Variable Assist Power Steering (VAPS)
Navigator vehicles include a variable assist power steering (VAPS) system that controls the level of power assistance available to the driver based on vehicle speed. Below 3.2 km/h (2 mph), full power steering assist is provided to lessen steering efforts and increase maneuverability. Between 3.2 km/h (2 mph) and 191 km/h (119 mph), steering assist will decrease gradually at a calibrated rate to raise steering efforts for increased directional stability and greater road feel. Above 191 km/h (119 mph), steering assist is constant.
For vehicles equipped with air suspension and VAPS, the electronics for both systems are contained in the air suspension module.
The air suspension control module outputs a pulse-width modulated (PWM) current to the control valve actuator. The control valve actuator controls the hydraulic valve that determines the amount of hydraulic assist provided to the steering gear. The amount of assistance provided varies with the control valve actuator current, which is based on vehicle speed according to a look-up table internal to the module. The amount of hydraulic steering assistance provided by the VAPS subsystem is proportional to the average control valve current.
The vehicle speed is provided to the air suspension module by the PCM over a dedicated vehicle speed signal circuit, this circuit also provides the vehicle speed information to other modules. The air suspension module also receives engine RPM information from the PCM but this information is sent over the universal asynchronous receiver-transmitter (UART) based protocol (UBP) communications network.
When whomever disabled the air bags as well as the air bag module however they did so they also disabled the variable assist power steering which is a pressure control actuator mounted on the steering gear which adjusts the pressure of the hydraulic fluid going into the rack and pinion. This is why the steering effort has increased because the actuator has defaulted to more effort required.
