Jaguar AJV8 engine / 5HP24 transmission. Manual - part 13

AJ-V8/5HP24

53

Engine Management

Electronic Throttle

The electronic throttle enables the ECM to
perform the following functions:
•

engine air flow (ie. power) control

•

engine idle speed control

•

vehicle cruise control

•

stability/traction control (in response to
ABS commands)

•

engine power limiting

•

vehicle speed limiting

•

reverse gear torque reduction

Engine Air Flow Control

During normal operation, when the accelerator
pedal is above idle, the ECM positions the
throttle valve in response to the inputs from the
accelerator pedal position sensor. The throttle
valve follows or leads at a minimum distance the
mechanical guard as the guard is moved by the
accelerator pedal. Thus engine power output is
directly related to driver demand. With the
engine running, the time for the throttle to travel
from idle to fully open is approximately 120 ms
maximum, and from fully open to idle
approximately 140 ms maximum.

Idle Speed Control

During engine idle speed control, the ECM
operates in a closed speed control loop to
maintain a given target idle speed. The ECM
uses gear selection, intake air temperature and
battery voltage to determine the required target
idle speed.
Gear selection is derived from the input of the
Park/Neutral switch in the transmission rotary
switch.
Battery voltage is monitored and, if low voltage is
detected (11.8 V and below), target idle speeds
are increased by up to 50 rpm to increase the
output from the generator.
When the ECM selects a change of target idle
speed, overshoot is kept to 50 rpm maximum.

Certain vehicle services/functions cause a
significant change in engine load as they are
selected on and off. To minimize the effect on
idle speed, the ECM anticipates the change in
load rather than correct the subsequent change in
rpm. The services/functions the ECM anticipates
load changes for are:
•

A/C compressor clutch

•

gear shifts between Park/Neutral and
Reverse/Drive

•

headlamps and side lamps

•

windshield and backlight heaters

•

radiator cooling fans.

The ECM detects gear shifts, headlamp status
and side lamp status from their related status
messages on the CAN data bus. The ECM
directly controls the A/C compressor clutch and
the radiator cooling fans. The windshield and
backlight heaters are controlled by the ACCM
upon consent of the ECM via the heater
request/load inhibit circuits.
If a fault develops in the idle control function, the
ECM goes to open loop control. During open loop
control, rpm is within +100 and -0 rpm of the
target speed for a warm engine (ie. when the
coolant temperature is >30 ˚C (86˚F).

Idle Target Speeds

Intake Air Temperature, ˚C (˚F)                 Engine Speed, (rpm)

Drive/Reverse

Park/Neutral

>-10 (14)

600 ± 25

650 ± 25

-10 to >-20 (14 to >-4)

625 ± 25

675 ± 25

-20 to >-30 (-4 to >-22)

650 ± 25

700 ± 25

≤

-30 (

≤

-22)

675 ± 25

725 ± 25

AJ-V8/5HP24

54

Engine Management

Vehicle Cruise Control

The ECM operates the vehicle cruise control
system, in all forward gears, in response to driver
inputs through the master, SET+, SET-, CANCEL
and RES switches.
When the master switch is pressed, two 12 V
ignition supplies are connected to the ECM, one
direct and one via the brake switch. The LED in
the switch comes on to indicate the system is
armed.
With the system armed, a momentary press of
either the SET+ or the SET- switch engages
cruise control, provided the vehicle speed is
28 km/h (17.5 mph) or above. The ECM
memorizes the current road speed as the `set’
speed and signals the vacuum system to operate
the mechanical guard. The inputs from the
position sensors in the electronic throttle allow
the ECM to monitor and adjust the mechanical
guard and the throttle valve to maintain the set
speed.
The vacuum components are installed in a line
connected between the induction elbow and the
vacuum actuator on the electronic throttle. In the
vacuum system:
•

the check valve isolates the system from
the induction elbow at wide open throttle
when there is little or no vacuum available

•

the reservoirs extend the time (to
approximately 20 minutes) that, at wide open
throttle, sufficient vacuum can be maintained
in the system to operate the vacuum
actuator

•

VSV 1 regulates the vacuum applied to the
vacuum actuator to control the position of
the mechanical guard

•

VSV 2 connects the vacuum actuator to
atmosphere to release the mechanical guard
when cruise control is disengaged

•

VSV 3 is a back-up for VSV 2 (because cruise
control disengagement is safety critical)

•

filters on the atmospheric ports of VSV 2 and
VSV 3 prevent moisture ingress.

