Principles of Operation
The vehicle has two module communication networks: the standard corporate protocol (SCP), which is an
unshielded twisted pair cable (data bus plus, [circuit 914 (TN/OG)] and data bus minus, [circuit 915 (PK/LB)]
and the international standards organization (ISO) 9141 communications network, which is a single wire
network (circuit 70 [LB/WH]). Both networks are connected to the data link connector (DLC). This makes
diagnosis and testing of these systems easier by allowing one smart tester to be able to diagnose and control any
module on the two networks from one connector. The DLC can be found under the instrument panel between
the steering column and the radio.
The SCP communications network will remain operational even with the severing of one of the bus wires.
Communications will also continue if one of the bus wires is shorted to ground or voltage, or if some but not all
termination resistors are lost.
The ISO 9141 communications network does not permit inter-module communication. When the diagnostic tool
communicates to modules on the ISO 9141 communication network, the diagnostic tool must ask for all
information; the modules cannot initiate communications.
The ISO 9141 communications network will not function if the wire is shorted to ground or battery voltage.
Also, if one of the modules on the ISO 9141 communications network loses power or shorts internally,
communications to that module will fail.
The powertrain control module (PCM) is on the SCP communication network. The PCM controls the engine for
better fuel economy, emissions control, and failure mode detection and storage. Refer to POWERTRAIN
CONTROL MODULE for diagnosis and testing of the PCM.
The natural gas vehicle module (NGVM) if equipped is on the SCP communication network. The NGVM
calculates fuel level based on inputs from the fuel tank pressure sensor and the fuel tank temperature sensor.
The NGVM then provides a voltage signal to the instrument cluster to drive the fuel gauge. The NGVM also
has separate injector drivers to run the injectors at the higher operating pressure of natural gas. The NGVM does
not perform any calculations for this function; calibration for the injectors is maintained in the PCM. Refer to
NATURAL GAS VEHICLE MODULE for diagnosis and testing of the NGVM.
The restraint control module (RCM) is on the ISO 9141 network. The RCM controls the deployment of the air
bags and pretensioners based on sensor input. Refer to AIR BAG RESTRAINT SYSTEMS - F150 PICKUP .
The generic electronic module (GEM) is on the ISO 9141 network. The GEM is equipped on vehicles with 4-
wheel drive or vehicles with power windows. The GEM controls additional functions that include:
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speed-dependent windshield wipers
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4-wheel electronic shift on-the-fly
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one-touch down power window
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belt minder (if equipped)
Refer to MULTIFUNCTION ELECTRONIC MODULE .
2003 Ford Pickup F150
2003 ACCESSORIES AND EQUIPMENT Module Communications Network - F150 Pickup