Ford F150 Pickup. Instruction - part 1613

The available input signals for Compressed Natural Gas (CNG) include the following: 

ALT/GAS Fuel Switch 

The ALT/GAS switch is located on the instrument panel and allows the driver to pre-select the operating fuel 
before starting the vehicle. The switch only operates with the ignition switch in the OFF position. If the switch 
is pressed while the vehicle is running, the input is ignored. The gaseous fuel system will NOT automatically 
change fuel modes with the vehicle running. 

The ALT/GAS switch has two operating positions: GAS and ALT. 

When the GAS position is selected, the engine will operate in the gasoline mode. Selecting the ALT position 
sends a signal to the AFCM requesting ALT fuel mode. If the temperature and pressure conditions are correct, 
the vehicle will switch to the gaseous fuel mode. A Green indicator will also illuminate on the switch to verify 
the vehicle is operating in the CNG mode. If no signal is received by the AFCM, the system defaults to gasoline 
mode. 

Fuel Gauge 

Fuel gauge operation in a CNG bi-fuel vehicle behaves similarly to a standard gasoline fuel gauge. The fuel 
gauge provides a linear reading from full down to empty. Fuel gauge operation for a CNG bi-fuel vehicle is 
based on the pressure of the natural gas in the fuel tank. A full fuel gauge reading will occur at a pressure of 
approximately 3600 psi (24,800 kPa) at a temperature of approximately 70°F (21°C). If a fault with the fuel 
tank pressure sensor occurs, the AFCM will drive the fuel gauge to empty. For more information on fuel gauge 
performance during the fast-fill method of CNG fueling, refer to the owner literature. 

Fuel Rail Pressure Sensor 

The Fuel Rail Pressure (FRP) sensor is a pressure transducer that converts pressure to voltage through a piezo-
resistive element. The FRP sensor is used to adjust the injector combination according to supply pressure and 
allows the vehicle to start in ALT fuel mode at a minimum of 75 psi (517 kPa). The resistance of the sensor 
increases as the pressure increases and decreases as the pressure decreases. 

Fuel Rail Temperature Sensor 

The Fuel Rail Temperature (FRT) sensor is used to calculate fuel density (the amount of vapor within the fuel 
rail). The FRT sensor is a thermistor style element that converts temperature into voltage. The resistance of a 
thermistor decreases as the temperature increases and increases as the temperature decreases. 

Fuel Tank Pressure Sensor 

The Fuel Tank Pressure (FTP) sensor is integral to the gas regulator. It provides information on CNG fuel tank 
pressure to the AFCM and is also used to determine the alternative fuel gauge reading. 

Fuel Tank Temperature Sensor 

Fuel Tank Temperature (FTT) sensor is a thermistor mounted on CNG fuel tank and is used along with Fuel 

 

2003 Ford Pickup F150 

2003 ENGINE PERFORMANCE Theory & Operation - Bi-Fuel - Gasoline/CNG

  

Tank Pressure (FTP) sensor to calculate fuel density.

OUTPUT DEVICES 

ALT/GAS Switch Indicator 

The ALT/GAS switch indicator is integral to the ALT/GAS switch. The indicator is controlled by the AFCM 
and is illuminated when operating in CNG mode. If an emission-critical Diagnostic Trouble Code (DTC) is 
recorded, the indicator will flash with a steady ON, then OFF pulse. 

Bi-Fuel Relay Module 

See FUEL CONTROL under FUEL SYSTEMS. 

Fuel Injectors 

See FUEL CONTROL under FUEL SYSTEMS. 

Fuel Rail Solenoid 

See FUEL CONTROL under FUEL SYSTEMS. 

Self-Diagnostics 

See SELF-DIAGNOSTIC SYSTEMS . 

Fuel Tank Solenoid 

See FUEL CONTROL under FUEL SYSTEMS. 

FUEL SYSTEMS 

The bi-fuel systems consist of Compressed Natural Gas (CNG) fuel tanks, coalescent fuel filter, a high pressure 
regulator, Alternative Fuel Control Module (AFCM), fuel rails, fuel injector sleeves, an ALT/GAS fuel switch 
and ALT/GAS fuel switch indicator, and a bi-fuel relay module.

NOTE:

Only alternative fuel output devices for CNG bi-fuel vehicles are listed in this 
article. For additional information on gasoline-related output devices for bi-fuel 
vehicles, see OUTPUT DEVICES under COMPUTERIZED ENGINE CONTROLS in 
THEORY & OPERATION - CNG, FLEX-FUEL & GASOLINE article.

NOTE:

Vehicles are equipped with different combinations of computer-controlled 
components. Not all components listed below are used on every vehicle. For 
theory and operation on each output component, refer to system indicated after 
component.

 

2003 Ford Pickup F150 

2003 ENGINE PERFORMANCE Theory & Operation - Bi-Fuel - Gasoline/CNG

  

COMPRESSED NATURAL GAS SYSTEM

Fuel from the Compressed Natural Gas (CNG) fuel tank flows through the solenoid tank valve and coalescent 
fuel filter to the gas regulator. The gas regulator provides a consistent system operating fuel pressure of 
approximately 105 psi (724 kPa). The fuel is passed to the fuel rail solenoid and is then introduced into the fuel 
rails. The fuel in the fuel rails is introduced into the intake air system through the fuel injectors and fuel injector 
sleeve assemblies. See Fig. 1 . 

