Electronic Power Steering (Diagnostics - Introduction): Overview Lexus IS II facelift

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1. HANDLING PRECAUTIONS When handling the electronic parts: Avoid any impact to electronic parts such as ECUs and relays. Replace these parts with new ones if dropped or subjected to a severe blow. Do not expose any electronic parts to high temperature or humidity. Do not touch the connector terminals in order to prevent deformation or malfunctions due to static electricity. If the power steering ECU assembly has been replaced with a new one, initialize the rotation angle sensor value and calibrate the torque sensor zero point . Refer to «CALIBRATION»(ref-387428-S08117833522011022400000) . When handling the power steering gear assembly: Avoid any impact to the power steering gear assembly, especially to the motor or torque sensor. Replace these parts with new ones if dropped or subjected to a severe blow. Do not pull on the wire harness when moving the power steering gear assembly. If the power steering gear assembly has been replaced with a new one, clear the rotation angle sensor calibration value, initialize the rotation angle sensor value, and calibrate the torque sensor zero point . Refer to «CALIBRATION»(ref-387428-S08117833522011022400000) . When disconnecting and reconnecting the connectors: Turn the engine switch on (IG), center the steering wheel, and turn the engine switch off again before disconnecting the connectors related to the electronic power steering system. Ensure that the engine switch is off before reconnecting the connectors related to the electronic power steering system. After reconnection, center the steering wheel and turn the engine switch on (IG). NOTE: Do not turn the engine switch on (IG) when the steering wheel is not centered. If the above operations are not carried out properly, the steering center point (zero point) will deviate, which may lead to a difference in steering effort between right and left. If there is a difference in steering effort between right and left, perform steering zero point calibration . Refer to «CALIBRATION»(ref-387428-S08117833522011022400000) .
2. PRECAUTIONS FOR CAN COMMUNICATION CAN communication is used to receive information from the skid control ECU, power steering ECU, ECM, and steering angle sensor and to transmit warnings to the combination meter. It is also used for communication between terminals Tc and Ts of the DLC3. If there are any problems in the CAN communication lines, DTCs indicating communication line malfunctions are output. Perform troubleshooting for communication line problems if CAN communication DTCs are output. Be sure to start troubleshooting on the electric power steering system only when data communication is normal. Temporary fixing or repair with bypass wiring, etc. is impossible because the length and path of each CAN communication line is specified.
3. BUS LINE REPAIR After repairing the bus line with solder, wrap the repaired part with electrical tape. NOTE: The CANL bus line and CANH bus line must be installed together all the times. When installing, make sure to braid them. CAN bus lines are likely to be influenced by electrical noise if they are not braided. The difference in length between the CANL bus line and CANH bus line should be 100 mm (3.937 in.) or less. Leave approximately 80 mm (3.150 in.) loose in the braided wires around the connector. Do not use bypass wiring between the connectors. NOTE: The ability of the braided bus lines to resist interference will be lost if bypass wiring is used.

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1. DESCRIPTION The EPS system generates assist torque to assist steering effort through the operation of the motor installed on the power steering gear assembly. The direction and amount of power assistance are determined by signals from the torque sensor, and controlled in accordance with vehicle speed. As a result, steering effort is controlled to be light during low speed driving and moderately high during high speed driving.
2. FUNCTIONS OF COMPONENTS Components Function Power Steering ECU Assembly Calculates degree of assistance needed based on steering torque and vehicle speed. Drives the motor based on motor electric angle obtained from the motor rotation angle sensor. Boosts battery voltage to generate motor drive voltage. Restricts power assistance to protect the system if the motor and ECU overheat. Torque Sensor Detect steering effort generated when the steering wheel is turned. Power Steering Motor Located coaxially with the rack shaft. Its rotation is converted to the rectilinear motion of the steering rack via the ball screw nut and ball. Motor Rotation Angle Sensor Detects motor rotation angle. Combination Meter (PS Warning Light) Upon receiving a signal from the power steering ECU in the event of a system malfunction, the meter ECU illuminates the PS warning light. Skid Control ECU Transmits speed sensor signals to the power steering ECU. ECM Transmits engine speed signals to the power steering ECU.
3. OPERATION EXPLANATION The torque sensor are mounted on the input shaft to the main shaft and on the output shaft to the pinion shaft. The input and output shafts are joined by a torsion bar. If the steering wheel is turned, the torsion bar twists, resulting in a difference in the rotation angle detected by torque sensor. The power steering ECU assembly calculates torque based on this difference. The power steering ECU assembly calculates proper assisting torque, according to vehicle speed, based on the torque obtained in the previous step. Then the ECU controls the motor drive circuit in order to cause it to generate assisting torque. A ball screw nut is attached to the motor shaft. It converts the rotational motion of the motor into the rectilinear motion of the steering rack via the ball. The assisting force generated in the above process will reduce the steering effort required by the driver.