Dodge Ram 2004 Service Manual page 21

different module connections. The modules are
wired in parallel. Connections are made in the
harness using splices.
The following modules are used on this vehicle:
• Front Control Module
•
Airbag Control Module
•
Controller Antilock Brake
• A/C Heater Control Module
• Powertrain Control Module (gas only)
•
Engine Control Module (diesel only)
• Radio
• Amplifier (if equipped)
• Hands Free Module
•
Transfer Case Control Module (if equipped)
• Sentry Key Immobilizer Module
• Compass Mini-Trip Computer (if equipped)
• Electro/Mechanical Instrument Cluster
Each module provides its own bias and termina-
tion in order to transmit and receive messages. The
bus voltage is at zero volts when no modules are
transmitting and is pulled up to about seven and a
half volts when modules are transmitting.
The bus messages are transmitted at a rate
averaging 10800 bits per second. Since there is only
voltage present when the modules transmit and the
message length is only about 500 milliseconds, it is
ineffective to try and measure the bus activity with
a conventional voltmeter. The preferred method is
to use the DRBIII lab scope. The 12v square wave
selection on the 20-volt scale provides a good view of
the bus activity. Voltage on the bus should pulse
between zero and about seven and a half volts.
Refer to the following figure for some typical dis-
plays.
The PCI Bus failure modes are broken down into
two categories. Complete PCI Bus Communication
Failure and individual module no response. Causes
of a complete PCI Bus Communication Failure
include a short to ground or battery on the PCI
circuit. Individual module no response can be
caused by an open PCI circuit at the module, or an
open battery or ground circuit to the affected mod-
ule.
Symptoms of a complete PCI Bus Communication
Failure would include but are not limited to:
• All gauges on the EMIC stay at zero
•
All telltales on EMIC illuminate
•
EMIC backlighting at full intensity
• Dashed lines in the overhead console ambient
temperature display
•
No response received from any module on the PCI
bus (except the ECM/PCM)
GENERAL INFORMATION
•
No start (if equipped with Sentry Key Immobi-
lizer)
Symptoms of Individual module failure could
include any one or more of the above. The difference
would be that at least one or more modules would
respond to the DRBIII .
Diagnosis starts with symptom identification. If a
complete PCI Bus Communication Failure is sus-
pected, begin by identifying which modules the
vehicle is equipped with and then attempt to get a
response from the modules with the DRBIII . If any
modules are responding, the failure is not related to
the total bus, but can be caused by one or more
modules PCI circuit or power supply and ground
circuits. The DRBIII may display "BUS
NAL OPEN" or "NO RESPONSE" to indicate a
communication problem. These same messages will
be displayed if the vehicle is not equipped with that
particular module. The CCD error message is a
default message used by the DRBIII and in no way
indicates whether or not the PCI bus is operational.
The message is only an indication that a module is
either not responding or the vehicle is not equipped.
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