Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
ECM Power Source Circuit (Ignition Keyed): LS400
1998 Lexus SC 300SECTION LS400
- Access ECM behind glove box. Leave electrical connectors attached to ECM. Turn ignition on. 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 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.
- 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.
- Measure voltage between chassis ground and terminal IGSW at ECM connector. If 9-14 volts is present, go to step 6). If 9-14 volts is not present, remove and inspect IGN fuse located in instrument panel junction block, at rear of parking brake release lever. If fuse is blown, check for short to ground in wiring. Repair as necessary.
- If IGN fuse is okay, disconnect ignition switch connector. With ignition switch in ON position, check for continuity between Black/White wire and Black/Red wire terminals at ignition switch. If continuity does not exist, replace ignition switch. If continuity exists, repair open circuit between battery and ignition switch, or between ignition switch and ECM.
- 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 junction block, in front of left shock tower. If EFI fuse is blown, repair circuit.
- If fuse is okay, check EFI main relay. See RELAYS under MODULES, MOTORS, RELAYS & SOLENOIDS. If EFI main relay is okay, check circuit between chassis ground and terminal MREL at ECM connector. Repair as necessary. 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.