Pinpoint Test F: The Defrost System Will Not Shut Off Automatically

Refer to appropriate SYSTEM WIRING DIAGRAMS article, Heated Window for schematic and connector information.

Normal Operation 

The heated rear window is controlled by the HVAC module. The ignition switch must be in RUN for the rear window defrost system to operate. When the rear window defrost button is pressed, the HVAC provides a ground for the rear window defrost relay coil through circuit 688 (GY/LB). The rear window defrost relay coil is energized, providing battery voltage to the antenna module on circuit 186 (BN/LB). The antenna module then supplies voltage and ground to the heated rear window grid.

This pinpoint test is intended to diagnose the following:

• Wiring, terminals or connectors
• Heated rear window relay
• HVAC module

PINPOINT TEST F: THE DEFROST SYSTEM WILL NOT SHUT OFF AUTOMATICALLY 

1. F1 CHECK THE INPUT FOR A SHORT TO GROUND 
   • Key in OFF position.
   • Disconnect: Rear Window Defrost Relay
   • Key in ON position.
   Fig 1: Measuring Resistance
   Courtesy of FORD MOTOR CO.
   • Measure the resistance between rear window defrost relay pin 85, circuit 688 (GY/LB), harness side and ground.
   • Is the resistance greater than 10,000 ohms? 
   1. YES  : Go to  F3.
   2. NO  : Go to  F2.
2. F2 CHECK CIRCUIT 688 (GY/LB) FOR A SHORT TO GROUND 
   • Key in OFF position.
   • Disconnect: HVAC Module C228b
   Fig 2: Measuring Resistance
   Courtesy of FORD MOTOR CO.
   • Measure the resistance between rear window defrost relay pin 85, circuit 688 (GY/LB), harness side and ground.
   • Is the resistance greater than 10,000 ohms? 
   1. YES  : INSTALL a new HVAC module. REFER to CLIMATE CONTROL SYSTEM - GENERAL INFORMATION & DIAGNOSTICS article. TEST the system for normal operation.
   2. NO  : REPAIR the circuit. TEST the system for normal operation.
3. F3 CHECK CIRCUIT 186 (BN/LB) FOR A SHORT TO VOLTAGE 
   Fig 3: Measuring Voltage
   Courtesy of FORD MOTOR CO.
   • Measure the voltage between rear window defrost relay pin 87, circuit 186 (BN/LB), harness side and ground.
   • Is any voltage present? 
   1. YES  : REPAIR the circuit. TEST the system for normal operation.
   2. NO  : INSTALL a new rear window defrost relay. TEST the system for normal operation.