DESCRIPTION
General
Steering power assistance is provided by an electric motor mounted on the steering column which drives the column through a worm gear arrangement. The level of steering assistance is governed by the power output of the motor. The EPAS ECU controls the current to the motor and hence the level of steering assistance. The EPAS ECU measures the steering column torque input from the driver and the road speed of the vehicle, supplying the motor
with current to achieve the required level of assistance. The motor is connected to the column via an electromagnetic clutch which is controlled by the EPAS ECU. If the EPAS ECU detects a fault in the system, the motor will be disconnected from the column by disengaging the clutch. The steering reverts to a normal unassisted system with the clutch disengaged.
The driver steering torque input and direction of torque is monitored by a sensor mounted on the steering column. The sensor measures the angular displacement of a torque tube. Outputs from the sensor are processed by two independent systems, these readings being compared by the EPAS ECU for accuracy. A difference in the two calculated values indicates a fault. The road speed signal, from a speed transducer on the gearbox, is also processed by two independent systems.

OPERATION
Power supplies
Power for the EPAS motor is supplied from the in-line EPAS fuse (C0318) to the EPAS ECU (C0317) on a N wire.
Fusible link 4 in the under bonnet fuse box supplies battery power to the ignition switch (C0028) on a NP wire. When the ignition switch is in position II, the power is fed to fuse 1 in the passenger compartment fuse box on a BY wire. Fuse 1 supplies the feed to the EPAS ECU on a G wire. A system earth is provided by a B wire connected between the EPAS ECU and an earth point.

General
When the ignition switch is first turned to position II, the EPAS ECU performs an integrity check to ensure that the system is operating correctly. During the integrity check, the EPAS ECU earths the KU wire from the instrument pack to illuminate the EPAS warning lamp. The warning lamp remains illuminated until the EPAS ECU receives an engine speed signal from the ECM
(C0158 on VVC MEMS 2J models, C0159 on MPi MEMS 1.9 models, C0634 on MPi MEMS 3 models) on a WB wire. The road speed signal is transmitted on a WO wire from the speed transducer (C0195) to the EPAS ECU . The EPAS ECU outputs a 5 volts supply to the torque sensor on an O wire and connects the OY wire to earth to complete the circuit. Two independent torque signals are input from the torque sensor to the EPAS ECU on the OU and W wires.

NOTE: Although installed in the harness, the UG and OW wires are not used.
When, from the torque sensor signals, the EPAS ECU detects a steering input, it switches the supply from the in-line EPAS fuse to the EPAS motor. For a right turn, the supply is switched to the NU wire between the EPAS ECU and the EPAS motor , and the NR wire is connected to earth. For a left turn the polarity is reversed. The EPAS ECU regulates the supply to the EPAS motor between approximately 0.5 and 12 volts, depending
on the level of assistance required.
The clutch in the EPAS motor has a power feed from the EPAS ECU on a US wire. The earth side of the clutch is connected to the EPAS ECU by a UK wire. The EPAS ECU continually monitors the performance of the system. If a fault is detected, the EPAS ECU energises the clutch, to disengage the motor drive from the steering column, and illuminates the EPAS warning lamp in the instrument pack; a fault code is then stored
in the memory of the EPAS ECU and the steering operates without power assistance. Fault codes can be read using TestBook, which communicates with the system on the K wire connected between the EPAS ECU and the diagnostic socket . For certain faults, the EPAS ECU may temporarily reduce power to the motor instead of disengaging the clutch.

Possible causes for clutch disengagement or reduced power assistance include:
- Motor overheat -
Repeated turning of the steering wheel from lock to lock causes a temperature build up in the EPAS motor. The EPAS ECU monitors the power supply to the EPAS motor and, if the level of EPAS motor activity will cause an unacceptable temperature, reduces the current of the power supply. Reducing the current of the power supply limits the power available for steering assistance and prevents permanent damage to the EPAS motor.
- Engine racing -
If the engine speed is in excess of 2500 rev/min for 30 seconds and no vehicle speed is detected, the EPAS ECU suspects a fault with either the engine speed sensor or the road speed sensor. The EPAS ECU then disengages the clutch and illuminates the warning lamp. If the vehicle subsequently moves off and a road speed signal is received, the warning lamp is extinguished and the clutch engaged.
- Low battery voltage -
If the battery voltage is below 8.4 volts, the clutch will be disengaged and the warning lamp illuminated.
- Poor power supply -
A fault in the power supply to the motor may produce poor steering feel. Low battery voltage may cause steering wheel loads to increase as the assistance is impaired. Fluctuating voltage due to poor battery condition or an alternator fault may cause torque fluctuations to be felt through the steering wheel.