Wipers and Washers 2-69
Description and Operation
Wiper/Washer System Description
and Operation
Wiper/Washer System Components
The wiper/washer system consists of the following
electrical components:
• Windshield Wiper Relay
• Windshield Wiper Speed Control Relay
• Windshield Washer Pump Relay
• Windshield Washer Fluid Pump
• Windshield Wiper Motor
• Window Wiper/Washer Switch
• Windshield Wiper Motor Fuse
• Windshield Washer Fluid Pump Fuse
• Body Control Module
Windshield Wiper System
The body control module (BCM) determines the front
wipe/wash system mode of operation by monitoring
several signals from the front wipe/wa sh switch as
indicated in the wiper switch.
The front wipe/wa sh switch receives a reference
ground signal from the BCM. Each input of the BCM
provides a switched battery pull-up for each front wiper/
washer switch output signal it receives . All the BCM
inputs are recognized as active when the wiper switch
provides a path to the referenced ground signal. The
first signal received by the BCM is the result of 6
resistors in the front wiper switch configured as a
resistor ladder network. This signal is connected to a
BCM analog to digital input. Dependin g on the function
selected (High, Low, Intermittent 1 thru 5, Mist, Off), the
front wiper control switch connects a different set of
resistors into the circuit resulting in different voltages
appearing on the BCM A/D input. By monitoring this
voltage, the BCM determines how to control the wiper
motor On/Off Relay. It should be noted that High, Low,
and Mist all have the same value on this signal circuit.
The second signal received from the front wiper switch
is active only when the front wiper switch is in the high
speed wiper position. When the wiper switch is not in
the high speed position, the switch is open and the
signal circuit is pulled to battery by the BCM. When the
wiper switch is in the high speed position, the switch
pulls the circuit low. The BCM determines how to
control the Wiper high/low speed relay from this input.
The third signal received from the front wiper switch is
from the momentary windshield wash control switch.
When the washer switch is not active the switch is open
and the signal circuit is pulled to battery by the BCM.
When the washer switch is active, the switch pulls the
circuit low. The BCM controls the windshield wash and
windshield wash activated wiper operation based on
this input.
The BCM controls front wiper motor operation through
two output signals and the monitoring of one input
signal. The two outputs (one high side drive, one low
side drive) are used to control two external wiper motor
relays: front wiper motor on/off relay: which provides
the wiper motor with battery power when it is activated
by the high side drive signal (switched battery) from the
BCM. When left deactivated, the normally closed
contacts provide a ground to the wiper motor. Wiper
high/low speed relay: when activated by a low side
drive signal (ground) from the BCM, it switches the
power supplied by the wiper motors on/off relay to the
motors high speed input. When left deactivated, the
normally closed contacts connect the power supplied
by the wiper motors on/off relay to the motors low
speed input. The input used by the BCM is from the
park switch located in the wiper motor assembly. When
the wiper blades are not in the park position, the wiper
park switch is open and the circuit is pulled up to
battery by the BCM. When the wiper blades are in the
park position at the bottom of the glass, the wiper park
switch closes to ground pulling the park signal
circuit low.
To initiate low speed operation, the BCM only energizes
the front wiper motor on/off relay. This allows battery
voltage from the wiper fuse to be applied through the
switched contacts of the wiper motor on/off relay,
through the normally closed contacts of the wiper high/
low speed relay, to the low speed control circuit of the
windshield wiper motor.
Redundant high speed switch pass through. The BCM
provides redundant circuitry which places battery
power on its wiper motor on/off relay output with
activation of its low assertion high speed wiper switch
input. The BCM shall be capable of doing this, even if
the module has lost all microproce ssor control. This
redundant circuit shall supply power while in the RUN
and CRANK power modes. however; while in the
CRANK power mode, the pass through shall only be
active if the BCM is NOT in a computer operating
properly state.
To initiate high speed operation, the BCM energizes
both the front wiper motor on/off relay and the wiper
high/low speed relay . This allows battery voltage from
the wiper fuse to be applied through the switched
contacts of the wiper motor on/off relay, through the
switched contacts of the wiper high/low speed relay, to
the high speed control circuit of the windshield wiper
motor.
Parking the wiper motor. In order to perform an
accurate read of the park switch and to ensure the
wipers will come to rest while still in the park position,
parking of the wipers only occurs while in a low speed
wiper mode. This requires that if the wipers are
performing a high speed wiper operation at the time
they are required to park, the BCM shall transition the
Wipers to low speed by deactiva ting the wiper high/low
relay before attempting to park. In order to park the
wipers, the BCM monitors the park circuit until the park
switch pulls the park circuit to ground. At this time, the
BCM will immediately deactivate the wiper motor on/off
relay. The relay contacts will switch back to their
normally closed position and will apply ground to the
wiper motor power inputs through the normally closed
contacts of the wiper high/low relay. This deactivates
and dynamically brakes the wiper motor in the park
position. When the wiper switch is turned to the OFF
position while the wiper motor is somewhere in
mid-cycle, the BCM will continue to operate the motor
until the wipers reach the park position. If the BCM is
running the wiper motor and does not see a state
transition of the park switch after 8 s, the wipers will
stop immediately when the wiper switch is turned to
2021 - Silverado 4500HD, 5500HD, and 6500HD Electrical Body Builder Manual
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