Flow Chart No. 3: Defrost Always On

Flow Chart No. 3 - Symptom - Defrost always on.

NOTE
- The air handling system is designed to provide defrost when no vacuum is applied to any of the three air door vacuum motors. This is done to prevent a situation where defrost cannot be obtained due to a system vacuum leak. Instead, a leak in the vacuum control circuit will send all airflow to the defroster outlets. This condition may occur during acceleration (slow leak), may exist at all times (large leak) and may happen only when certain specific functions are selected, indicating a leak in that portion of the circuit.
- The vacuum hoses used in the passenger compartment control circuit are constructed from PVC plastic material. The vacuum hoses used in the engine compartment are constructed of Hytrel. Because of the materials used, the vacuum hoses should never be pinched off during diagnosis to locate a leak. Use a vacuum tester to locate vacuum leaks. A wood golf tee can be used as a plug when it is necessary to plug one end of a vacuum hose for leak test purposes.

Step 1
Check system airflow.




1. With engine running and blower motor on high, check system airflow in each function selector position to determine which position(s) have incorrect airflow.




2. Is airflow from the defroster outlets for all positions?

Step 2
Check system vacuum supply hose.
1. Check vacuum supply hose to be sure it is connected to both the engine manifold and vacuum check valve.




2. Is hose disconnected?

Step 3
Check vacuum supply hose for leak.
1. Disconnect vacuum supply hose from engine manifold fitting and from check valve. Plug one end of hose and leak test with vacuum pump.




2. Does hose leak?

Step 4
Check vacuum check valve.

NOTE
- Engine must be running for this test.




1. Check vacuum check valve for correct installation by removing the reservoir hose and the control assembly source hose (one at a time) from check valve and check for vacuum.




2. Is vacuum available at both check valve ports?

Step 5
Inspect vacuum check valve.
1. Inspect vacuum check valve for an obstruction and/or proper operation. Airflow through the check valve should be in the direction toward engine (engine connection side is marked VAC).




2. Is check valve plugged or obstructed?

Step 6
Leak test vacuum check valve.




1. Remove vacuum check valve from vehicle. Connect two hoses and a tee fitting to outlet ports of check valve. Connect vacuum tester to tee fitting.




2. Pump 50 kPa (15 in. Hg) on check valve and observe gauge reading. If vacuum loss exceeds 3.3 kPa (1 in. Hg) per minute, remove check valve from tester and plug vacuum hoses. Pull a vacuum with the tester to certain hoses are not cause of leak.
3. If tester and hoses do not leak, check valve is cause of leak.




4. Does check valve lose more than 3.3 kPa (1 in. Hg) in one minute?

Step 7
Check vacuum reservoir.
1. Connect vacuum tester to vacuum reservoir and leak test reservoir. Reservoir must hold vacuum.




2. Does reservoir leak?

Step 8
Check supply hose for leak.




1. Disconnect and plug black supply hose at control assembly. Disconnect other end of supply hose from check valve and leak test hose with vacuum tester.




2. Does supply hose leak?

Step 9
Check control assembly.
1. Disconnect vacuum harness at in-line multiple connector near control. Plug all vacuum hoses except black hose. Select each function position, apply 50 kPa (15 in. Hg) and check for vacuum to drop. Drop should not exceed 3.3 kPa (1 in. Hg) per minute for any function position.




2. Does drop exceed 1 inch vacuum per minute?

Step 10
Leak test control assembly.
1. Remove vacuum harness from control assembly. Connect vacuum tester to control supply port and plug control port that indicated a leak in Step 9. Select function positions noted in Step 9 and apply 50 kPa (15 in. Hg) after selecting each position. Vacuum drop should not exceed 1.6 kPa (1/2 in. Hg) per minute.




2. Does vacuum drop exceed 1.6 kPa (1/2 in. Hg) per minute?

Step 11
Check supply hose.
1. Disconnect black supply hose from check valve. Connect vacuum tester to supply hose and operate tester as it pulling from a vacuum. If tester can pull a vacuum, hose is plugged. If tester can pull a partial vacuum, hose is restricted.




2. Is hose plugged or restricted?

Step 12
Leak test jumper vacuum harness.
1. Plug one end of vacuum hose that indicated a leak in Step 9. Apply 50 kPa (15 in. Hg) to hose and observe vacuum gauge. Vacuum should not drop.




2. Does vacuum drop?

Step 13
Evaluate system airflow.




1. Is the airflow in Step 1 correct for each function position?

Step 14
Isolate leaking vacuum circuit.
1. Repeat Step 1 and accelerate engine speed for each function position.




2. Does airflow go through defrost during acceleration?

Step 15
Review vehicle history.




1. Did system function properly prior to this complaint?

Step 16
Check vacuum hoses.
1. Check each vacuum hose to be sure it is not plugged.




2. Is a hose plugged?

Step 17
Check vacuum circuit for kinks.
1. Check vacuum circuit for pinched or kinked vacuum hose.




2. Is hose pinched or kinked?

Step 18
Check vacuum circuit.
1. Check each vacuum hose connection to determine if it is partially connected or disconnected.




2. Is vacuum hose partially connected or disconnected?

Step 19
Check vacuum hose.
1. Disconnect both ends of vacuum hose in suspected circuit. Plug one end and leak test with vacuum tester.




2.Does vacuum hose leak?

Step 20
Check vacuum motor.
1. Check vacuum control motor for leaks with vacuum tester.




2. Does motor hold vacuum?

Step 21
Check vacuum motor for installation.
1. Check attachment of vacuum motor arm to damper door.




2. Is motor arm attached to door or door to crank arm?