Steering System - Electronic Lexus LS I

DESCRIPTION & OPERATION

The electronically controlled power steering system includes a speed sensor, ECU, a solenoid valve and other conventional power steering components. (Scheme 8)

The ECU controlled solenoid valve reduces hydraulic pressure to the rack at high speeds to increase steering effort and increases pressure to the rack at low speeds or when the vehicle is stationary to reduce steering effort.

Scheme 8

TROUBLE SHOOTING

A malfunction in the electronically controlled power steering system will usually cause hard steering at idle or low-speed driving, or too sensitive steering during high-speed driving. However, these problems do not necessarily indicate a malfunction in the electronically controlled power steering system. Always check following components first before trouble shooting the electronically-controlled power steering system.

1. Tire pressure.
2. Suspension and steering linkage lubrication.
3. Front wheel alignment.
4. Steering system and suspension ball joints.
5. Bent steering column.
6. Power steering system leakage.
7. Power steering pump pressure.

If all of the above have been checked and are found to be okay, the problem in the system can probably be traced to the electronically-controlled power steering.

SYSTEM TEST

Note. For terminal identification referenced in testing procedure (Scheme 9)

Scheme 9

1. Turn ignition on. Check ECU-IG fuse. If fuse is blown, check for short circuit between fuse box and ECU. If fuse is okay, disconnect ECU harness connector.
2. Using a DVOM, check for voltage between B (+) terminal and chassis ground. If battery voltage exists, go to next step. If battery voltage does not exist, check for open circuit between ECU-IG fuse and ECU harness connector. See appropriate WIRING DIAGRAMS article: WIRING DIAGRAMS - ES 250 WIRING DIAGRAMS - LS 400
3. Using an ohmmeter, check for continuity between GND terminal and chassis ground. If continuity exists, go to next step. If continuity does not exist, check for open circuit in harness between ECU and chassis ground or poor ground connection. See appropriate WIRING DIAGRAMS article: WIRING DIAGRAMS - ES 250 WIRING DIAGRAMS - LS 400
4. Jack up and support one front wheel. Connect ohmmeter leads between SPD and GND terminals. Spin front wheel and observe ohmmeter reading. Ohmmeter reading should switch between zero and infinite resistance.
5. If reading is as specified, go to next step. If reading is not as specified, check for open circuit between speed sensor and ECU SPD terminal. If wiring is okay, replace speed sensor. See appropriate WIRING DIAGRAMS article: WIRING DIAGRAMS - ES 250 WIRING DIAGRAMS - LS 400
6. Check for continuity between GND terminal and SOL (-) terminal, and between GND terminal and SOL (+). If NO continuity exists, go to next step. If continuity exists, check for short between SOL (-) and SOL (+) wires and ground. If wiring is okay, replace solenoid valve. See appropriate WIRING DIAGRAMS article: WIRING DIAGRAMS - ES 250 WIRING DIAGRAMS - LS 400
7. Measure resistance between SOL (-) terminal and SOL (+) terminal. Resistance reading should be 6-11 ohms. If resistance is as specified, test is complete. Substitute known good ECU and test power steering operation. If resistance is not as specified, check for open circuit or faulty solenoid valve. Repair or replace as required.

Power Steering ECU

1. Raise vehicle and support on chassis stands. Remove instrument panel No. 1 undercover. DO NOT disconnect ECU connector. Start engine. Using a voltmeter, check voltage between computer terminals SOL (-) and ground with engine idling. (Scheme 9) Reading should be.32-.44 volt.
2. Place transmission in gear and raise vehicle speed to 37 MPH (60 KM/H) Reading should now be.14-.40 volt. If voltage is not as specified, replace ECU.

Scheme 10

1. Disconnect wiring connector from solenoid valve. Using an ohmmeter, measure resistance between solenoid valve terminals. (Scheme 10) Resistance should be 6-11 ohms. If resistance is not as specified, solenoid valve is defective.
2. Connect battery positive terminal to solenoid terminal SOL (+) and battery negative terminal to solenoid terminal SOL (-). (Scheme 10) Ensure that solenoid clicks. If operation is not as specified, replace solenoid.

WIRING DIAGRAMS

For additional information, see appropriate chassis wiring diagram in appropriate WIRING DIAGRAMS article

1. WIRING DIAGRAMS - ES 250
2. WIRING DIAGRAMS - LS 400

Electronic Power Steering Wiring Diagram (ES250). Scheme 11

Electronic Power Steering Wiring Diagram (LS400). Scheme 12