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Rear Window Will Not Defog

WARNING: This page is about a different car, the 1997 Ford Expedition. However, it is still accessible from the selected car via links, so may be relevant.
  1. Check Power At Fuses  - Turn ignition off. Check fuse No. 6 (5-amp) in interior fuse relay panel. Check fuse No. 27 (40-amp) in power distribution box. If fuses are okay, go to next step. If fuse(s) are blown, replace as necessary. If fuse repeatedly blows, repair short to ground in Black wire between fuse panel and rear defogger relay.
  2. Check Ignition PID  - Using New Generation Star (NGS) scan tester or equivalent, access Generic Electronic Module (GEM). Check IGN PID data while turning ignition switch to each position. If PID values agree with actual switch positions, go to next step. IF PID values disagree with actual switch positions, check fuses No. 6, 8 and 20 in fuse relay panel and replace as necessary. If fuses are okay, check Black/Light Green, Red/Light Blue wire and Gray/Yellow wires between ignition switch and fuse relay panel.
  3. Check DTCs  - Retrieve Diagnostic Trouble Codes (DTCs) from GEM. For additional information on DTC retrieval procedure, see MODULE COMMUNICATIONS NETWORK article. If no DTCs are displayed, go to next step. If DTC B1347 or B1349 is displayed, go to step 10). If DTC B1345 is displayed, go to next step. If DTC B1342 is displayed, replace GEM and retest system.
  4. Check RDEF_SW PID  - Monitor RDEF_SW PID while pressing rear defogger switch. If RDEF_SW PID indicates ON when pressed and held and OFF when released, go to step 10). If RDEF_SW PID always indicates OFF, go to next step. If RDEF_SW PID always indicates ON, go to step 8).
  5. Check Ground To Switch  - Turn ignition off. Disconnect rear defogger switch. Measure resistance between Black wire at rear defogger switch harness connector and ground. If resistance is less than 5 ohms, go to next step. If resistance is greater than 5 ohms, repair open ground (Black) wire.
  6. By-Pass Switch  - Connect a jumper wire between rear defogger switch harness connector terminals No. 3 (Dark Blue/Orange wire) and No. 4 (Black wire). See Fig 1 . Monitor RDEF_SW PID data. If RDEF_SW PID indicates ON, replace rear defogger switch and retest system. If RDEF_SW PID indicates OFF, go to next step.
    Fig 1: Identifying Rear Defogger Switch Harness Connector Terminals
    G96E29342Courtesy of FORD MOTOR CO.
  7. Check Dark Blue/Orange Wire For Open  - Turn ignition off. Disconnect 26-cavity harness connector from GEM located in left side of instrument panel, above fuse relay panel. Measure resistance of Dark Blue/Orange wire between rear defogger switch connector pin No. 3 and GEM harness connector pin No. 1. If resistance is less than 5 ohms, replace GEM and retest system. If resistance is greater than 5 ohms, repair open Dark Blue/Orange wire.
  8. Check Rear Defogger Switch Signal  - Disconnect rear defogger switch harness connector. Monitor RDEF_SW PID data. If RDEF_SW PID indicates ON, go to next step. If RDEF_SW PID indicates OFF, replace rear defogger switch and retest system.
  9. Check Dark Blue/Orange Wire For Short To Ground  - Turn ignition off. Disconnect 26-cavity harness connector from GEM. With rear defogger switch disconnected, measure resistance between rear defogger switch harness connector pin No. 3 (Dark Blue/Orange wire) and ground. If resistance is greater than 10 k/ohms, replace GEM and retest system. If resistance is less than 10 k/ohms, repair short to ground in Dark Blue/Orange wire and retest system.
  10. Check Rear Defog Relay PID  - Turn ignition on. Access GEM and monitor RDEFRLY PID data. Toggle active command RLY CNTRL ON and OFF. If RDEFRLY PID data agrees with command from scan tool, go to next step. If PID data indicates OFFO-G, go to next step. If PID data indicates ON-B-, go to step 14).
  11. Check Power To Rear Defogger Relay  - Remove rear defogger relay. Measure voltage between ground and defogger relay harness connector pin No. 85. See Fig 2 . If battery voltage exists, go to next step. If battery voltage does not exist, replace fuse No. 6 in interior fuse relay panel. If fuse repeatedly, check for short to ground in fuse relay panel.
    Fig 2: Identifying Rear Defogger Relay Harness Connector Terminals
    G96G29344Courtesy of FORD MOTOR CO.
    Fig 3: Checking Continuity Of fuse Relay Panel (Defogger Relay Circuit)
    G96F29343Courtesy of FORD MOTOR CO.
  12. Check Rear Defogger Relay  - Remove Rear defogger relay. Test relay. See REAR DEFOGGER RELAY under COMPONENT TESTING  . Replace relay if necessary. If relay is okay, go to next step.
