Each vehicle has at least one, or possibly several, alarm systems that alert the driver to system malfunctions. At start-up, all of the warning indicators will turn on as a bulb check. An alarm module controls the alarm systems on any vehicle. Refer to the following alarm and module information.

The brake fluid level switch turns on the PRIMARY BRAKE warning lamp on the instrument panel if the fluid in the brake master cylinder is low. If the flow of fluid from the power steering pump is low, the module turns on the PRIMARY BRAKE lamp and sounds a continuous alarm.

If the module detects a malfunction in the auxiliary electric pump system, the module turns on the AUX BRAKE warning lamp and sounds a continuous alarm.

If the coolant level becomes very low, the module also turns on the LOW COOLANT warning lamp and sounds an alarm at five beats per second.

The low air pressure switch turns on the PRIMARY BRAKE warning lamp when the engine starts, and the lamp remains on until the system reaches normal pressure.

If the system air pressure drops below 503 kPa (73 psi), when the ignition is on, the low air pressure switch turns on the warning lamp and the module sounds a constant tone alarm.

If the coolant level becomes very low, the module also turns on the LOW COOLANT warning lamp and sounds an alarm at five beats per second.

This module applies to vehicles with the optional CHECK GAUGES warning lamp and the engine shutdown feature.

This alarm module monitors the following information:

The module may also shut down the engine under the following conditions:

If the air brake pressure drops below 503 kPa (73 psi) when the engine is running, the air pressure switch closes, and the module turns on the PRIMARY BRAKE warning lamp and sounds a continuous alarm.

If the engine coolant temperature rises above 107°C (225°F), the coolant temperature switch turns on the CHECK GAUGES warning lamp and sends a signal to the alarm module and sounds a tone alarm at two beats per second.

If the coolant temperature increases above 111°C (232°F), or if the oil pressure drops below 55 kPa (8 psi), the CHECK GAUGES warning lamp remains on and the tone alarm increases to five beats a second.

If the coolant level falls below the level of the coolant probe, the module allows 25 seconds for any splashing effect, and then the module sounds a tone alarm at five beats per second. This then turns on the LOW COOLANT warning lamp.

The five beats per second tone alarm signals the activation of the engine shutdown system. The engines runs for 25 seconds before the module interrupts the current flow through the fuel flow solenoid. This shuts down the engine. Restarting the engine resets the timer, but after 25 seconds, the engine will again shut off.

This module applies to vehicles with the optional CHECK GAUGES feature.

The brake fluid level switch turns on the PRIMARY BRAKE warning lamp on the instrument panel if the fluid in the brake master cylinder is low. If the flow of fluid from the power steering pump is low, the module turns on the PRIMARY BRAKE lamp and sounds a continuous alarm.

If the flow of fluid from the power steering pump is low, the module turns on the PRIMARY BRAKE warning lamp and sounds a continuous alarm.

If the module detects a malfunction in the auxiliary electric pump system, the module turns on the AUX BRAKE warning lamp and sounds a continuous alarm.

If the coolant level becomes very low, the module also turns on the LOW COOLANT warning lamp and sounds an alarm at five beats per second.

The module monitors the oil pressure, the coolant temperature, and the coolant level. The module also shuts down the engine as described in the air brake, low coolant, and engine shutdown module.

The hydraulic brake booster motor drives a hydraulic pump in order to provide a reserve power source in the event of a loss of primary hydraulic power in the brake system.

Two electrical switches (flow and stoplamp) control the booster motor. The switches are wired in series through the brake booster relay.

The flow switch senses fluid flow through the primary hydraulic power system. If the primary system drops below a minimum level (about 0.75-1.50 gallons per minute), the switch closes and provides a ground signal to the coil in the brake booster relay.

This ground signal is also applied to the alarm/brake booster module as a signal that the primary system is not operating correctly.

Whenever the brakes are applied, a brake switch inside the stoplamp switch assembly closes. Closing this switch allows battery voltage from the HYD BRK circuit breaker to be applied to the coil of the brake booster relay. This battery voltage signal is also applied to the alarm/brake booster module as an input that the brake switch is closed.

The following signals control the brake booster relay:

The relay energizes whenever the primary hydraulic booster system is not operating correctly, and the brakes are applied. Energizing the relay applies battery voltage to the brake booster motor in order to activate the secondary brake booster system.

The fluid level switch closes when hydraulic fluid in the reservoir drops below the switch level. The ground signal from the switch turns on the PRIMARY BRAKE warning lamp in order to alert the driver of a low fluid condition.

The alarm/brake booster module performs the following functions:

The PNK wire provides power to the alarm/brake booster module, and the BLK wire provides the system ground.

