1999 Lexus SC 400FACTORY OEM SERVICE PROCEDURE

Throttle System Operation (LS400)

  1. Remove engine cover. Ensure throttle linkage moves smoothly. Turn ignition on. Rotate accelerator pedal position sensor lever and listen for running sound from throttle motor. Ensure friction sound does not exist. If running sound exists, go to step  3. If running sound does not exist, go to next step.
  2. Check throttle control motor. See THROTTLE CONTROL MOTOR  under MOTORS, RELAY & SOLENOIDS. If throttle control motor is okay, check for open or short circuit in wiring harness between throttle control motor and ECM. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair wiring as necessary. If wiring is okay, ECM may be defective. Check ECM. See PIN VOLTAGE CHARTS article. Repair as necessary.
  3. Ensure all accessories are off. If not using Lexus scan tool, go to step  5. If using Lexus scan tool, connect scan tool to Data Link Connector (DLC) No. 3. See Figure. Using scan tool, select CURRENT DATA. Rotate accelerator pedal position sensor lever to full open position. Throttle opening percentage (THROTTLE POS on Lexus scan tool) should indicate 60 percent or more. If results are as specified, go to step  7. If results are not as specified, go to next step.
  4. Check accelerator pedal position sensor. See ACCELERATOR PEDAL POSITION SENSOR  under ENGINE SENSORS & SWITCHES. If accelerator pedal position sensor is okay, check for open or short circuit in wiring harness between accelerator pedal position sensor and ECM. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair wiring as necessary. If wiring is okay, ECM may be defective. Check ECM. See PIN VOLTAGE CHARTS article. Repair as necessary.
  5. Access ECM behind glove box. Turn ignition on. Backprobing, measure voltage between terminals No. 21 (Pink wire) and No. 18 (Brown wire) at ECM E13 connector. See Figure. With throttle fully closed, voltage should be .3-.9 volt. With throttle fully open, voltage should be 3.2-4.8 volts. If voltage is as specified, go to step  7. If voltage is not as specified, go to next step.
  6. Check accelerator pedal position sensor. See ACCELERATOR PEDAL POSITION SENSOR  under ENGINE SENSORS & SWITCHES. If accelerator pedal position sensor is okay, check for open or short circuit in wiring harness between accelerator pedal position sensor and ECM. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair wiring as necessary. If wiring is okay, ECM may be defective. Check ECM. See PIN VOLTAGE CHARTS article. Repair as necessary.
  7. Start engine and allow it to reach normal operating temperature. Place transmission in Neutral position. Turn A/C on and then off and check idle speed. Idle speed should be 700-800 RPM. Drive vehicle to ensure vehicle performs properly.
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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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