The Alternative Fuel Control Module (AFCM) meters fuel according to demand from the Powertrain Control 
Module (PCM). The AFCM will enable the ALT fuel injectors and disable the gasoline injectors when in ALT 
fuel mode. The AFCM monitors the fuel tank pressure and communicates the fuel level to the instrument cluster 
on the SCP network. The fuel gauge, integral to the instrument cluster, will display the appropriate fuel level 
when in ALT fuel mode. 

The PCM calculates the spark advance for maximum power. A combination of sensors and inputs from the 
PCM are used to monitor the engine and environment to provide closed loop fuel control. Through speed 
density calculations, the PCM determines fuel flow and air flow while adjusting the air/fuel mixture. The 
air/fuel adjustment is required for each spark event. 

FUEL DELIVERY 

CNG Fuel Tank 

The CNG fuel tank is a refillable storage tank requiring replacement only when damaged, failing an inspection 
or when the expiration date stamped on the tank is past. The CNG fuel tank has an operating pressure rating of 
3600 psi (24,822 kPa). The CNG tank is equipped with a non-repairable Pressure Relief Device (PRD) designed 
to relieve all fuel pressure within the alternative fuel tank if exposed to fire (tank replacement is required once 
the PRD safety valve has been released). The CNG tank construction consists of a metal liner reinforced with a 
resin impregnated continuous filament shell. The tank is refueled through the use of a fuel fill valve (located 
behind the ALT fuel filler door) containing a safety valve that only allows one-way fuel flow to the tank. The 
CNG fuel tank is connected to the fill valve by a high-pressure hose. 

Coalescent Fuel Filter 

The coalescent fuel filter removes water and particulate contamination from the CNG fuel system. The fuel 
filter housing has a drain plug allowing excess moisture to be removed during scheduled maintenance. 

Gas Regulator 

High-pressure CNG enters the high-pressure regulator from the CNG fuel tank. The gas regulator delivers fuel 
at approximately 105 psi (724 kPa) to the fuel rail solenoid. 

FUEL CONTROL 

Bi-Fuel Relay Module 

NOTE:

Compressed natural gas may also be referred to as natural gas.

 

2003 Ford Pickup F150 

2003 ENGINE PERFORMANCE Theory & Operation - Bi-Fuel - Gasoline/CNG

  

The ALT fuel relay is a standard mini-ISO type. The fuel pump relay and ALT fuel relay are controlled by the 
AFCM. The ALT fuel relay is normally open, while the fuel pump relay is normally closed. When ALT fuel 
operation is enabled the AFCM will energize both relays. VPWR from the inertia switch is then directed from 
the fuel pump to the CNG fuel tank and alternative fuel solenoids. 

Fuel Injectors & Injector Sleeves 

The AFCM contains the drivers for both the ALT fuel and gasoline injectors. In CNG mode, the AFCM 
disables the gasoline fuel injectors and enables the ALT fuel injectors. The AFCM controls the ALT fuel 
injectors based on the corresponding input signals from the PCM fuel injector drivers. The ALT fuel injectors 
require an increased driver due to the higher operating current requiring greater heat dissipation. 

The injector sleeves are placed in the injector seats. The gasoline fuel injectors are placed in the injector sleeve 
seats. When the gasoline fuel injectors are de-energized and the CNG injectors are energized, the sleeves bypass 
the gasoline fuel injectors, delivering alternative fuel into the intake system and providing a path of fuel flow 
from the gasoline injectors. 

Fuel Rail Solenoid 

The fuel rail solenoid is used to control the alternative fuel flow into the left and right fuel rails. The solenoid is 
normally closed. When the ALT fuel switch is placed in the ALT position, the solenoid receives VPWR from 
the Inertia Fuel Shutoff (IFS) switch through the ALT fuel relay. This opens the solenoid allowing CNG fuel 
flow to the ALT fuel injectors. 

Fuel Tank Solenoid 

The tank valve solenoid is normally closed. When the ALT fuel switch is placed in the ALT position, the 
solenoid receives VPWR from the Inertia Fuel Shutoff (IFS) switch through the ALT fuel relay. This opens the 
solenoid allowing CNG fuel flow to the fuel rail solenoid. The AFCM will close the tank valve if the gaseous 
fuel system pressure drops below 75 psi (517 kPa). 

IDLE SPEED 

For additional information, see IDLE SPEED in THEORY & OPERATION - CNG, FLEX-FUEL & 
GASOLINE article. 

IGNITION SYSTEMS 

Alternative fuel systems are gaseous fuels systems that require a higher spark plug firing voltage to ignite the 
fuel. This increases the load on the vehicle ignition system. A marginally operating secondary ignition system 
may exhibit problems when operating on CNG such as backfires or stalling, even though these problems do not 
exist while operating on gasoline. 

CNG burns slower than gasoline under normal operating conditions. When required, the CNG system advances 
the ignition timing over the base gasoline timing by 0 to 15 degrees to optimize driveability and emissions of 
the vehicle. The system may or may not advance spark timing at idle depending on calibration. This timing 
adjustment is accomplished by the PCM. 

 

2003 Ford Pickup F150 

2003 ENGINE PERFORMANCE Theory & Operation - Bi-Fuel - Gasoline/CNG