Defender Electric Diagrams. Instruction - page 14

CIRCUIT OPERATION

DEFENDER 90 NAS

11

OTHER FEATURES

Rough road detection ECU

When running on rough roads, it is possible for a false indication of engine misfire to be
detected by the ECM. To prevent false fault codes being stored in the ECM, a rough road
detection ECU is fitted to the vehicle. When the rough road detection ECU signals rough
road conditions, the ECM temporarily ignores engine misfire signals.

The rough road detection ECU monitors and compares the speed of all 4 wheels. When
the vehicle is traveling on rough roads, the wheels will rotate at varying speeds as
obstacles are negotiated and wheel slippage occurs. The variance in wheel speeds is
detected by the ECU which signals rough road conditions to the ECM.

Inertia switch

Power to the fuel pump is supplied via the inertia switch. In the event of extreme
deceleration, as would be experienced in a collision, the inertia switch trips, isolating
electrical supply from the fuel pump. The switch is reset by pressing the rubber button on
top of the switch.

CIRCUIT OPERATION

12

DEFENDER 90 NAS

SEAT BELT WARNING

DESCRIPTION

If the ignition is switched on without the seat belt being fastened, a warning light will be
illuminated on the instrument pack and an audible warning will sound.

OPERATION

Feed from the positive battery terminal is connected to the engine compartment fuse box
(C632-1) by the brown wire. Current passes through fusible link 1 and fusible link 5 of
the engine compartment fuse box which are connected in series. Fusible link 5 (C570-2)
is connected to the ignition switch (C28-1) by the brown wire.

With the ignition switch in position 'II', power flows through the ignition switch (C94-1)
to fuse 1 of the passenger compartment fuse box (C580-1) on the white wire. Fuse 1
(C580-2) supplies the multi-function ECU sounder (C16-5) and instrument pack seat belt
warning light (C230-4) on white/green wires. The ECU (62-7) and warning light (C230-
1) are connected to the seat belt buckle switch (C100-2) by white/black wires. With the
seat belt unfastened, the switch is closed and provides a route to ground (C100-1) for the
sounder and warning light on the black wire. Fastening the seat belt opens the switch and
breaks the circuit, extinguishing the warning light and silencing the sounder.

CIRCUIT OPERATION

DEFENDER 90 NAS

13

AIR CONDITIONING

BLOWER OPERATION

With the ignition in position 2, a battery feed passes from the ignition switch (C94-1) to
Fuse 17 (C581-13) on a white wire. Fuse 17 (C581-14) provides a feed to the Blower
Relay coil (C55-4) on a green wire. The coil is grounded on a black wire and the relay is
energized whenever the ignition switch is at position II.

With the relay energized, a battery feed from Fuse 15 (C581-10) on a purple and slate
wire passes through the closed relay contacts (C55-1 & 3) to the blower switch (C102-5)
on a brown wire.

When the Blower Motor Switch is turned to position 1 (C102-1), the blower motor (C52-
1) is fed on a blue wire. Current passes through both blower motor resistors and the
blower operates at slow speed.

When the Blower Motor Switch is turned to position 2 (C102-2), the blower motor (C64-
1) is fed on a green wire. Current now passes through only one resistor, so the motor
operates at intermediate speed.

With the Blower Motor Switch in position 3 (C102-3), the blower motor (C65-1) is fed
on a red wire. Current flows directly to the motor winding and the motor operates at fast
speed.

The Blower Motor is grounded on a black wire in all switch positions.

AIR CONDITIONING REQUEST

Operation of the air conditioning system is controlled by the Generic Engine Management
System (GEMS) Engine Control Module (ECM). The ECM will decide whether to
operate the Air Conditioning Compressor Clutch on receipt of an air conditioning request
signal. The request signal takes the form of a ground signal applied to pin 28 of ECM
connector C636. The signal is dependent on the following:

A pressure of not greater than 30 Bar (435 psi) and not less than 2.4 Bar (35 psi) must
exist at the high pressure and low pressure switch contacts within the Trinary Switch
(C51-1 & C51-2).

The Air Conditioning Thermostat Switch must be closed. The switch will be closed
whenever its probe, positioned in the vanes of the evaporator, senses a warmer
temperature than that set at the switch.

The logic relay must be energized. The relay is energized with a feed on the blue and
yellow wire from the blower switch (C102-4) whenever the Blower Motor is operating.
The relay coil is grounded on a black wire.

CIRCUIT OPERATION

14

DEFENDER 90 NAS

When all of the above conditions are met, the ground path for the air conditioning request
signal is completed. The ECM then registers receipt of the request signal and will issue an
Air Conditioning Grant Signal when the necessary changes have been made to idle speed.
Similarly, when the Air Conditioning Request signal is removed, the ECM will delay
disengagement of the compressor clutch until the correct conditions are met. The purpose
of this Request/Grant process is to ensure that idle speed remains as near constant as
possible during engagement & disengagement of the compressor clutch.

AIR CONDITIONING GRANT

When the GEMS ECM decides that the necessary conditions have been met to allow
engagement of the Air Conditioning (A/C) Compressor Clutch, and hence, grant operation
of the air conditioning system, it applies a ground (C634-1) on the black and slate wire.
At the A/C harness connector (C47-4), the wire color changes to blue and black, where it
continues to the Air Conditioning Compressor Clutch Relay (C60-2). The relay coil (C60-
4) is fed from fuse 17 (C581-14) on a green wire when the Ignition Switch is at position
II.

With the clutch relay energized, a battery feed from Fuse 16 (C581-12) passes to the
compressor clutch relay (C60-1) on a brown and white wire. The feed passes to the
compressor clutch (C49-1) through the closed relay contacts (C60-3) on a brown and pink
wire. The compressor clutch is grounded on a black wire.

High Temperature Disable

The ECM can disable compressor clutch operation to reduce engine load during high
engine temperatures. The purpose of this system is to prevent engine overheat.

When the ECM senses a high coolant temperature (above 112

deg C - 234 deg F) via the

Engine Coolant Temperature (ECT) sensor (C636-14), Air Conditioning Grant is
overridden by removing the ground signal from C634-1. The Air Conditioning
Compressor Clutch Relay is then de-energized, disabling the Air Conditioning
Compressor Clutch. This condition will remain until coolant temperature drops to
acceptable levels.

CONDENSER FAN REQUEST

Operation of the condenser fan is controlled by the GEMS ECM using a Request/Grant
process, similar to that used in Air Conditioning Compressor Clutch circuit. The request
signal is received as a ground applied to the ECM (C636-29) through the medium
pressure switch contacts in the Trinary Switch (C51-3 & C51-4). The Trinary Switch
medium pressure contacts close at pressures greater than 21 Bar (305 psi) and reopen
when pressure falls below 17 Bar (247 psi).

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