Dual Function Pressure Switch

Fig. 4 - Dual Function Pressure Switch Wiring:

The dual function pressure switch is located on the bottom of the hydraulic assembly and monitors accumulator pressure. The primary function is to control operation of the pump/motor assembly and thus maintain proper accumulator operating pressure. When accumulator pressure falls to or below 11,032 kPa (1600 psi,) the pump/motor switch (internal to the dual function pressure switch) will close. This provides a ground, through the starter solenoid, to the pump/motor relay coil. The energized coil pulls the relay contacts closed, providing battery voltage to run the pump/motor. When accumulator pressure reaches 13,790 kPa (2,000 psi,) the switch opens, de-energizing the pump/motor relay which turns off the pump/motor.
NOTE: THE (CAB) DOES NOT REGULATE OR CONTROL ACCUMULATOR PRESSURE.
The second purpose of the Dual Function Pressure Switch is to provide a signal to the (CAB) when the accumulator pressure falls below 6,895 kPa (1,000 psi.) A warning pressure switch, internal to the dual function pressure switch, is normally closed above 6,895 kPa (1,000 psi.) This sends a ground signal to pin 17 at the (CAB). At or below 6,895 kPa (1,000 psi.) the warning pressure switch opens. Internally, the (CAB) (pin 17) detects a voltage and thus low pressure. At this warning pressure, the (CAB) will disable ANTI-LOCK braking functions, light the BRAKE warning lamp and the ANTI-LOCK warning lamp. After two minutes of continuous detection, a low accumulator fault is stored.

Ground for the Dual Function Pressure Switch is provided through the starter solenoid. This prevents the energizing of the pump/motor relay while the starter motor is engaged. This reduces total battery current draw during cranking.

This starter signal is also used by the (CAB) to detect when the starter motor is engaged to light the ANTI-LOCK warning lamp. The lighting of this lamp provides a CRANK-MODE bulb check.