The duct doors control the airflow through the duct box. The types of duct doors include the following examples. Fresh/Recirculated Air Controls where the duct box air is pulled from. The two positions for this door include fresh air and recirculated air.
Blend Air Controls the temperature of the duct box air by forcing it through or around the heater core. When the blend door forces the air through the heater core, the air is warmed up, but when blend door causes the air to bypass the heater core, the air is not heated. This door can be full hot, full cold, or at any point between these two limits.
Mode Controls where the air is distributed into the cab. The three mode options include defrost, vent, and floor. Many systems allow for bi-level or tri-level operation, which allows the air to be distributed at two or three locations at once. For example, it is possible to have air coming from the defrost port as well as the floor, or all three locations.
These doors can be moved by either cables, vacuum actuator, or electric actuators. See the steps below for troubleshooting each type of system.
Testing Duct Door Operation
1. Cable-operated duct doors: Quickly move the selector to both ends of the range on the control head and listen for a snapping sound under the dash. If a distinct snap is heard at both ends of the range, then the cable is operating correctly and the door is hitting the limits of travel. If a snap is only heard on one end of the travel, then the cable length will need to be adjusted. The adjustment is correct when both ends of the travel causes the door to move to its limits. A reliable service information database can be consulted to determine how to adjust the cable length. The HVAC control head will need to be removed to replace one of the duct door cables.
2. Vacuum-operated duct doors: Test for vacuum being supplied to the HVAC control head. If no vacuum is present, seek the problem closer to the vacuum source, such as the engine or vacuum pump. Check the one-way check valve operation. Use a vacuum pump at the engine area to see if the system will hold vacuum. Test each vacuum actuator to see if it holds vacuum for a minute. Look for damaged vacuum tubing.
3. Electric-operated duct doors: Test for voltage at the duct door actuator while moving the controls on the control head. If voltage is present and the actuator does not move, it is faulty. The actuator can also be removed from the duct box to see if the duct door can be moved by hand.
These electric actuators also need to be calibrated if vehicle’s battery power is lost or the actuator has been replaced. Calibration can sometimes be performed by pressing a series of buttons on the control head or by using a scan tool connected to the vehicle data link connector. Scan tools are very useful in checking the system for faults and running output tests as well.
A vacuum pump test kit is handy to use when testing the vacuum-controlled components of the HVAC system. The gauge is useful when trying to measure vacuum levels and the pump can be used to create vacuum to test vacuum circuits and components. (Photo Courtesy Lincoln
The vacuum actuators on the duct box can be tested by using a vacuum pump to see if each port can be held in place. The hose of the vacuum pump should be connected to each port of the vacuum actuator and then the pump should be actuated to create a vacuum. The vacuum actuator should not leak off for several
A vacuum gauge or a vacuum pump can be used to measure the vacuum supplied by the engine or other vacuum source. Vacuum-operated HVAC systems need to have about 17 to 18 inches of vacuum present to operate
A vacuum pump can be used to create a vacuum to test to see if there are leaks in the hoses that connect to the HVAC control head and actuators. The pump should be actuated until about 18 inches of vacuum is created and it should not leak off for several
A voltmeter can be used to measure the voltage supplied to the duct door actuator. This test is performed by back-probing the terminals at the actuator with the voltmeter leads while adjusting the controls on the HVAC control head. If voltage is present during the test, the control head and wiring is good and the likely problem is the
The data link connector is in the location where a scan tool can be connected, which will allow the operator to view data activity and trouble codes as well as perform output tests on the
The power cord for the scan tool connects to the data link connector. The operator should be careful when connecting and disconnecting the scan tool to not damage the connector or the scan tool power
The scan tool allows for many diagnostic functions to be performed by the operator. Having a quality scan tool greatly increases the diagnostic capabilities of someone working on the HVAC
The scan tool allows the operator to view live data from the many sensors and switches that the AC and heating system
