FUEL SYSTEM
FUEL DELIVERY
Automatic Shutdown Relay
The Automatic Shutdown (ASD) relay is located in power
distribution center near the battery or next to radiator coolant
recovery bottle.
The ASD relay is used by the PCM to supply voltage to fuel
pump, fuel injectors and ignition coil. The relay contacts are
normally open.
Power is supplied to relay coil when the ignition switch is
turned on. The PCM controls the ground circuit, which energizes the
coil and closes the relay contacts.
The PCM will only ground the relay when ignition switch is in
the RUN or START positions and activity is sensed through the
crankshaft position sensor and the camshaft position sensor in the
distributor. If the PCM senses the RPM signal has stopped, it will
remove the ground from relay coil, which will cause the contacts to
open and remove power from the circuit.
Ballast Resistor (Cherokee)
A ballast resistor, located between fuel pump relay and the
fuel pump, is used to reduce voltage to the fuel pump. This reduces
fuel pump noise during operation. Ballast resistor is mounted on
fender panel, next to washer fluid reservoir.
When fuel pump relay is energized, voltage is supplied to
fuel pump through the ballast resistor. During start and wide open
throttle conditions, ballast resistor is by-passed and fuel pump
receives its voltage from ballast resistor by-pass relay.
NOTE: Wrangler DOES NOT use a ballast resistor or ballast resistor
by-pass relay in the fuel pump circuit. The PCM operates
fuel pump through the fuel pump relay during all operating
conditions.
Ballast Resistor By-Pass Relay (Cherokee)
A ballast resistor by-pass relay is located on a bracket next
to power distribution center (next to coolant recovery bottle). By
switching the ground circuit on or off, the PCM can control fuel pump
(power) feed. The ballast resistor by-pass relay receives its voltage
from fuel pump relay.
Normally, voltage is supplied to fuel pump through a ballast resistor. At wide open throttle, fuel pump receives voltage through
the ballast resistor by-pass relay, which speeds up fuel pump to
compensate for higher fuel demand.
Fuel Pump (Electric)
All models are equipped with a gear/rotor type electric pump.
Pump is driven by a permanent magnet, 12-volt electric motor. The intank
pump is an integral part of the fuel gauge sending unit.
Fuel system pressure is maintained at about 31 psi (2.2
kg/cm�� ) when pump is operating and vacuum is applied to fuel pressure
regulator. With no vacuum applied to fuel pressure regulator, fuel
pressure should be 39-41 psi (2.7-2.9 kg/cm �� ) or higher. When fuel
pump is not operating, fuel pressure is maintained at 19-39 psi (1.3-
2.7 kg/cm�� ) by fuel pump outlet check valve and the fuel pressure
regulator.
Fuel Pump Relay
On Cherokee, fuel pump relay is located in the power
distribution center, next to coolant recovery bottle. On Wrangler and
Grand Cherokee, the fuel pump relay is located in the power
distribution center next to battery.
The feed side of the relay coil is powered by the ignition
switch. The relay is energized by the PCM by grounding the other side
of the relay coil. The relay contacts are normally open and will close
when the PCM provides a ground path for the relay coil.
The fuel pump circuit is completed during cranking and
whenever the engine is running. If the ignition switch is turned to
the RUN position, the fuel pump will operate for 1-3 seconds and then
shut off. If the PCM does not receive a crank or run signal, it
deactivates the fuel pump by opening the relay coil ground circuit.
The 1-3 second time limit is used to prevent unnecessary operation of
the fuel pump once the system is pressurized. If the engine were
running, the PCM would maintain the fuel pump relay coil ground
allowing continuous operation of the fuel pump.
FUEL CONTROL
Fuel Injectors
The fuel injectors are controlled electronically by the PCM.
Because each injector is connected to 12 volts, the injector is
energized when connected to ground through the PCM. The PCM also
controls the amount of time the injector is energized (pulse width).
Pulse width is based on various inputs and is calculated by the PCM.
The fuel injectors are sequentially energized (in firing order) by the
PCM.
With injector connected to a pressurized fuel supply, a fine
mist will spray from the injector nozzle into the intake manifold. The
injector uses an electromagnet and spring pressure to open or close
the fuel metering plunger. When connected to battery voltage, the coil
of wire in the injector becomes an electromagnet. The magnetic field
generated will overcome spring pressure and raise the plunger off its
seat. When the injector circuit is opened by the PCM, the magnetic
field collapses and spring pressure forces the plunger against its
seat.
Whenever an injector is opened, it will always spray a
consistent amount of fuel for a given amount of pressure. Because
pressure drop across the injector is fixed and the fuel flow rate
constant, the only control variable is the amount of time injector is
open. By controlling the time the injector is open (pulse width), the
PCM can decrease pulse width for engine idle or it can increase pulse
width at wide open throttle.
Injection Timing
All engines use a sequential port fuel injection system. This
means that the injectors have a specific firing order and fuel
injection is timed to piston movement. The spark plugs and injectors
are fired in the same order: 1-3-4-2 on 2.5L and 1-5-3-6-2-4 on 4.0L.
In order for the PCM to fire the injectors in a specific
order timed to crankshaft and piston movement, it has to establish a
reference point. Establishing the reference point requires PCM inputs
from the crankshaft position sensor and camshaft position sensor.
The crankshaft position sensor is located on transmission
bellhousing and provides the PCM with crankshaft angle and speed. The
PCM converts crankshaft speed into engine RPM and crankshaft angle
into piston position.
On 2.5L engine, the slotted flywheel/drive plate, rotating
past the sensor, contains 2 groups of 4 slots located 180 degrees
apart. Each group of slots represents the position of 2 of the
pistons. Pistons No. 1 and 4 approach TDC at the same time and use the
same flywheel slot, while piston No. 3 is matched with piston No. 2.
On 4.0L engine, the slotted flywheel/drive plate, rotating
past the sensor, contains 3 groups of 4 slots located 120 degrees
apart. Each group of slots represents the position of 2 of the
pistons. Pistons No. 1 and 6 approach TDC at the same time and use the
same flywheel slot. Pistons No. 2 and 5 are matched, while piston No.
3 is matched with piston No. 4.
The PCM, through the crankshaft position sensor, knows that 2
pistons are approaching TDC and uses the sync signal generator on the
camshaft position sensor to determine which injector/spark plug to
fire.