Electronically Controlled Power Steering System: Fail-Safe

Electronically controlled power steering system enters the fail-safe mode (that allows the steering force to be controlled without impairing the drive ability) if any of the input/output values to/from electronically controlled power steering system (power steering control unit) deviate from the standard range.

Fig 1: Solenoid Valve Voltage Versus Engine Speed Graph
Courtesy of NISSAN NORTH AMERICA, INC.
Power steering control unit controls the driving voltage to power steering solenoid valve for maintaining the power steering assist force when the fail-safe function is activated. (The engine speed signals control electronically controlled power steering system if any vehicle speed signal error is detected.)

Error area and root cause	Cancel condition
Engine speed is 1, 500 rpm or more and there is no vehicle speed signal input for over 10 seconds during vehicle travel.	When a vehicle speed signal of 2 km/h (1.2 MPH) or more is inputted. Key switch is turned OFF to ON.
Vehicle speed signal has abruptly dropped from 30 km/h (19 MPH) or more to 2 km/h (1.2 MPH) or less within 1.4 seconds.