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.
Throttle System Operation (LS400)
1998 Lexus SC 300SECTION Throttle System Operation (LS400)
- 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, check throttle control motor. See THROTTLE CONTROL MOTOR under MODULES 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, go to next step.
- Ensure all accessories are off. If not using Lexus scan tool, go to step 5). If using Lexus scan tool, connect Lexus scan tool to Data Link Connector (DLC) No. 3. See Figure. Switch Lexus scan tool to CURRENT DATA. Ensure MIL does not illuminate. Rotate accelerator pedal position sensor lever to full open position. 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, 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.
- Access ECM behind glove box. Backprobing, measure voltage between ECM E13 connector terminal No. 21 (Pink wire) and No. 18 (Brown wire). See appropriate wiring diagram in WIRING DIAGRAMS article. See Figure-10. With ignition switch in ON position and throttle fully closed, voltage should be .3-.9 volts. With ignition switch in ON position and throttle fully open, voltage should be 3.2-4.8 volts. If voltage is as specified, go to next step. If voltage is not as specified, 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.
- Clear trouble codes. See CLEARING DIAGNOSTIC TROUBLE CODES under SELF-DIAGNOSTIC SYSTEM in TESTS W/CODES article. Start engine and ensure MIL does not illuminate. Allow engine 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.