During the engine crank mode of operation, input on the YEL wire alerts the module to the START process. When the engine starts, the module receives pulsed signals from the generator. If these signals are not received, the module will not operate.

During the engine crank mode, the PRIMARY BRAKE and the AUX BRAKE warning lamps energize for four seconds in order to verify continuity. Also, during the pre-engine running stage of operation, the alarm/brake booster module performs a brake booster system check. Prior to the engine running, the flow switch is closed because there is no primary power steering fluid flow. With the switch closed, the module senses ground on the LT BLU/BLK wire. If the switch is stuck open, the module provides a signal to turn on the AUX BRAKE warning lamp. The module will also sound a constant tone alarm.

This ground signal from the flow switch is also sent to the brake booster relay on the LT BLU/BLK wire. With the brake switch open, this ground signal passes through the relay coil to the GRY wire and then to the module. If the module does not sense this ground signal, this indicates an open in the relay coil circuit. The module then provides an output to turn on the AUX BRAKE warning lamp and sounds a constant tone alarm.

With the brake booster in a de-energized state, the module senses a ground signal on the ORN wire through the motor windings and the BLK wire. If this ground is not sensed, this turns on the AUX BRAKE warning lamp and sounds a tone alarm.

When the engine is running, and the pulsed signal is received from the generator on the YEL wire, the module changes to RUN mode monitoring. If the flow switch does not open when the engine is running, the module provides an output turning on the AUX BRAKE warning lamp. This also sounds a constant tone alarm. If the ground signal on the ORN wire to the module stops, this indicates an open in the circuit through the motor windings to ground. The AUX BRAKE warning lamp then comes on and sounds a constant tone alarm.

If the flow switch closes when the engine is running, the module provides an output turning on the PRIMARY BRAKE warning lamp and sounds a constant tone alarm.

Also, during the engine-running mode of operation and when the brakes are applied, the closed stoplamp switch sends a battery voltage signal to the alarm/brake booster module on the GRY wire. At the same time, the module senses battery voltage on the WHT wire from the brake switch. If the battery voltage signal is not sensed, this indicates an open in the GRY wire to the brake booster relay. If this condition occurs, the module provides an output to turn on the AUX BRAKE warning lamp and sounds a constant tone alarm.

The alarm/brake booster module operates as a monitor for the primary and the secondary hydraulic brake booster systems. This module also monitors the following outputs:

The module provides a tone signal and turns on the tone signal under the following conditions:

The alarm module also turns on the CHECK GAUGES lamp. This tone signal is on a 50 percent duty cycle.

This alarm module monitors air pressure through the air pressure switch.

Whenever the engine is running, and the air pressure drops below 455 kPa (73 psi), the air pressure switch closes. The LOW AIR warning lamp then turns on and the module sounds a continuous alarm.

A coolant probe in the radiator surge tank uses coolant in order to provide a constant ground for alternating current. The low coolant circuit in the alarm module then changes the direct current from the vehicle electrical system into alternating current. The module then activates a 25 second timer, providing a ground for the tone alarm and the circuit for the LOW COOLANT warning lamp in the instrument cluster. The lamp then turns on and a tone alarm sounds at five beats per second.

The module has a built-in 25 second delay in order to compensate for the effects of coolant splashing in the radiator and the surge tank.

The engine low coolant indicator probe is part of the coolant surge tank. If the engine low coolant indicator does not function properly, replace the coolant surge tank. Refer to Surge Tank (Diesel) Replacement in Engine Cooling.

The alarm/brake booster module operates as a monitor for the primary and the secondary hydraulic brake booster systems. The module also monitors the following outputs:

The module provides a tone signal and turns on the tone signal under the following conditions:

The alarm module also turns on the CHECK GAUGES lamp. This tone signal is on a 50 percent duty cycle.

This optional system monitors the following information:

If the engines encounters a high temperature or a low pressure condition, the CHECK GAUGES warning lamp comes on and the switches send a signal to the alarm module in order to activate the tone alarm. The alarm sounds at two beats per second.

However, this system may also shut down the engine should the following conditions occur:

If the oil pressure drops below 41 kPa (6 psi), the oil pressure switch closes. If the coolant temperature increases above 103°C (232°F), the coolant temperature switch closes. Either, or both, of these conditions sound a tone alarm at five beats per second.

If the coolant level drops below the coolant probe, the module allows 25 seconds for any splashing effects and then sounds the tone alarm at five beats per second.

Whenever the tone alarm sounds at five beats per second, a 25 second timer in the alarm module activates. The module then interrupts the current flow through the #1 relay coil, stopping the fuel flow through the solenoid. Then the module shuts down the engine.

You can start the engine again, but the engine will only run for another 25 seconds before the module shuts down the engine again.