Discovery 2. Manual - part 84

ENGINE MANAGEMENT SYSTEM - V8

DESCRIPTION AND OPERATION 18-2-65

SLABS ECU

The SLABS ECU provides the road speed signal to the cruise control ECU.  This is the same speed signal provided 
to the ECM.  Cruise control will only operate between 28 - 125 mph (45 - 200 km/h).  Cruise control will not operate if 
a road speed signal is not present.

Input/Output

The input from the SLABS ECU to the cruise control ECU is a square wave oscillating between 0 - 12 Volts at a 
frequency of 8,000 pulses per mile (1.6 km).

ECU operating parameters (connector connected and cruise control master switch on)

Pin No.

Condition

Volts

Ohms

15

Road wheels stopped

0

15

Road wheels turning

0 - 12 Volts with a  

frequency of 8,000 

pulses per mile 1.6 km)

ENGINE MANAGEMENT SYSTEM - V8

18-2-66 DESCRIPTION AND OPERATION

Cruise control ECU

The cruise control ECU controls the cruise control system.

Most functions of the cruise control ECU are described under other components.

Input/Output
The diagnostic line for the cruise control system is between cruise control ECU and diagnostic socket.

The cruise control ECU does not generate fault codes however the following system information is available via 
TestBook:

l

Last switch off reason, which was due to unacceptable speed input.

l

Speed signal detected.

l

Below minimum speed threshold.

l

Current vehicle speed.

l

Recorded SET road speed.

ECU operating parameters (connector connected)

Pin No.

Condition

Volts

Ohms

18

All conditions

Less than 0.5 to earth

ENGINE MANAGEMENT SYSTEM - V8

DESCRIPTION AND OPERATION 18-2-67

SET+/RES switches

The cruise control system uses two steering wheel switches labelled SET+ and RES.

The SET+ switch performs the set speed, tap up and accelerator functions. The RES switch performs the resume and 
suspend functions.

With the cruise control master switch on and the vehicle in the cruise control operating speed range, one press of the 
SET+ switch stores a speed value in the cruise control ECU.  If the switch is pressed and held while the vehicle is 
under cruise control operation, speed increases until the switch is released.  At this point the cruise control ECU stores 
the new speed value. If the switch is tapped (held down for less than 0.5 second) the cruise control ECU increases 
vehicle speed by 1 mph (1.5 km/h).

If the RES switch is pressed while the systems is inactive (no stored values) the system will not respond.  If there is 
a stored value in the cruise control ECU memory and the switch is pressed, the cruise control system operates and 
holds the vehicle at the stored road speed.  If the cruise control system is active and the RES switch is depressed, 
the cruise control ECU deactivates cruise operation but maintains the current set speed value.

Input/Output
The input from the SET+ switch to the cruise control ECU is either 12 Volts or an open circuit.

The input from the RES switch to the cruise control ECU is either 12 Volts or an open circuit.

The following diagnostic information is available through TestBook:

l

The state of operator switch SET+.

l

The state of operator switch RES.

ECU operating parameters (connector connected)

Pin No.

Condition

Volts

Ohms

4

Ignition in position II, SET+ switch 
released

More than 10,000

4

Ignition in position II, SET+ switch 
pressed

12

2

Ignition in position II, RES switch 
released

More than 10,000

2

Ignition in position II, RES switch 
pressed

12

ENGINE MANAGEMENT SYSTEM - V8

18-2-68 DESCRIPTION AND OPERATION

Brake pedal switch

The cruise control ECU has two inputs from the brake pedal switch that determine the position of the brake pedal. 
One input comes through the BCU and is low when the brake pedal is not pressed. The second input comes directly 
from the brake pedal switch. This input is high when the brake pedal is not pressed. On vehicles with a manual 
gearbox, the input from the clutch pedal switch to the cruise control ECU is connected in series with the direct signal 
from the brake pedal switch.

If the cruise control ECU receives a changed signal from either source, it deactivates cruise control, removing power 
to the vacuum pump and activating the vacuum control valve releasing all vacuum in the system.

The brake pedal switch also provides the signal to illuminate the brake lamps and the brake input to the SLABS ECU.

Input/Output
With the brake pedal and the clutch pedal in the rest position, the cruise control ECU receives 12 Volts. 

With the brake pedal pressed, the cruise control ECU receives 0 Volts and a low voltage logic signal from the BCU. 

NOTE: If the clutch pedal is pressed, 0 Volts are present at the cruise control ECU irrespective of brake pedal position. 

ECU operating parameters (connector connected)

Pin No.

Condition

Volts

Ohms

1

Ignition in position II, brake pedal 
released, clutch pedal released

12

1

Ignition in position II, brake pedal 
pressed, clutch pedal released

More than 10,000

1

Ignition in position II, brake pedal 
released, clutch pedal pressed

More than 10,000