Speed Proportional Steering

The Speed Proportional Steering automatically adjusts steering effort based on the vehicle speed. The amount of effort required to turn the steering wheel is determined by a 500-Hz Pulse Width Modulated (PWM) output signal. The desired duty cycle of the PWM is selected from a calibration table internal to the Body Control Module (BCM).

To obtain the desired solenoid current the duty cycle of the solenoid can be compensated according to a current measuring feedback circuit. The speed proportional steering system is always active with the ignition ON. The 500Hz PWM output signal is capable of generating a duty cycle from 0% (full OFF - minimum steering assist) to approximately 50% duty cycle (full on - maximum steering assist) in small increments. The duty cycle is determined by vehicle speed and is continuously modified by solenoid current feedback information. With the engine running and vehicle speed between 0 kph (0 mph) and approximately 32 kph (20 mph) the duty cycle will be about 50%. Between approximately 32 kph (20 mph) and 100 kph (60 mph) the duty cycle will gradually decrease to 0% based upon vehicle speed. Above approximately 100 kph (60 mph) there is no assist provided by the speed proportional steering system. Maximum solenoid current will be limited to one amp but the system operates at about 0.5 amp. Speed Proportional Steering Solenoid resistance is 5.7-6.3 ohms at 20 °C (68 °F).

The Speed Proportional Steering System can detect three failure modes relating to the Speed Proportional Steering PWM driver located in the BCM. If any DTC is set, the solenoid is Put OFF unless the condition goes away before approximately 10 seconds. Once the DTC is set, the solenoid will remain OFF for the remainder of the ignition cycle.
1. An open or shorted to ground circuit fault is detected by monitoring the feedback circuit. If the BCM detects that the feedback circuit has zero current when the software indicates the solenoid circuit should have current, the DTC will be set. This test is performed every 250 milliseconds.
2. A short to voltage fault is detected by monitoring the feedback circuit. The DTC will be set when the BCM has sensed that there was current on the solenoid feedback circuit when the solenoid was turned OFF
3. An over temperature or thermal shutdown DTC will be set by default if the internal diagnostic pin of the solenoid driver is at a low level, but the feedback measurement does not show a high PWM output current.