Why does my 99 Pathfinder heater/AC mode switch not change where the blower directs the airflow. Mine is stuck on defrost and does not change when I select floor or dash vents?

There are several possible reasons why your blower, vent, defrost, AC, and heater might stop blowing air:

1. Blower Motor Failure: The blower motor is responsible for blowing the air through the vents. If the blower motor fails, you will not get any airflow from the vents.
2. Blower Motor Resistor Failure: The blower motor resistor controls the speed of the blower motor. If the resistor fails, the blower motor may stop working or only work at one speed.
3. Fuse or Relay Failure: A blown fuse or faulty relay in the blower motor circuit can prevent the blower motor from working.
4. Control Module Failure: The HVAC (heating, ventilation, and air conditioning) control module is responsible for controlling the blower motor and other components. If the control module fails, it can prevent the blower motor from working.
5. Vacuum Leak: If your vehicle uses a vacuum system to control the vents, a vacuum leak can prevent the system from working properly.
6. Broken Blend Door: The blend door controls the direction of the airflow. If the blend door breaks or gets stuck, the air may not flow through the correct vents.
7. Wiring or Connector Issues: Faulty wiring or connectors in the blower motor circuit can prevent the blower motor from working.

COULD BE BLOWER MOTOR OR RESISTER

This is caused by a low engine vacuum, the engine vacuum is low when you accelerate, the HVAC system loses vacuum to the mode control vacuum switch and defaults to the defrost ducts for safety. You likely have a leaking mode control vacuum switch in the control panel of the heater AC system (HVAC).

What A/C problem did you have , original complaint for taking to the dealer ? Does the blower motor blow air out of the vents , when switching from floor to center vent to defrost does air change direction ? What make model an year is your vehicle . Does your vehicle have manual or automatic climate control . Were there any codes stored in the control panel ? Mode door or temperature control problem ? Do you here any clicking behind the dash ?

. Air won't blow out of the floor or center vents just defrost ? Do you have manual or automatic climate control ?
Air Flow Control
Air flow control is accomplished in the following manner:

• Primary control is through the function selector knob mounted on the A/C-heater function selector switch, which is part of the integrated control panel.
• The function selector knob has the following positions: OFF, MAX A/C, A/C, PANEL, PANEL/FLOOR, FLOOR, FLOOR/DEFROST and DEFROST. The A/C-heater function selector switch combines a vacuum selector valve with an internal electrical switch.
• The vacuum selector valve directs source vacuum to various vacuum control motors (18A318). Refer to the Vacuum Control Diagram , System Airflow Schematics and Vacuum Control Chart .
• Two internal, single-pole electrical switches are also controlled by the A/C-heater function selector switch. The combination of these electrical switches controls the electrical supply to the A/C clutch and heater blower motor switch (18578).
• The position of the function selector knob determines the manner in which the system will operate.
• Each position of the function selector knob is detented for positive engagement.
• Vacuum Control Motors The vacuum control motors (18A318):
  
  • are located on the A/C evaporator housing, A/C recirculation air duct and heater air plenum chamber (18471).
  • direct system airflow to the vehicle interior as determined by the remote climate control module or manual override.
  • The five-passenger seating configuration has an additional vacuum control motor mounted on the heater air plenum chamber to control airflow to the rear seat A/C register (19893).
  
  

Here is the info. I hope the pictures show up. please rate assistance. thx.

Air distribution within the vehicle is determined by the function selector switch position. Two airflow mode doors are used to direct airflow within the heater core and evaporator core housing. The function selector switch/cable assembly is used to position these airflow mode doors. Refer to Section 412-00 for description and operation of each of the system functions.
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Function Selector Cable Assembly
The function selector cable assembly moves the airflow mode doors to the desired position using a 2-sided pull-pull type cable assembly. The function selector controls the cable movement. The end of the cable assembly attaches to the airflow mode door/temperature blend door actuating assembly which transfers the cable movement to the 2 airflow mode doors.
An electrical switch is incorporated into the function selector to notify the A/C switch assembly if DEFROST or FLOOR/DEFROST is selected. When DEFROST or FLOOR/DEFROST is selected, the A/C switch assembly will disable the RECIRC request button and send an A/C request to the powertrain control module (PCM) regardless of the A/C request button indicator ON/OFF status.
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SECTION 412-01: Climate Control 2005 Focus Workshop Manual REMOVAL AND INSTALLATION Procedure revision date: 02/25/2004
Function Selector Switch Printable View (173 KB) Removal and Installation

1. Remove the 4 bolts and position aside the lower steering column cover.

1. Remove the function selector knob.

1. Remove the center instrument panel finish panel.
   • Disconnect the cigar lighter electrical connector.

1. Detach the function selector switch.
   1. Disconnect the electrical connector.
   1. Remove the screw and detach the function selector switch.

1. Detach the function selector cable and remove the function selector switch/cable assembly.

1. To install, reverse the removal procedure.

When the engine is cold,check the coolant level.

Make sure it's on max.

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A coolant system should be flush every two years.

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If you did a coolant flush recently then air pocket can be the cause of the un-even heating problem.

The DIY method is to drive you car onto a ramp or a steep hill.

Iet the engine cool off and top off the coolant tank.

Set MAX heat + MAX fan

Let the engine ran for 20 minutes wih the cap off and the trap air will escape from the coolant reserve tank,

Check coolant level and top off coolant when the engine cools down.



A shop will charge $75+ for this but using an air operated excavate tool!

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A car with over 90K miles may experience linkage problem to the heater control valve.

The heater control valve (near the firewall) may be stick over time.

Open the hood and move the heater to max and min. Back and forth.

Clean and lube the linkage with WD-40 to free the heater control valve.

If it's a vacuum type heater control valve then cash for leaky vac. hoses.


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Move the fan switch back and forth to re-new the electrical conductors.

Pass a vacuum over the control panel to prevent dust build up behind the console.

Defrost is the default mode so if the system fais, you alays have defrost. The mode doors are controlled by vacuum motors. I would suspect a vacuum leak in the engine compartment on the hose going from the engine through the firewall and into the heater controls. Some systems have a vacuum storage canister that can also leak. The canister stores vacuum when vacuum drops going up a hill.
Or, you have a bad heater control head that is not passing vacuum to the vacuum motors on the mode doors.

Here's a diagram of the system for reference.

there is a plug/electrical that is almost directly below the dash on the hump of the floor board. right hand side of it. below carpet though. move connection there and see if it affects it. there is a wiring issue on I know 03 and on where harness that connects to blower motor melts. also has a "resistor" that goes bad. you can buy both from gm dealer.