is on cold but you get only hot air from the vents.

How to check the A/C air blend motor flap

The following video will show you how to check the operation of the A/C air blend heater flap. You can also manually switch the stuck flap to COLD settings if you need a temporary solution. Otherwise, buy the part above and replace it.

First make sure it really is the flap

This is worth a minute because the actuator is not a cheap part, and the symptom it causes looks identical to a couple of other faults. The test costs nothing and uses the same flap you can reach behind the glove box. With the engine running and the fan on, move the flap by hand to the cold position, hold it there, and feel the air at the vents for a minute.

If the air turns cold, the flap and its actuator are the problem. The mechanism itself works and simply was not being driven to the right position, which is the fault this page describes and what the part listed above fixes.

If the air stays warm while you hold the flap in the cold position, stop before ordering anything. The flap is doing its job and the air is not being cooled in the first place, so the question is on the refrigerant side and a new blend motor will not change the temperature at the vents. The reverse test works the same way: if you cannot get heat with the flap held toward hot, what matters is whether hot coolant is reaching the heater core, not whether the actuator moves.

Solution

1. Start by removing the glove box. Open the glove box and you will see the four screws that hold it in place.
2. Once you remove the screws pull out the glove box.
3. Behind the glove box (on the left), you will see a white plastic arm. This is the flap that switches the airflow from hot to cold. It gets stuck in the HOT position. After you remove the glove box, put your hand through and manually move the flap a couple of times. If you want to switch it to cold, you need to push the flap (white arm) toward the front of the car.
4. You can turn the AC to hot and cold a couple of times. Pay attention to the white arm on top of the blend motor.

   

Replacing the blend motor

First, try to operate it manually a couple of times. Hopefully, it will start to work again. If the flap will not hold a position or will not move at all, the actuator behind it has failed and needs to be replaced. Note that this is the blend motor, the small actuator that drives the flap. It is not the blower motor, which is the fan that pushes air through the vents and is a different part in a different place. To replace the blend motor, you need a set of 3/8-Inch Drive Star Bit Socket Set to remove the glove box and your toolbox.

This video may help you if you plan to replace the air blend motor.