2002 LAND ROVER DISCOVERY Workshop Manual

ENGINE MANAGEMENT SYSTEM - TD5
18-1-24 DESCRIPTION AND OPERATION
Fuel pump relay
The fuel pump relay is located in the engine compartment fuse box. It switches on the fuel pump to draw fuel from
the fuel tank to the electronic unit injectors (EUI).
Input/Output
The fuel pump relay is a 4 pin normally open relay. Voltage input to the fuel pump relay comes from the main relay
switching contacts. When the main relay is energised the switching contacts close and the fuel pump relay windings
are supplied a voltage. The ECM provides the earth for the relay windings to close the relay contacts and operate the
fuel lift pump. The fuel pump relay switching contacts are supplied voltage via fuse 10 located in the engine
compartment fuse box. Output from these switching contacts is supplied directly to the fuel pump. When the ECM
interrupts the earth the return spring in the relay pulls the contacts apart and the fuel lift pump stops operating. The
earth path is via pin 5 of ECM connector C0658.
The fuel pump relay can fail in the following ways:
lRelay open circuit.
lShort circuit to vehicle supply.
lShort circuit to vehicle earth.
lBroken return spring.
In the event of a fuel pump relay failure any of the following symptoms may be observed:
lEngine will crank but not start.
lIf the engine is running it will stop.
The MIL will not illuminate in a fuel pump relay failure.

ENGINE MANAGEMENT SYSTEM - TD5
DESCRIPTION AND OPERATION 18-1-25
Main relay
The main relay is located in the engine compartment fuse box and supplies battery voltage to the following:
lECM.
lMAF.
lFuel pump relay.
lCruise control master switch.
lCruise control RES switch.
lCruise control SET+ switch.
It is a 4 pin normally open relay and must be energised to provide voltage to the ECM.
Input/Output
The earth path for the main relay is via a transistor within the ECM. When the earth path is completed, the main relay
energises to supply battery voltage to the ECM. Interrupting this earth path de-energises the main relay, preventing
battery voltage reaching the ECM.
Input to the main relay is via pin 1 of connector C0632, located at the engine compartment fuse box. Output from the
main relay is via fuse 1 to the ECM connector C0658 pins 3, 22 and 27. The earth path is via pin 21 of ECM connector
C0658.
The main relay can fail in the following ways:
lRelay open circuit.
lShort circuit to vehicle supply.
lShort circuit to vehicle earth.
lBroken return spring.
In the event of a main relay failure any of the following symptoms may be observed:
lEngine will crank but not start.
lIf the engine is running it will stop.
For the ECM start up to take place the ignition 'on' (position II) voltage must be greater than 6.0 volts.

ENGINE MANAGEMENT SYSTEM - TD5
18-1-26 DESCRIPTION AND OPERATION
Brake pedal switch
The brake pedal switch is located above the pedal box assembly and activates when the brake pedal is operated. The
ECM uses the signal from the brake pedal switch for the following functions:
lTo activate the brake lamps.
lTo limit fuelling during braking.
lTo cancel cruise control if operating.
The brake pedal switch consists of two component parts, one normally open switch and one normally closed switch.
Input/Output
With the brake pedal in the off position the two switches will give opposite signals to the ECM. When the pedal is
operated the signals exchange operating state.
With the brake pedal pressed the ECM receives the following:
l12 volts via pin 10 of the ECM connector C0658.
l0 volts via pin 16 of the ECM connector C0658.
With the brake pedal released the ECM receives the following:
l0 volts via pin 10 of the ECM connector C0658.
l12 volts via pin 16 of the ECM connector C0658.
The brake pedal switch can fail in the following ways:
lSwitch open circuit.
lShort circuit to vehicle supply.
lShort circuit to earth.
In the event of a brake pedal switch failure the ECM will disable cruise control.

ENGINE MANAGEMENT SYSTEM - TD5
DESCRIPTION AND OPERATION 18-1-27
Clutch switch
The clutch switch is located at the rear of the engine compartment on the RH side. The switch is operated by hydraulic
pressure when the clutch pedal is pressed. The ECM uses the signal from the clutch switch for the following functions:
lTo cancel cruise control if operating.
lTo provide surge damping during gear change.
Surge damping stops engine speed rising dramatically (engine flaring) during gear change. Surge damping assists
driveability as follows:
lSmoother gear change.
lGreater exhaust gas emission control.
lImproved fuel consumption.
Input/Output
The clutch switch receives battery voltage from the BCU. With the clutch pedal in the rest position the switch is closed,
allowing battery voltage to pin 35 of the ECM connector C0658. When the clutch pedal is pressed the switch contacts
open, interrupting the power supply to the ECM. The ECM receives 0 Volts.
The clutch switch can fail in the following ways:
lSwitch open circuit.
lShort circuit to vehicle supply.
lShort circuit to earth.
In the event of a clutch switch failure the ECM will react as follows:
lSurge damping will be inactive.
lCruise control will be inactive.
High/Low ratio switch
Refer to transfer box for description of the high/low ratio switch.

