Why does the AC blow cold air on the driver’s side but hot air on the passenger’s side?
By
Vroomouk
This blog will explain the causes of hot air coming from one side and cold air from the other. The answers to the following questions are also provided in the blog: What to do if AC blows cold air from one side but hot air on the other?
Why does the AC blow cold air on the driver’s side but hot air on the passenger’s side?
The AC blows cold air from the driver’s side but hot air on the passenger’s side due to the following reasons:
- Faulty flap actuating motor
- Mechanical fault in the blending air flap
- Open circuit or short circuit in the electrical line
- Temperature sensor malfunction
Let’s take a detailed look at each of the reasons and the possible remedies:
Faulty flap actuating motor
The air conditioning housing consists of up to 20 actuating motors that are used to control blending air flaps. These motors can open or close a flap so the air can be directed towards or bypass the evaporator coil.
In vehicles with more than one zone of climate control, these flaps are used to blend the air. The air conditioning control unit uses a LIN signal to control the position of the flap. Hence, when cold air is coming from the driver’s side and hot air from the passenger side, it means that the motor did not open the blending air flap.
The air conditioning control unit runs a self-check routine when the ignition is turned on and if the position of the flaps is not okay, a fault code will be registered in the control unit. The fault code can be read -out by any OBDII scanner.
According to the fault code, the faulty actuating motor can be easily identified and further diagnosis can be performed. The further diagnosis steps include checking the power supply of the motor and the LIN signal from the control unit.
The power supply of the actuating motor must be between 11-14.5 volts and the LIN signal is around 7-12 V. When the electrical coupler is opened, the air condition control unit must be reset in order to check the LIN signal.
If the power supply and the LIN signals are within range then the actuating motor can be replaced. After replacing the actuator motor, normalization of all the motors must be carried out from the adaptation menu in the air conditioning control unit.
The normalization routine consists of storing the open and closed positions of the flaps in the air conditioning housing. The air conditioning control unit uses this data to actuate the motors in the housing.
Mechanical fault in the blending air flap
The blending air flap in the air conditioning housing is used to mix the cold and the hot air in order to blow the air at the desired temperature. The flap is controlled by an electrical actuating motor.
If the normalization of the actuating motors is unsuccessful and a consequential fault code is stored in the control unit then the flap must be inspected for a mechanical fault. The actuating motor is either directly connected to the flap or is connected to a linkage.
If the linkage that is used to operate the blending air flap is broken, the actuator motor continues to rotate and the flap will remain closed. The linkage can break due to a dislocated flap in the air conditioning housing.
If a mechanical fault is present in the air conditioning housing, the entire housing must be replaced. It is not possible to open the housing and repair the dislocated or mechanically blocked flap.
Replacing the entire housing is a complicated task as the complete dashboard must be removed in order to access the air conditioning housing. The refrigerant is also extracted because the evaporator coil is also replaced along with the air conditioning housing.
After the replacement is complete, the air conditioning control unit is again normalized as all the actuating motors are new. If the normalization routine is not performed the air conditioning control unit cannot function properly.
Open circuit or short circuit in the electrical lines
The air condition control unit is the master control unit that controls the actuating motors installed on the air conditioning housing. The actuating motors act as slaves and respond according to the signal from the air conditioning control unit.
All the actuating motors are connected in series and the electrical lines from the air condition control unit form the series for sending and receiving the LIN signal. If there is an open circuit or a short circuit in the LIN electrical line, there is a break in the LIN network.
The last motor in the series that is connected to the control unit will send a signal and the control unit will register a fault code. The motors after the open circuit cannot operate as the LIN signal cannot be transferred.
The electrical lines can be repaired according to the manufacturer’s repair instructions or the entire wiring harness can be replaced. It is an expensive repair as the center console along with the dashboard must be removed to inspect or replace the wiring.
Temperature sensor malfunction
The air conditioning control unit is connected to the in-car temperature sensor. In vehicles with multi-zone climate control, there are temperature sensors at the outlet of the air conditioning vents.
The control measures the temperature of the air blowing from the ac vents and adjusts the air blending flaps accordingly. All the temperature sensors are negative temperature coefficient types. As the temperature of the sensor decreases, its resistance will increase.
The air conditioning control unit will not be able to detect the temperature of air blowing from the driver or passenger side AC vent if the temperature sensor is not working. Measuring the resistance of the temperature sensor and comparing it with the specified values can help to determine the malfunctioning sensor.
Replacing the sensor can be easy as most of the air conditioning vent outlets are accessible after removing the ac vent or nozzle. The cost of the temperature sensor is also very low.
What to do if AC blows cold air from one side but hot air on the other?
If the AC blows cold air from one side but hot air on the other side then performs the following steps.
- Turn off the AC and select the different airflow modes on the control unit.
- Actuate the defrost modes on the front and rear windshields.
- Turn off the ignition and lock the vehicle.
- Unlock the vehicle and check if the AC blows cold air from both sides.
If the fault is still present then the air condition system must be checked by the authorized workshop.
Conclusion
This blog explained the causes and the possible remedies that can cause the air conditioning system to blow hot air from one side and cold air from the other. These types of faults are only present in vehicles with multi-zone climate control systems.
