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Test No. 11: Taillights Do Not Operate With Headlight Control Switch In Tail Position, Or Taillights Do Not Shut Off With Headlight Control Switch In Off Position

  1. Check taillight relay. See TAILLIGHT RELAY  under COMPONENT TESTS. If taillight relay is okay, go to next step. If taillight relay is defective, replace taillight relay and recheck taillight operation.
  2. Check wiring harness and electrical connectors between taillight relay and passenger's side Junction Block (J/B) Electronic Control Unit (ECU), between battery and taillight relay, and between taillight relay and taillight. See WIRING DIAGRAMS in EXTERIOR LIGHTS article. For additional power supply, see POWER DISTRIBUTION article in WIRING DIAGRAMS. Passenger's side J/B ECU is located at passenger's side of instrument panel. See Figure. If necessary to check TAIL fuse (7.5-amp) or TAIL/PANEL fuse (15-amp), these fuses are located in fuse/relay box at driver's side of instrument panel. See Figure. If wiring harness and electrical connectors are okay, go to next step. If wiring harness or electrical connector are defective, repair as necessary and recheck taillight operation.
  3. Check headlight control switch. See HEADLIGHT CONTROL SWITCH  under COMPONENT TESTS. If headlight control switch is okay, go to next step. If headlight control switch is defective, replace headlight control switch and recheck taillight operation. See HEADLIGHT CONTROL SWITCH  under REMOVAL & INSTALLATION.
  4. Check wiring harness and electrical connectors between headlight control switch and passenger's side J/B ECU for taillight. See WIRING DIAGRAMS in EXTERIOR LIGHTS article. If wiring harness and electrical connectors are okay, go to next step. If wiring harness or electrical connector are defective, repair as necessary and recheck taillight operation.
  5. Check passenger's side J/B ECU for proper voltage and input signals. See PASSENGER'S SIDE JUNCTION BLOCK ELECTRONIC CONTROL UNIT  under PIN VOLTAGE TESTS. If proper voltage and input signals exist, go to next step. If proper voltage and input signals do not exist, repair as necessary and recheck taillight operation.
  6. Replace passenger's side J/B ECU with a known-good passenger's side J/B ECU and recheck taillight operation. If taillights operate properly, original passenger's side J/B ECU was defective. If taillights still do not operate properly, check wiring harness and electrical connectors for passenger's side J/B ECU, taillights, taillight relay and headlight control switch for intermittent problem. See WIRING DIAGRAMS in EXTERIOR LIGHTS article. Repair wiring harness or electrical connector as necessary.
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