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.
1998 Lexus SC 400FACTORY OEM SERVICE PROCEDURE
Position Sensor Circuit Test
NOTE:
Power seat ECU applies an 8-volt reference signal to each position sensor. When a power seat motor turns one revolution, a pulse signal returns to driver's seat ECU (20 pulses per revolution for reclining motor). Power seat ECU uses pulse signals to determine position of seat in relation to memorized position. If driver's seat ECU does not receive a signal, it prevents seat from returning to memorized position, even if seat motor is okay.
- At driver's seat, remove bolts for front vertical adjuster. Lift front end of seat cushion slightly for access to driver's seat ECU (do not lift too far; harness clamp may come loose). Remove driver's power seat ECU, leaving wiring harness connectors attached.
- Connect a jumper wire between chassis ground and ECU connector "B", terminal No. 13 (no wire). See Figure. This puts the driver's seat ECU into a check mode (diagnostic mode). 3) Connect an analog (needle-type) voltmeter between chassis ground and terminal No. 17 (Black wire) of driver's seat ECU connector "B". A "ready" code should be output by driver's seat ECU (needle deflects once every second).
- While watching voltmeter, use manual seat switch to operate each seat motor (on vehicles with lumbar support, do not operate lumbar motor because system does not monitor its position).
- While motor is running and seat is moving, voltmeter needle should deflect every .25 second (okay indication). If okay indication is displayed, return to TROUBLE SHOOTING . If voltmeter needle deflects every .5 second (no good indication) go to next step.
- If voltmeter indicates a "no good" code for any motor while it is running, disconnect wiring harness connectors from position sensor and motor. Connect battery voltage between terminals No. 3 (positive lead) and No. 2 (negative lead) at position sensor connector. See Fig 1. Connect voltmeter between ground and terminal No. 1 of position sensor connector. While watching voltmeter, connect battery voltage between terminals No. 2 (positive lead) and No. 1 (negative lead) at motor connector.
- While motor is running, voltage should be about half of battery voltage. While motor is stopped, voltage should be zero or battery voltage, depending on where seat is stopped within its range of travel.
- If voltage is not as specified, replace position sensor. If voltage is okay, check for faulty circuit between position sensor and driver's seat ECU. See WIRING DIAGRAMS . If circuit is okay, replace driver's seat ECU.
Factory Service Tools
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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.