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Low Voltage Disconnect (LVD), Function
Description

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Internal Content

The Low Voltage Disconnect unit is avaliable in 11.8 and 12.3
voltages. Before trouble shooting verify which unit is installed to
the truck.
The LVD is an electronic unit that prevents the batteries from being
excessively discharged by electrical accessory loads.
When the battery voltage drops to 11.8/12.3 volts for three seconds, the LVD
automatically performs an alarm sequence and disconnects accessory loads
from the battery. The LVD automatically reconnects accessory loads to the
battery when battery voltage rises to 12.9 volts. Refer to the alarm signal and
disconnect sequence below for more information about the operation of the
LVD.
The LVD is an electronic unit designed to protect the batteries by
disconnecting uncritical loads when the battery voltage drops below a preset
limit. This ensures that the batteries have sufficient voltage to crank the
engine. It automatically disconnects and reconnects the loads based on the
battery voltage.
The LVD electronic unit also protects components from reverse voltage.

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A
P1

Connector
Pin, alarm signal

P4
P5
P7
1
2
3

Pin, voltage sensor
Pin, ground
Pin, override
LVD
Input power
Output power

Electrical Diagram
Diagram

G01
S169
FEM01
X10
S15
A131B
A46
FRC
RLY01
ACC
1
2
3
Live
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4UI
5

Battery
Battery main switch
Fuse, main power
Cab pass-through stud
Key switch
LVD
LECM (Living Environment Control Module)
Fuse and relay center
Relay, accessory power (located in the FRC (Fuse and Relay
Center))
Accessories
Dash sockets
Interior aux fan
DCP (Door Control Panel)
Radio
HVAC (Heating, Ventilation and Air Conditioning)

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Heated seats

Alarm signal, low voltage indicator LEDs (Light Emitting Diode), and audible
alarm
When battery voltage drops to 11.8/12.3 volts for three seconds, the LVD
allows one minute to elapse and then sends an alarm signal to the LECM.
When the LECM receives the alarm signal, the low voltage indicator LED on
the LECM flashes. The LED continues to flash until the LVD is overridden or
battery voltage increases to 12.9 volts.
When the LED on the LECM is flashing, the LED on the fan speed control
(on the dashboard) will also flash.
When the LED on the LECM flashes, the audible alarm (located in the
LECM) will sound at the same frequency for one minute. Refer to the alarm
signal and disconnect sequence below for more information about the
audible alarm.
To turn the audible alarm on/off, perform the following:
1. Press the MENU button five times. The LVD icon is blinking on the display.
2. Use the ON/OFF button to set the audible alarm to on or off. The LVD icon
is blinking in the display and On or Off is displayed.
3. Press and release the SET button to confirm the setting.
Refer to the alarm signal and disconnect sequence below for more
information about the low voltage indicator LED and the audible alarm.

LVD override
The LVD can be overridden by pressing and releasing the fan speed control
switch on the LECM or by pressing and releasing the fan speed control
switch on the dash. The override has a duration of ten minutes. During the
ten-minute time period, the LECM provides battery voltage to the LVD on the
LVD override circuit. During the ten-minute time period, the LVD will not send
the alarm signal to the LECM and the LVD will not disconnect battery power.
When the ten-minute time period has elapsed, the LECM stops sending
battery voltage to the LVD on the LVD override circuit. If battery voltage is
below 12.9 volts when the LECM stops sending battery voltage to the LVD
on the LVD override circuit, the LVD will begin the alarm signal and
disconnect sequence again.

Parking heater (parking heater off with LVD feature)
The parking heater is controlled by the LECM. When the LECM receives the
alarm signal from the LVD, the LECM turns off the parking heater unless this
feature (parking heater off with LVD feature) has been turned off.
To turn the parking heater off with the LVD feature on/off, perform the
following:
1. Press the MENU button five times. The LVD icon is blinking on the display.
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2. Press and release the SET button. The LVD icon and the HEAT icon are
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blinking in the display.
3. Rotate the increment/decrement knob (around the MENU button) to set
this feature to On/Off. When the LECM display screen displays On, the
LECM is set to turn off the parking heater when the LECM receives the alarm
signal from the LVD. When the LECM display screen displays Off, the LECM
is set to allow the parking heater to operate when the LECM receives the
alarm signal from the LVD.
4. Press and release the SET button to confirm the setting.

Alarm signal and disconnect sequence
1. The LVD senses 11.8/12.3 volts for three seconds on the LVD input power
circuit or on the LVD voltage sense circuit. The disconnect timer in the LVD
begins a two-minutes countdown.
2. After one minute has elapsed (from the time that the LVD sensed
11.8/12.3 volts), if the LVD still senses the voltage less than 12.3 volts, the
LVD sends an alarm signal (voltage signal - 1.0 Hertz, 50% duty cycle) to the
LECM. The low voltage indicator LED on the LECM flashes at a frequency of
one flash per second. An audible alarm is emitted from the LECM at the
same frequency as the flashing LED. This occurs for thirty seconds.
3. After one minute and thirty seconds has elapsed (from the time that the
LVD sensed 11.8/12.3 volts), the LVD sends an alarm signal (voltage signal 2.5 Hertz, 50% duty cycle) to the LECM. The low voltage indicator LED on
the LECM flashes at a frequency of approximately two flashes per second.
An audible alarm is emitted from the LECM at the same frequency. This
occurs for thirty seconds.
4. When two minutes have elapsed (from the time that the LVD sensed
11.8/12.3 volts), the LVD disconnects battery voltage from the relay (located
in the FRC). After disconnection, the LVD sends an alarm signal (voltage
signal - 0.5 Hertz, 50% duty cycle) to the LECM. The low voltage indicator
LED on the LECM flashes at a frequency of one flash every two seconds, but
the audible alarm emitted from the LECM is mute. The low voltage indicator
LED continuous to flash at this frequency until the LVD is overridden or
battery voltage increases to 12.9 volts.

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Tags

lvd

low voltage disconnect

battery

alarm

low voltage indicator

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