Spicer TPCS Troubleshooting Guide: Older Systems (ECU's w_Dual Connectors) - Manual (Page 5)

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Component Description
The following describes how each component of the Tire Pressure
Control System functions. Figure 1 shows the approximate location of
each component.
Wheel Valve
All axles under tire pressure control incorporate a wheel valve at each
wheel end. Dual wheels are typically connected to one wheel valve at the
outer wheel to provide tire pressure balance. When the system is idle, the
wheel valve isolates the tire(s) from the system, thereby extending seal life
since the seals are not under constant pressure. The valve also ensures
fail safe operation should the system become disabled or inoperable. The
wheel valve provides for inflation of the tires from the vehicle air supply
via the pneumatic controller, and deflation of the tires upon system
1emand.
Electronic Control Unit (ECU)
The Electronic Control Unit is the control center for the entire Tire
Pressure Control System. The Electronic Control Unit receives
commands from the driver through the Operator Control Panel and
transmits appropriate signals throughout the system. The Electronic
Control Unit is typically mounted in the cab behind the passenger seat
near the Pneumatic Control Unit.
Operator Control Panel (OCP)
By using the Operator Control Panel keys, the operator selects tire
pressures for the conditions encountered. The panel also displays such
system parameters as current tire pressures, selected modes, and system
status. The Operator Control Panel is typically mounted on the dash
within view and reach of the driver.
Pneumatic Control Unit (PCU)
The Pneumatic Control Unit is a solenoid controlled manifold that
receives commands from the Electronic Control Unit and controls the
air system. It also contains the pressure transducer which transmits the
pressure readings to the Electronic Control Unit. The Pneumatic Control
Unit delivers the proper control signal to the appropriate channel (steer/
drive/trailer).
Speed Sensor
The speed sensor provides the Electronic Control Unit with vehicle
speed information. If the vehicle speed is above programmed limits,
the system will display an overspeed indication on the Operator Control
Panel. Continued operation in this condition will cause the system to
automatically inflate the tires to a more appropriate pressure.
Pressure Switch
The pressure switch acts as an electronic brake priority switch. It
prevents the Tire Pressure Control System from consuming air from the
wet tank until the brake system is fully charged. This prevents the Tire
Pres sure Control System from allowing the primary and secondary tanks
to go below recommended operating pressures for braking.
Operation