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Engine Control Module Power Source Circuit: Prius

WARNING: Read service precautions before proceeding with service procedure. See SERVICE PRECAUTIONS .
  1. Remove glove box for access to Engine Control Module (ECM). For illustration of ECM location, see THEORY & OPERATION article. Note ECM electrical connector terminals. See Figure.
  2. Turn ignition on. Using voltmeter, check voltage between terminals No. 4 (Black wire) at ECM electrical connector E8 and No. 17 (Brown wire) at ECM electrical connector E8. This is the +B and E1 terminals at ECM. Voltage should be 9-14 volts. If voltage is within specification, diagnose by symptom. See TROUBLE SHOOTING - NO CODES article. If voltage is not within specification, check for open in wiring harness and electrical connectors between terminal No. 17 (Brown wire) at ECM electrical connector E8 and body ground. See WIRING DIAGRAMS article. If wiring harness and electrical connectors are okay, go to next step. If wiring harness or electrical connector is defective, repair as necessary.
  3. Turn ignition off. Check for open and short in Black wire between EFI main relay and terminal No. 4 at ECM electrical connector E8. See WIRING DIAGRAMS article. EFI main relay is located in fuse/relay box at driver's side front corner of engine compartment, just in front of strut tower. If wiring is okay, go to next step. If wiring is defective, repair as necessary.
  4. Turn ignition on. Using voltmeter, check voltage between body ground and terminal No. 9 (Black/White wire) at ECM electrical connector E10. This is the IGSW terminal at ECM. Voltage should be 9-14 volts. If voltage is not within specification, go to next step. If voltage is within specification, go to step  8.
  5. Check for defective AM2 fuse. AM2 fuse is located in fuse/relay box at driver's side front corner of engine compartment. If AM2 fuse is okay, go to next step. If AM2 fuse is defective, repair short in wiring harness and components connected to AM2 fuse. See WIRING DIAGRAMS article.
  6. Check ignition switch. See IGNITION SWITCH  under ENGINE SENSORS & SWITCHES. If ignition switch is okay, go to next step. If ignition switch is defective, replace ignition switch. See IGNITION SWITCH under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - 4-CYLINDER article.
  7. Check for open in wiring harness and electrical connectors between battery and ignition switch and between ignition switch and terminal No. 9 (Black/White wire) at ECM electrical connector E10. See WIRING DIAGRAMS article. Repair wiring harness or electrical connectors as necessary.
  8. Ensure ignition is on. Using voltmeter, check voltage between body ground and terminal No. 25 (Green/Red wire) at ECM electrical connector E9. This is the MREL terminal at ECM. Voltage should be 9-14 volts. If voltage is within specification, go to next step. If voltage is not within specification, check ECM ground circuit. See WIRING DIAGRAMS article. If ECM ground circuit is defective, repair ground circuit as necessary. If ECM ground circuit is okay, replace ECM.
  9. Check for defective EFI fuse. EFI fuse is located in fuse/relay box at driver's side front corner of engine compartment. If EFI fuse is okay, go to next step. If EFI fuse is defective, repair short in wiring harness and components connected to EFI fuse. See WIRING DIAGRAMS article.
  10. Check operation of EFI main relay. See EFI MAIN RELAY  under MODULES, MOTORS, RELAYS & SOLENOIDS. If EFI main relay operates properly, go to next step. If EFI main relay is defective, replace EFI main relay.
  11. Check for open and short in wiring harness and electrical connectors between terminal No. 25 (Green/Red wire) at ECM electrical connector E9 and EFI main relay, and for open in White/Black wire between EFI main relay and body ground. See WIRING DIAGRAMS article. If wiring harness and electrical connectors are okay, go to next step. If wiring harness or electrical connector is defective, repair as necessary.
  12. Check wiring harness and electrical connectors between battery and EFI fuse, and between EFI fuse and EFI main relay. See WIRING DIAGRAMS article. Repair wiring harness or and electrical connectors as necessary.
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