CRUISE CONTROL STEERING WHEEL
SWITCHES

CRUISE CONTROL MASTER SWITCH

303-131

303-132

AJ-V8/5HP24

55

Engine Management

M

CRUISE CONTROL SCHEMATIC

B+ Ignition

Driver’s Facia Fuse Box

To Dimmer Switch

Master Switch

Ignition Switched
Ground

Brake
Switch

Steering Wheel
Switches

Parking
Brake
Switch

Engine
Speed
Sensor

-VEHICLE SPEED
-TRACTION STATUS
-GEAR POSITION SELECTED

CAN

ECM

VSV 2

VSV 3

VSV 1

Reservoirs

Check Valve

Throttle Valve
Position Sensor

Accelerator Pedal
Position and
Mechanical Guard
Position Sensors

Vacuum
Actuator

dc Motor

RES

CANCEL

SET-

SET+

F14

303-133

Brake
Cancel
Switch

AJ-V8/5HP24

56

Engine Management

Once cruise control is engaged, a further
momentary press of the SET+ or SET- switch
increments or decrements the set speed by
1.6 km/h (1.0 mph). The ECM then accelerates or
decelerates the vehicle to the new set speed.
The ECM stores a maximum of five incremental
or decremental commands at any one time. Once
the ECM has actioned the first stored command,
a further command can be added. On momentary
operation of the opposite sense switch, the ECM
deletes the last command from memory.
Pressing and holding the SET+ or SET- switch
causes the ECM to increase or decrease the set
speed, and smoothly accelerate or decelerate the
vehicle, until the switch is released. To protect
against switch failure in the closed position, if
either switch is held for longer than 10 minutes,
the ECM disables cruise control until the next
ignition cycle.
The throttle pedal can be used to accelerate the
vehicle beyond the set speed, without
disengaging cruise control (since the diaphragm
actuator is holding the mechanical guard, there is
a noticeable reduction in accelerator pedal load
up to the point at which the input shaft in the
electronic throttle begins to turn the mechanical
guard). When the pedal is released the ECM
returns the vehicle to the set speed in a
controlled manner. Alternatively, a momentary
press of the SET+ or SET- switch causes the
increased vehicle speed to be adopted as the
new set speed.
On receipt of an input from the CANCEL switch,
the ECM disengages cruise control and clears the
set speed from memory. Disengagement is in a
controlled manner to provide a smooth return of
throttle control to the driver.
The ECM disengages cruise control, clears the
set speed from memory and immediately returns
control of the throttle to the driver if any of the
following occur:
•

the master switch is pressed, to switch the 
system off

•

a fault is detected in the electronic throttle
system, the brake switch or the cruise
control switches

•

the parking brake is applied

•

the engine overspeeds.

The ECM disengages cruise control, immediately
returns control of the throttle to the driver and
retains the set speed in memory, if any of the
following occur:
•

the brake pedal is pressed

•

the vehicle decelerates too fast (ie. as
under heavy braking, to guard against
sticking contacts in the brake switch)

•

the gear selector moves to neutral, park
or reverse

•

after resuming cruise control the vehicle
accelerates to only 50% of the set speed
(eg. due to a steep hill)

•

the stability/traction control system
operates

•

vehicle speed decreases below 26 km/h
(16 mph).

On receipt of an input from the RES switch, if a
set speed is stored in the ECM memory, the
cruise control function is re-engaged and the
vehicle accelerated or decelerated to resume the
set speed in a controlled manner.
The ECM continuously monitors the cruise
control switches. If a switch fails closed, on the
initial failure the input is treated as a driver
command and the system responds accordingly.
However, after subsequent disengagement then
re-engagement of the system, the input from the
failed switch is diagnosed as a fault. On detection
of a switch fault, operation of cruise control is
inhibited until the fault is corrected.
The ECM also conducts internal checking
procedures to ensure that only permitted outputs
are generated for given input conditions.