1998 Lexus SC 400FACTORY OEM SERVICE PROCEDURE

ECM Power Source Circuit (Ignition Keyed): Es300

  1. Access ECM behind glove box. Turn ignition on. Leave ECM connectors attached. Backprobing connector, connect voltmeter negative lead to terminal E1 and positive lead to terminal "B" at ECM connector. See appropriate wiring diagram in WIRING DIAGRAMS article. See Figure-10. If 9-14 volts is present, ECM power circuit is okay. Diagnose by symptom. See TESTS W/O CODES article. If 9-14 volts is not present, go to next step.
  2. Check for an open circuit in wiring harness between ground and terminal E1 at ECM connector. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair as necessary. If wiring is okay, go to next step.
  3. Measure voltage between chassis ground and terminal IGSW at ECM connector. If 9-14 volts is present, go to step 5). If 9-14 volts is not present, check the IGN fuse in instrument panel junction block at lower left finish panel.
  4. If fuse is blown, repair cause of blown fuse. If fuse is okay, check ignition switch. See STEERING COLUMN SWITCHES article. Replace ignition switch as necessary and retest. If ignition switch is okay, repair open circuit between battery and ignition switch, or between ignition switch and ECM.
  5. Turn ignition on. Measure voltage between chassis ground and terminal MREL at ECM connector. If 9-14 volts is not present, replace ECM. If 9-14 volts is present, check EFI fuse located in engine compartment fuse/relay block, at left front of engine compartment, in front of strut tower. If EFI fuse is blown, repair cause of blown fuse.
  6. If fuse is okay, check EFI main relay located in engine compartment fuse/relay block, at left front of engine compartment, in front of strut tower. See RELAYS  under MODULES, MOTORS, RELAYS & SOLENOIDS. If EFI main relay is okay, check circuit between chassis ground and terminal M-REL at ECM connector. If circuit is okay, repair circuit between battery and EFI fuse.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

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