1994 Lexus SC 400FACTORY OEM SERVICE PROCEDURE

ECM Power Source Circuit (Ignition Keyed)

  1. Ensure ECM ground circuits are okay. See ECM GROUND CIRCUITS . Pull back the right front floor mat. Remove ECM protective cover. Connect Check Harness (09990-01000) between ECM and harness. See USING CHECK HARNESS  under INTRODUCTION. Turn ignition on.
  2. Connect voltmeter negative lead to terminal No. 69 (Brown wire) of ECM connector "B". Connect voltmeter positive lead to terminals No. 31 and 32 (Black/Red wires) of ECM connector "A". To identify connectors and terminals, see Figure.
  3. If battery voltage is present, ECM keyed (ignition) power circuit is okay. If battery voltage is not present, measure voltage between chassis ground and terminal No. 1 (Black/Orange wire) of ECM connector "A".
  4. If battery voltage is present, go to step 6). If battery voltage is not present, check the following fuse. See COMPONENT LOCATIONS  under INTRODUCTION.
    • IGN fuse
  5. If fuse is blown, repair circuit. If fuse is okay, disconnect ignition switch connector. With ignition switch in ON position, check for continuity between White/Red and Black/Orange wire terminals of ignition switch. If there is no continuity, replace ignition switch. If there is continuity, repair open circuit between battery and ignition switch, or between ignition switch and ECM.
  6. Turn ignition on. Measure voltage between chassis ground and terminal No. 24 of ECM connector "A" (Black/Orange wire). If battery voltage is not present, replace ECM. If battery voltage is present, check EFI fuse. If EFI fuse is blown, repair circuit.
  7. 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 No. 24 of ECM connector "A". 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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