General
ECU (Electronic Control Unit)
1 This component is the heart of the entire
engine management system, controlling the
fuel injection, ignition and emissions control
systems. It also controls sub-systems such as
the air conditioning and automatic
transmission, where appropriate. Refer to
Section 2 of this Chapter for an illustration of
how it works.
Air mass meter
2 The air mass meter fitted to models
equipped with Sequential Electronic Fuel
Injection (SEFI) is based on a “hot-wire”
system, sending the ECU a constantly-varying
(analogue) voltage signal corresponding to the
mass of air passing into the engine. Since air
mass varies with temperature (cold air being
denser than warm), measuring air mass
provides the ECU with a very accurate means
of determining the correct amount of fuel
required to achieve the ideal air/fuel mixture
ratio.
Crankshaft speed/position sensor
3 This is an inductive pulse generator bolted
(in a separate bracket) to the cylinder
block/crankcase, to scan the ridges between
36 holes machined in the inboard (right-hand)
face of the flywheel/driveplate. As each ridge
passes the sensor tip, a signal is generated,
which is used by the ECU to determine engine
speed.
4 The ridge between the 35th and 36th holes
(corresponding to 90º BTDC) is missing - this
step in the incoming signals is used by the
ECU to determine crankshaft (ie, piston)
position.
Camshaft position sensor
5 This is bolted to the rear left-hand end of
the cylinder head on Zetec engines, to register
with a lobe on the inlet camshaft. It functions
in the same way as the crankshaft
speed/position sensor, producing a series of
pulses (corresponding to No 1 cylinder at 46º
ATDC); this gives the ECU a reference point,
to enable it to determine the firing order, and
operate the injectors in the appropriate
sequence.
Coolant temperature sensor
6 This component is an NTC (Negative
Temperature Coefficient) thermistor - that is, a
semi-conductor whose electrical resistance
decreases as its temperature increases. It
provides the ECU with a constantly-varying
(analogue) voltage signal, corresponding to
the temperature of the engine coolant. This is
used to refine the calculations made by the
ECU, when determining the correct amount of
fuel required to achieve the ideal air/fuel
mixture ratio.
Inlet air temperature sensor
7 This component is also an NTC thermistor -
see the previous paragraph - providing the
ECU with a signal corresponding to the
temperature of air passing into the engine.
This is used to refine the calculations made by
the ECU, when determining the correct
amount of fuel required to achieve the ideal
air/fuel mixture ratio.
Throttle position sensor
8 This is mounted on the end of the throttle
valve spindle, to provide the ECU with a
constantly-varying (analogue) voltage signal
corresponding to the throttle opening. This
allows the ECU to register the driver’s input
when determining the amount of fuel required
by the engine.
Vehicle speed sensor
9 This component is a Hall-effect generator,
mounted on the transmission’s speedometer
drive. It supplies the ECU with a series of
pulses corresponding to the vehicle’s road
speed, enabling the ECU to control features
such as the fuel shut-off on the overrun, and
to provide information for the trip computer,
adaptive damping and cruise control systems
(where fitted).
Manifold absolute pressure (MAP)
sensor
10 The manifold absolute pressure sensor
measures inlet manifold vacuum, and supplies
this information to the ECU for calculation of
engine load at any given throttle position.
Power steering pressure switch
11 This is a pressure-operated switch,
screwed into the power steering system’s
high-pressure pipe. Its contacts are normally
closed, opening when the system reaches the
specified pressure - on receiving this signal,
the ECU increases the idle speed, to
compensate for the additional load on the
engine.
Oxygen sensor
12 The oxygen sensor in the exhaust system
provides the ECU with constant feedback -
“closed-loop” control - which enables it to
adjust the mixture to provide the best possible
conditions for the catalytic converter to
operate.
13 The sensor has a built-in heating element
which is controlled by the ECU, in order to
bring the sensor’s tip to an efficient operating
temperature as rapidly as possible. The
sensor’s tip is sensitive to oxygen, and sends
the ECU a varying voltage depending on the
amount of oxygen in the exhaust gases; if the
intake air/fuel mixture is too rich, the sensor
sends a high-voltage signal. The voltage falls
as the mixture weakens. Peak conversion
efficiency of all major pollutants occurs if the
intake air/fuel mixture is maintained at the
chemically-correct ratio for the complete
combustion of petrol - 14.7 parts (by weight)
of air to 1 part of fuel (the `stoichiometric’
ratio). The sensor output voltage alters in a
large step at this point, the ECU using the
signal change as a reference point, and
correcting the intake air/fuel mixture
accordingly by altering the fuel injector pulse
width (injector opening time).
Air conditioning system
14 A pressure-operated switch and
compressor clutch solenoid are connected to
the ECU, to enable it to determine how the
system is operating. The ECU can increase
idle speed or switch off the system, as
necessary, so that normal vehicle operation
and driveability are not impaired. See Chap-
ter 3 for further details, but note that diagnosis
and repair should be left to a dealer service
department or air conditioning specialist.
Testing
ECU (Electronic Control Unit)
15 Do not attempt to “test” the ECU with any
kind of equipment. If it is thought to be faulty,
take the vehicle to a Ford dealer for the entire
electronic control system to be checked using
the proper diagnostic equipment. Only if all
other possibilities have been eliminated
should the ECU be considered at fault, and
replaced.
Air mass meter
16 Testing of this component is beyond the
scope of the DIY mechanic, and should be left
to a Ford dealer.
Crankshaft speed/position sensor
17 Unplug the electrical connector from the
sensor.
18 Using an ohmmeter, measure the
resistance between the sensor terminals.
Compare this reading to the one listed in the
Specifications Section at the beginning of this
Chapter. If the indicated resistance is not
within the specified range, renew the sensor.
19 Plug in the sensor’s electrical connector
on completion.
Camshaft position sensor
20 The procedure is as described in
paragraphs 17 to 19 above.
Coolant temperature sensor
21 Refer to Chapter 3, Section 6.
Inlet air temperature sensor
22 Unplug the electrical connector from the
sensor which is located either in the air
cleaner (carburettor engines) or in the CFi unit
or inlet manifold (fuel-injected engines).
23 Using an ohmmeter, measure the
resistance between the sensor terminals.
Depending on the temperature of the sensor
tip, the resistance measured will vary, but it
should be within the broad limits given in the
Specifications Section of this Chapter. If the
sensor’s temperature is varied - by placing it
in a freezer for a while, or by warming it gently
- its resistance should alter accordingly.
24 If the results obtained show the sensor to
be faulty, renew it.
Throttle position sensor
25 Remove the air cleaner assembly or
plenum chamber as necessary for access (see
Chapter 4) then unplug the sensor’s electrical
connector.
6•6 Emissions control systems