MG MG6 Series 2011 Service Manual page 326

Engine
Failure
TheECT sensor could fail, or provide an incorrect signal, in
the following ways:
• Lack of coolant in system due to leakage
• External wiring open circuit
• External wiring short circuit to ground
• External wiring short circuit to 5V
• External wiring high resistance
• ECM not providing 5V reference
• ECM not providing ground path
• ECT thermistor damaged
Electronic throttle body(ETB)
The ETB uses the signal from the PPS in the throttle pedal
to determine the position of the throttle butterfly, this is
monitored by the ETB. This ensures that the engine receives
the correct quantity of air and fuel for any particular engine
loading, driveability is improved and emissions are reduced.
This system also allows finer control over idle speed and
cruise control (if fitted).
The ETB signal is required for control of the following engine
functions:
• Idle speed control
• Throttle damping
The ETB signal is used by the TCM to determine gear shift
points.
Ignition Coils
The ignition coils operate using the wasted spark principle.
The spark plugs are connected in series with the secondary
winding of the coil so a spark occurs in both cylinders at the
same time. When a spark occurs in the cylinder which is on
the compression stroke, the air/fuel charge is ignited. When
a spark occurs in the cylinder which is on the exhaust stroke,
the spark has no effect, hence the term 'wasted spark'.
Each coil is supplied with a 12V feed from the main relay via
fuse 2 in the engine compartment fuse box. TheECM controls
the primary winding current by supplying the ground path. Coil
1 is connected to theECM at pin H3 of connector EM056 and
coil 2 is connected to theECM at pin H1 of connector EM056.
The coils primary voltage can be checked by using an
oscilloscope. As a simple check to see if the coils are being
triggered by the ECM, connect a 12V test lamp between a 12V
supply and pin 2 on the coil connector. The test lamp should
flash when the engine is cranked. If an ignition coil fails, the
pair of cylinders may not fire at all or may suffer from misfiring
due to reduced spark voltages. This will be noticeable to the
driver as poor performance and will be detected by the misfire
monitor within the OBD strategy. TheECM will store fault
Version 2.0
Engine Fuel & Management System
codes, which can be retrieved using the diagnostic tool. In
the event of a permanent misfire condition, theECM will also
shut off fuelling to the affected cylinders, thereby protecting
theTWC from damaging exhaust pulses.
Failure
An ignition coil could fail in the following ways:
• Primary winding open circuit
• Primary winding short circuit
• Primary winding resistance outside tolerance
• Secondary winding open circuit
• Secondary winding short circuit
• Secondary winding short circuit to primary winding
• Secondary winding resistance outside tolerance
• Mechanical damage to coil tower insulation
Fuel Injectors
The injectors receive fuel under pressure from the fuel rail
and a 12V supply in parallel, through fuse 2 in the engine
compartment fusebox, from the main relay. The fuel injectors
operate at a pressure of 3.5 bar (50 lbf/in2). To deliver fuel to
the engine, theECM has to lift the needle off the injector seat
by energising the solenoid. To energise the solenoid theECM
supplies a ground path to the injector winding. Each injector
opens during the inlet stroke of the cylinder it supplies. The
injectors deliver fuel to the engine as an atomised spray to
maximise the benefits of the swirl and turbulence in the
manifold and cylinder head ports. The difference between
inlet manifold pressure and fuel pressure is calculated by the
ECM, using the TMAP sensor signal. The injector open time
is adjusted accordingly. This enables theECM to have total
control over the AFR at all times. If an injector fails, the
engine may lose power and driveability. TheECM will store
fault codes which can be retrieved using the diagnostic tool.
TheECM driver circuits can be monitored using an
oscilloscope and the injector opening time measured. The
opening time (duty cycle) of the injector varies according to
the fuel requirement of the engine. There are no specifications
for the duty cycle as each individual engine may vary but you
should be able to observe changes in duty cycle if you vary
the following conditions:
• Engine speed
• Engine load
• Engine temperature
• Throttle position
On road test the injector duty cycle should be 0% when
overrun fuel cut off conditions are met.
Failure
A fuel injector could fail in the following ways:
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