  13. Check Fuse Panel  - Turn ignition off. Disconnect fuse panel-to-GEM connector No. C267. See Fig 3 . Measure resistance between defogger relay harness connector pin No. 86 and fuse panel connector pin No. 12. See Fig 3 . Resistance should be less than 5 ohms. Measure resistance betwe en defogger relay harness connector pin No. 86 and ground. Resistance should be greater than 10 k/ohms. If resistance is greater than 5 ohms, replace fuse relay panel and retest system. If resistance is less than 5 ohms, replace GEM and retest system.
  14. Check Rear Defogger Relay  - Remove Rear defogger relay. Test relay. See REAR DEFOGGER RELAY under COMPONENT TESTING  . Replace relay if necessary. If relay is okay, go to next step.
  15. Check Fuse Panel For Short  - Disconnect 18-pin fuse panel connector C267. Turn ignition on. Measure voltage between defogger relay harness connector pin No. 30 and ground. See Fig 2 . If battery voltage exists, replace fuse relay panel and retest system. If battery voltage does not exist, replace GEM and retest system.
  16. Check Relay Operation  - Turn ignition on. Using scan tool, command defogger RLY CNTRL on. Measure voltage between fuse No. 17 (10-amp) in fuse relay panel and ground. Battery voltage should exists when relay is commanded on and zero voltage should exist when relay is commanded off. If voltage is as specified, go to next step. If voltage does not exist, go to next step. If battery voltage exists with relay commanded on or off go to step 20).
  17. Check Power To Relay  - Turn ignition off. Remove rear defogger relay. Measure voltage between defogger relay harness connector pin No. 30 and ground. See Fig 2 . If battery voltage exists, go to step 19). If battery voltage does not exist, go to next step.
  18. Check Power To Fuse Panel  - Disconnect Gray, 34-cavity harness connector (C242) from rear of fuse panel. Measure voltage between Pin No. 11 (Black wire) and ground. If battery voltage exists, replace fuse panel, clear DTCs and retest system. If battery voltage does not exist, repair open Black wire between power distribution box and fuse relay panel. Clear DTCs and retest system.
  19. Check Fuse Relay Panel For Open  - Turn ignition off. Remove fuse No. 17 (10-amp). Measure resistance between fuse cavity No. 17 (right terminal) and defogger relay pin No. 87A. See Fig 2 . If resistance is less than 5 ohms, replace rear defogger relay. If resistance is greater than 5 ohms, replace fuse relay panel and test system operation.
  20. Check Voltage To Fuse No. 17  - Turn ignition on. With rear defogger commanded on, measure voltage between fuse No. 17 and ground. If battery voltage exists, go to next step. If battery voltage does not exist, replace rear defogger relay and test system operation.
  21. Check Brown/Light Blue Wire For Short To Power  - Disconnect Gray, 34-cavity harness connector (C242) from rear of fuse panel. See Fig 4 . Turn ignition on. Measure voltage between ground and harness connector at right side of rear window defogger grid. If voltage exists, repair short to power in Brown/Light Blue wire between fuse relay panel and rear defogger grid. If battery voltage does not exist, repair Dark Green/Purple wire between rear defogger switch and rear defogger relay.
  22. Check Power To Defogger Grid & Heated Mirrors  - Using scan tool, command defogger RLY CNTRL ON. Check for battery voltage between ground and harness connector at right side of rear defogger grid. Check for battery voltage on Dark Green/Purple wire at right or left mirror harness connector located inside door. If battery voltage does not exist when commanded on, go to next step. If battery voltage exists when commanded on, go to step 24).
  23. Check Circuits Between Fuse Panel & Defogger Grid Or Mirrors  - Turn ignition off. Disconnect Gray, 34-cavity harness connector (C242) from rear of fuse panel. See Fig 4 . Measure resistance of Brown/Light Blue wire between fuse panel harness connector C242 and rear defogger grid connector. Measure resistance of Dark Green/Purple wire between fuse panel harness connector C242 and left and right mirror harness connectors. If resistance is less than 5 ohms for all measurements, replace fuse junction panel. If resistance is greater than 5 ohms, for any measurement, repair open wire in question.
  24. Check Rear Defogger Grid & Heated Mirror Ground  Measure resistance of ground circuit (Black wire) between harness connector on left side of rear defogger grid and ground. Measure resistance of ground circuit (Black wire) between left/right heated mirror harness connectors and ground. If resistance is greater than 5 ohms, repair ground wire in question. If resistance is less than 5 ohms, replace rear defogger grid or heated mirror in question.
Fig 4: Identifying Harness Connector C242 terminals
G96H29345Courtesy of FORD MOTOR CO.
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