1999 Mazda B3000 AC only coming through defrost

Hi loganb233,

There are many possibilities but the most common and likely cause would be vacumn fault.

Here are the diagnostic procedures.

TEST NO. 1: IMPROPER/ERRATIC DIRECTION OF AIRFLOW FROM OUTLETS
Check Vacuum Circuit

1.Start engine. Set blower speed to maximum setting. Check for proper airflow operation while selecting each mode. If airflow is from defrost outlets in all modes, go to next step. If airflow changes from defrost to different outlets, go to step 11 .

2.Turn ignition off. Inspect vacuum supply hose and connections between intake manifold and vacuum reservoir. If vacuum supply hose is not disconnected, go to next step. If vacuum supply hose is disconnected, reconnect and perform step 1 .

3.Start engine. Disconnect intake manifold vacuum hose. Check for vacuum at Black vacuum hose from vacuum reservoir. If vacuum exists, go to step
8 . If vacuum does not exists, go to next step.

4.Reconnect intake manifold vacuum hose. Using a vacuum pump, leak test vacuum supply hose (Black) at 15 in. Hg of vacuum. If vacuum does not hold or leaks down slowly, hose is blocked, restricted or damaged, replace vacuum supply hose. If vacuum is held, go to next step.

5.On 4WD models, disconnect transfer case Pulse Vacuum Hub (PVH) solenoid electrical connector located behind inner fender well. Disconnect PVH vacuum supply line located at rear of vacuum reservoir. Apply 15 in. Hg of vacuum to PVH vacuum supply line. If vacuum is held, go to next step. If vacuum reading drop exceeds 1 in. Hg per minute, repair transfer case/PVH vacuum circuit as necessary.
6.On all models, disconnect vacuum reservoir tank vacuum hose. Connect vacuum pump to one end of hose and plug other end. If vacuum hose leaks, is restricted or blocked, repair as necessary. If vacuum hose does not leak, remove vacuum reservoir tank. Connect vacuum pump to vacuum supply port of reservoir and plug remaining reservoir ports. Apply 15 in. Hg of vacuum to reservoir. If vacuum is held, go to next step. If vacuum drop exceeds 1 in. Hg per
minute, replace vacuum reservoir.

NOTE:
Vacuum hoses at vacuum reservoir are not interchangeable. Vacuum check valve is an integral part of reservoir tank.

7.Connect vacuum pump to vacuum supply port of reservoir. Apply 15 in. Hg of vacuum to reservoir. If vacuum is held, vacuum reservoir tank is plugged, replace reservoir tank. If vacuum drops slowly, reservoir is restricted, replace vacuum reservoir. If vacuum is not held, go to next step.

8.Disconnect mode switch vacuum harness from reservoir tank. Plug A/C-heater control panel vacuum supply line at reservoir. Disconnect in-line vacuum harness connector to A/C-heater control panel. Apply 15 in. Hg of vacuum to Black hose of supply side vacuum harness connector. If vacuum drops, repair or replace control panel vacuum supply line. If vacuum is held, go to next step.

9.Connect mode switch vacuum harness to reservoir tank. Disconnect in-line vacuum harness. Connect vacuum pump to Black hose of control panel side vacuum harness. Plug remaining vacuum hoses. Apply 15 in. Hg of vacuum in each A/C-heater control panel mode setting. If vacuum reading drop exceeds 1 in. Hg per minute, note function selector position and go to next step. If vacuum is held in each mode setting, go to step 11 .

10.Disconnect mode switch vacuum harness from control unit. Connect vacuum pump to vacuum supply port and plug control port that indicated leak. Apply 15 in. Hg of vacuum in each A/C-heater control panel mode setting. If vacuum reading drop exceeds 0.5 in. Hg per minute, replace function selector switch. If vacuum is held, replace vacuum jumper harness.

11.Connect in-line and reservoir tank vacuum lines. Disconnect mode switch vacuum line from control unit. Connect vacuum pump to Black supply hose of function supply switch. Apply 15 in. Hg of vacuum. If vacuum pulls down and holds or leaks slowly, hose is plugged or restricted. If hose is plugged or restricted, replace vacuum harness. If hose is not plugged or restricted, reconnect function selector switch vacuum harness and go to next step.

12.Perform step 1 again. If air flow is not correct for each function selector position, go to next step. If air flow is correct for each function selector position, go to step 14 .

13.Check for proper air flow in each function selector position during engine acceleration. If air flow changes to defroster vents during acceleration, go to next step. If air flow does not change to defroster vents during acceleration, go to step 17 .

14.If system operated properly prior to customer complaint, go to step 16 . If system did not operate properly prior to customer complaint, disconnect all actuators. Disconnect mode switch vacuum line. Connect vacuum pump to each hose and apply vacuum. If pump can pull and holds a vacuum or vacuum leaks down slowly, hose is plugged or restricted. If hose(s) are not plugged or restricted, go to next step . If hose(s) are plugged or restricted, replace appropriate hose(s) as necessary. Recheck system operation.

15.Verify vacuum hose color in each harness to function selector vacuum connector. If hose color agrees with color identification table, go to next step. If hose color does not agree with color identification table, replace vacuum harness. Recheck system operation.

16.Check A/C vacuum circuit for kinked or pinched vacuum hose. If hoses are not kinked or pinched, go to next step. If hoses are kinked or pinched, reposition vacuum lines. Recheck system operation.
17.On 4WD models, disconnect Pulse Vacuum Hub (PVH) solenoid 3-pin connector (if equipped). Plug 4WD hub solenoid at vacuum reservoir tank. Apply 15 in. Hg of vacuum to reservoir tank. If vacuum reading drop exceeds 1 in. Hg per minute, remove vacuum check valve from tester and plug vacuum hose. Apply 15 in. Hg of vacuum to ensure hose and pump are not source of leak. If vacuum reading drop exceeds 1 in. Hg per minute, replace vacuum tank. If vacuum is held, go to next step.

18.Turn ignition off. Ensure each vacuum hose connection is not partially disconnected or disconnected. Reconnect as necessary. If vacuum hoses are not disconnected or partially disconnected, go to next step.

19.Disconnect suspect hose. Plug one end of hose and attach vacuum pump to other end. Apply 15 in. Hg of vacuum to hose. If vacuum hose does not hold vacuum, replace hose. If hose holds vacuum, disconnect suspect vacuum actuator. Connect vacuum pump to vacuum control motor. Apply 15 in. Hg of vacuum. If vacuum is held, go to next step. If vacuum reading drop exceeds 0.5 in. Hg per minute, replace vacuum control motor.

20.Verify that vacuum control motor arm is attached to damper door. If vacuum motor control arm is attached to damper door or door crank arm, service damper door. If vacuum motor control arm is not connected to door crank arm, reconnect as necessary. Recheck system operation.