RAM 2500 Truck 2WD V8-5.7L VIN D (2004)
ABS Light: Description and Operation
ABS WARNING INDICATOR AND RED BRAKE WARNING INDICATOR (EBC 125 AND EBC 325)
The system is equipped with an amber ABS and a red Brake warning indicator to alert the driver of a malfunction it has detected. The CAB can signal
the operation of both the amber ABS warning indicator and the red Brake warning indicator via PCI BUS.
The CAB controls the ABS warning indicator by making it do one of three things:
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Light steady for 3.4 seconds during an initial test at the beginning of an ignition cycle to function as a bulb check.
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Light steady when a system malfunction exists.
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Flash if a vehicle tooth or tire parameter is faulty or missing.
The Instrument Cluster controls the amber antilock Brake system-warning indicator. All Dodge Truck Instrument Clusters have direct control over the
ABS warning indicator and Brake warning indicator. The Instrument Cluster transmits a message over the PCI BUS relating to diagnostics and current
lamp status for the ABS and Brake indicator. The Instrument Cluster will expect a PCI BUS message back from the CAB indicating whether the
indicator should be turned on or off. In the event that no signal is received from the CAB for a certain number of time, the Instrument Cluster will
illuminate the ABS warning indicator. The indicator provides notice of ABS system-related problems and the need to take diagnostic action.
The vehicle Instrument Cluster has a red Brake warning indicator to alert the driver to the following conditions:
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Brake fluid level low
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Parking Brake applied
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ABS system malfunction
The Instrument Cluster controls the red Brake warning indicator. There is a parallel path to chassis ground that will illuminate the red Brake warning
indicator:
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Through the parking Brake switch contacts
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Through the Brake fluid level switch
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Through the CAB via PCI BUS
The CAB can control the operation of both the amber ABS warning indicator and the red Brake warning indicator:
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If the CAB is disconnected, the amber ABS warning indicator and red Brake warning indicator will be illuminated
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If the axle type or tire size are not programmed properly, the amber indicator will blink
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If any Foundation Brake system problem exists
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If any ABS system DTC is set, both the amber and red indicators will be illuminated
The ABS Enabling Strategy (FMVSS105) is for vehicles above 10,000-lbs. gross vehicle weight. This allows different operators to be alerted that an
ABS fault existed in the last ignition cycle. When the ignition is cycled to the ON position the ABS system will be disabled if the vehicle speed was
zero on the last ignition cycle. The disabled ABS system will illuminate the amber ABS and red BRAKE warning lamps until the vehicle speed
reaches 7 km/in (4 mph) then the amber ABS and red BRAKE warning lamps will extinguish. If an ABS fault exists; the ABS is still disabled, the
amber ABS and red BRAKE warning lamps will still be illuminated, and a CAB DTC is then set.
BRAKE LAMP SWITCH CIRCUIT
This switch prepares the CAB for a possible antilock stop.
The antilock system uses an input signal from the brake pedal switch when activated. A released brake pedal will close the switch circuit and read 0
volts. When the driver applies the brake pedal, the circuit voltage is 12 volts. This signal tells the CAB that the pedal is depressed. The brake lamp
switch is located on the brake pedal under the driver dash.
BRAKE FLUID LEVEL SWITCH
CIRCUIT
The switch signals the CAB for a hydraulic fluid problem. The antilock system uses input from the brake fluid level switch when activate: d. A low
brake fluid condition in the master cylinder will close the switch circuit and read 0 volts. When the brake fluid level is at specifications, the switch will
open and circuit sense voltage will read 5 volts. This signal tells the CAB if there is enough brake fluid in the master cylinder to have an antilock
operation. The brake fluid level switch is locate in the master cylinder with 10k resistor. The switch is a Hall effect design and uses a float to
determine level.
HYDRAULIC CONTROL UNIT
The HCU on the EBC 125 has an integral valve body for controlling the front and rear Brakes. The HCU on the EBC 325 has an integral valve body
for controlling the front and rear Brakes.
Within the HCU are solenoids, valves, check valves, and a reset switch necessary to apply and release brake pressure as required to avoid wheel
lockup, keep the wheels rolling, and maintain optimum deceleration.
The Isolation Valve(s) is/are normally open, allowing unrestricted flow from the master cylinder to the wheels. When the CAB determines antilock
intervention is required, the valve(s) close to isolate the master cylinder hydraulic circuit(s). Fluid is trapped in the circuit(s) and then prevented from
reaching the wheels.
