Discovery 2. Manual - part 470

ENGINE MANAGEMENT SYSTEM - TD5

DESCRIPTION AND OPERATION 18-1-23

The FT sensor can fail the following ways or supply incorrect signal:

l

Sensor open circuit.

l

Short circuit to vehicle supply.

l

Short circuit to earth.

l

Sensor fitted incorrectly.

In the event of a FT sensor signal failure any of the following symptoms may be observed:

l

Difficult cold start.

l

Difficult hot start.

l

Driveability concern.

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:

l

Relay open circuit.

l

Short circuit to vehicle supply.

l

Short circuit to vehicle earth.

l

Broken return spring.

In the event of a fuel pump relay failure any of the following symptoms may be observed:

l

Engine will crank but not start.

l

If 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:

l

ECM.

l

MAF.

l

Fuel pump relay.

l

Cruise control master switch.

l

Cruise control RES switch.

l

Cruise 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:

l

Relay open circuit.

l

Short circuit to vehicle supply.

l

Short circuit to vehicle earth.

l

Broken return spring.

In the event of a main relay failure any of the following symptoms may be observed:

l

Engine will crank but not start.

l

If 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:

l

To activate the brake lamps.

l

To limit fuelling during braking.

l

To 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:

l

12 volts via pin 10 of the ECM connector C0658.

l

0 volts via pin 16 of the ECM connector C0658.

With the brake pedal released the ECM receives the following:

l

0 volts via pin 10 of the ECM connector C0658.

l

12 volts via pin 16 of the ECM connector C0658.

The brake pedal switch can fail in the following ways:

l

Switch open circuit.

l

Short circuit to vehicle supply.

l

Short circuit to earth.

In the event of a brake pedal switch failure the ECM will disable cruise control.