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Jeremy
Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 3362
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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2005 c5500 with brake warning light on

Customer Question

2005 c5500 with brake warning light on
Submitted: 2 years ago.
Category: Medium and Heavy Trucks
Customer: replied 2 years ago.
bleed brkaes no change, pedal still low
Expert:  Jeremy replied 2 years ago.
My name is ***** ***** I will assist you with your concerns.
It is possible you have a flow problem. Without the scanner code, it is hard to tell on this vehicle. I will post below the information straight from the manual which describes how the system works to help diagnose the issue. It's lengthy, but detailed.
Audible Warnings Operation
Each vehicle has at least one, or possibly several, alarm systems that alert the driver to system malfunctions.
An alarm module controls the alarm systems on any vehicle. Refer to the following alarm and module information.
On the B model, the multifunction alarm module mounts to the cowl by the body builder. On the C model the multifunction alarm module is behind the left side of the instrument panel.
The B and C models may use either hydraulic or air brakes. Upon startup, all the warning indicators will turn on as a bulb check.
This module turns on the PRIMARY BRAKE warning lamp on the instrument panel when the fluid level in the brake master cylinder is low, or when the flow of fluid from the power steering pump is low.
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.
The low air pressure switch turns on the PRIMARY BRAKE warning lamp when the engine is started, 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 vehicle is equipped with an optional CHECK GAUGES warning lamp, the alarm module also functions as part of the engine warning system.
The CHECK GAUGES logic in the instrument cluster monitors the engine coolant temperature and the engine oil pressure. If the oil pressure drops below 41 kPa (6 psi), or the engine coolant temperature rises above 121°C (250°F), the logic switch activates the CHECK GAUGES lamp and sends a signal to the alarm module. This signal from the logic switch causes the alarm module to sound a tone alarm at two beats per second.
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.
If the vehicle is equipped with an optional CHECK GAUGES warning lamp, the alarm module also functions as part of the engine warning system.
The CHECK GAUGES logic in the instrument cluster monitors the engine coolant temperature and the engine oil pressure. If the oil pressure drops below 41 kPa (6 psi), or the engine coolant temperature rises above 121°C (250°F), the logic switch activates the CHECK GAUGES lamp and sends a signal to the alarm module. This signal from the logic switch causes the alarm module to sound a tone alarm at two beats per second.
The low air pressure switch turns on the PRIMARY BRAKE warning lamp when the engine is started, 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.
If the vehicle is equipped with an optional CHECK GAUGES warning lamp, the alarm module also functions as part of the engine warning system.
The CHECK GAUGES logic in the instrument cluster monitors the engine coolant temperature and the engine oil pressure. If the oil pressure drops below 41 kPa (6 psi), or the engine coolant temperature rises above 121°C (250°F), the logic switch activates the CHECK GAUGES lamp and sends a signal to the alarm module. This signal from the logic switch causes the alarm module to sound a tone alarm at two beats per second.
This module applies to vehicles with the optional CHECK GAUGES warning lamp and the engine shutdown feature (RPO-KPJ).
This alarm module monitors the following information:
• Air brake system pressure
• Oil pressure
• Engine coolant temperature
• Coolant level
The module can also shut down the engine under the following conditions:
• The engine coolant gets too hot.
• The oil pressure drops below a safe level.
• The coolant level drops below the level of the coolant probe.
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
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:
• Battery voltage input from the brake switch in the stoplamp switch
• A ground signal from the flow switch
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:
• Monitors the primary and the secondary hydraulic brake booster systems.
• Monitors the output from the following switches:
• Oil pressure
• Coolant temperature
• Coolant Level
• Provides a tone signal.
• Turns on the tone signal under the following conditions:
• The oil pressure switch indicates low oil pressure.
• The temperature switch indicates high engine coolant temperature.
• The low coolant probe indicates a low coolant level.
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.