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HOMESERVICE MANUALSLEXUS1998SC 300REPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCETESTING & DIAGNOSISENGINE CONTROLS - SYSTEM/COMPONENT TESTSCOMPUTERIZED ENGINE CONTROLSECM POWER SOURCE CIRCUIT (IGNITION KEYED)SC300 & SC400
1998 Lexus SC 300

SC300 & SC400

1998 Lexus SC 300SECTION SC300 & SC400
  1. Access ECM behind glove box. Turn ignition on. Connect voltmeter negative lead to terminal E1 and positive lead to terminal "B". See appropriate wiring diagram in WIRING DIAGRAMS article. See Figure-10. If 9-14 volts is present, ECM keyed (ignition) 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 of 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 driver side junction block behind left kick 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 M-REL 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. 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. 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.