+ TRANSFER BOX - LT230SE, DESCRIPTION AND OPERATION, Description.

ENGINE MANAGEMENT SYSTEM - TD5
18-1-28 DESCRIPTION AND OPERATION
Exhaust Gas Regulator (EGR) modulator
The EGR modulator is located in the engine compartment on the offside inner wing. It regulates the vacuum source
to the EGR valve allowing it to open or close. The ECM utilises the EGR modulator to control the amount of exhaust
gas being recirculated in order to reduce exhaust emissions and combustion noise. Optimum EGR is usually obtained
when the vehicle is operating at light throttle openings, and the vehicle is cruising at approximately 2000 to 3000 rev/
min.
Input/Output
The EGR modulator receives battery voltage from fuse 2 in the engine compartment fuse box. The earth path is via
pin 3 of ECM connector C0158. The length of time that the ECM supplies an earth is how long the exhaust gases are
allowed to recirculate. The ECM decides how long to supply the earth for by looking at engine temperature and engine
load.
The EGR can fail in the following ways:
lSolenoid open circuit.
lShort circuit to vehicle supply.
lShort circuit to earth.
In the event of an EGR modulator failure the EGR system will become inoperative.
The MIL will not illuminate in the event of an EGR modulator failure.

ENGINE MANAGEMENT SYSTEM - TD5
DESCRIPTION AND OPERATION 18-1-29
Malfunction Indicator Lamp (MIL)
The MIL is located in the instrument cluster. It illuminates to alert the driver to system malfunctions. During ignition a
self-test function of the lamp is carried out. The lamp will illuminate for 3 seconds then extinguish if no faults exist. If
a fault is present the lamp will be extinguished for 1 second before illuminating.
Input/Output
The MIL is supplied with battery voltage from the instrument cluster. When the ECM detects a fault, it provides an
earth path to illuminate the MIL. The earth path is via pin 6 of ECM connector C0658.

ENGINE MANAGEMENT SYSTEM - TD5
18-1-30 DESCRIPTION AND OPERATION
Glow plug warning lamp
The glow plug warning lamp is located in the instrument cluster. It illuminates to alert the driver that the glow plugs
are being heated prior to the engine being started. The length of time that the lamp illuminates and the glow plugs
are operating prior to cranking is the pre-heat period, which is subject to battery voltage and ECT sensor signal
controlled by the ECM.
Input/Output
The instrument cluster supplies battery voltage to the glow plug warning lamp. The ECM provides an earth path to
illuminate the lamp. The earth path is via pin 30 of ECM connector C0658.
Glow plugs
The 4 glow plugs are located in the engine block on the inlet side, in cylinder 1 to 4. Cylinder 5 has no glow plug. The
glow plugs are a vital part of the engine starting strategy.

ENGINE MANAGEMENT SYSTEM - TD5
DESCRIPTION AND OPERATION 18-1-31
The purpose of the glow plugs is:
lAssist cold engine start.
lReduce exhaust emissions at low engine load/speed.
The main part of the glow plug is a tubular heating element that protrudes into the combustion chamber of the engine.
The heating element contains a spiral filament that is encased in magnesium oxide powder. At the tip of the tubular
heating element is the heater coil. Behind the heater coil and connected in series is a control coil. The control coil
regulates the heater coil to ensure that it does not overheat and cause a possible failure. The glow plug circuit has its
own control relay located in the engine compartment fuse box.
Pre-heat is the length of time the glow plugs operate prior to engine cranking. The ECM controls the pre-heat time of
the glow plugs based on battery voltage and coolant temperature information via the glow plug relay.
Post-heat is the length of time the glow plugs operate after the engine starts. The ECM controls the post-heat time
based on ECT information. If the ECT fails the ECM will operate pre/post-heat time strategies with default values from
its memory. The engine will be difficult to start.
Input/Output
The glow plugs receive voltage from the glow plug relay that is controlled by the ECM. The ECM provides the earth
path for the relay coil closing the relay contacts and supplying the glow plugs with battery voltage. The supply voltage
heats the coils to approximately 1000
°C (1832 °F). The glow plug circuit is wired in parallel, the body of each glow
plug is screwed directly into the engine block which provides each glow plug with an earth path.
The glow plugs can fail in the following ways:
lHeater coil open circuit.
lControl coil open circuit.
lPoor earth quality.
lShort circuit to vehicle supply.
lShort circuit to vehicle earth.
lWiring loom fault.
lRelay windings open circuit.
lIncorrect relay fitted.
In the event of a glow plug failure any of the following symptoms may be observed:
lDifficult starting.
lExcessive smoke emissions after engine start.