Jaguar AJV8 engine / 5HP24 transmission. Manual - part 15

AJ-V8/5HP24

61

Engine Management

Variable Valve Timing

The ECM energizes the valve timing solenoids to
advance the intake valve timing and de-energizes
them to retard it.
The ECM uses engine load and speed maps to
decide when to advance and retard the timing.
The maps incorporate hysteresis for both engine
load and speed to prevent "hunting".
Between 1250 and 4500 RPM (nominal), at
engine loads greater than approximately 25 % of
the maximum, the timing is advanced. At low
engine loads and at the two ends of the RPM
range, the timing is retarded.
System operation is inhibited at engine coolant
temperatures below -10˚C (14˚F). System
operation is monitored using the input from the
camshaft position sensor. If a fault is detected
the ECM defaults to the retarded (de-energized)
condition.

EGR System

The ECM operates the 4 pole stepper motor in
the EGR valve to control the recirculation of
exhaust gases. Unlike previous systems, there
are no temperature or position feedback signals
from the valve. The ECM monitors EGR operation
using changes of mass air flow.

Engine Starting

At ignition on, if the gear selector is in Park or
Neutral, the ECM enables the fuel injection and
ignition functions. It also outputs a hard wired
digital security acknowledge signal to the BPM to
enable engine cranking. While the engine cranks,
the BPM outputs a hard wired digital engine
cranking signal to the ECM, which employs
engine starting strategies for the duration of the
signal.
If the gear selector is not in Park or Neutral at
ignition on, the ECM inhibits the fuel injection and
ignition functions, and withholds the security
acknowledge signal to prevent cranking.

VARIABLE VALVE TIMING MAP

Engine Speed, RPM x 1000

Retarded

Advanced

Engine Load

1

2

3

4

5

6

7

303-135

AJ-V8/5HP24

62

Engine Management

H02S Heaters

The ECM energizes the heater elements of the
HO2S during engine warm-up to shorten the time
it takes for them to produce accurate outputs.

Instrument Cluster

The instrument cluster uses CAN messages from
the ECM to operate the trip computer,
tachometer, engine coolant temperature gauge
and the BRAKE, CHECK ENG and general
warning lamps.

Note:

The CHECK ENG lamp is commonly
known as the MIL.

A/C Compressor Clutch

Operation of the A/C compressor clutch is
controlled by the ECM, to prevent unnecessary
loads on the engine during unfavorable operating
conditions.
On receipt of an A/C request signal from the
A/CCM, the ECM immediately energizes the A/C
compressor clutch relay provided the engine is
not at idle speed, the coolant temperature is not
above 119˚C (246˚F) and the throttle valve is not
fully open. When the A/C compressor clutch relay
energizes, the relay output is sensed by the
A/CCM, confirming that the A/C compressor
clutch is engaged.
If the engine is at idle speed, the coolant
temperature is above 119˚C (246˚F) or the throttle
is fully open, the ECM outputs a load inhibit
signal to the A/CCM and delays energising the
A/C compressor clutch relay. At idle speed the
delay is only momentary (in the order of 50 ms)
while idle speed compensation is implemented,
after which the load inhibit signal is removed. At
coolant temperatures above 119˚C (246˚F) or with
a fully open throttle, the delay is for the duration
of the inhibiting condition.
Similarly, with the A/C compressor clutch relay
already energized, if the engine coolant
temperature exceeds 119˚C (246˚F) or the
throttle goes to fully open, the ECM de-energizes
the A/C compressor clutch relay and outputs the
load inhibit signal to the A/CCM until the
inhibiting condition is removed.

When the windshield heaters and/or the

backlight heater are requested on, the A/CCM
sends a screen request signal to the ECM.
Provided the engine is not at idle speed, the
coolant temperature is not above 119˚C (246˚F)
and the throttle valve is not fully open, the ECM
takes no action and the A/C control module
subsequently energizes the heaters.
If the engine is at idle speed, the coolant
temperature is above 119˚C (246˚F) or the throttle
is fully open, the ECM outputs the load inhibit
signal (the same one as used for the A/C
compressor clutch operation) to the A/CCM to
delay energising the heaters. At idle speed the
delay is only momentary while idle speed
compensation is implemented, after which the
load inhibit signal is removed. At coolant
temperatures above 119˚C (246˚F) or with a fully
open throttle, the delay is for the duration of the
inhibiting condition.
Similarly, with the heaters already energized, if
the engine coolant temperature exceeds 119˚C
(246˚F) or the throttle goes to fully open, the
ECM outputs the load inhibit signal to the A/CCM
and the heaters are de-energized until the
inhibiting condition is removed.

Radiator Cooling Fans

The ECM monitors inputs from the A/C single
and triple pressure switches, and from the ECT
sensor on the engine, to control the operation of
the two radiator cooling fans.  Outputs from the
ECM control two relays contained in the radiator
fans module, to operate the fans in off, slow or
fast mode. In the slow mode the fans are
connected in series; in the fast mode the fans are
connected in parallel. Hysteresis in the
temperature and pressure switching values
prevents "hunting" between modes.

Windshield and Backlight
Heaters

AJ-V8/5HP24

63

Engine Management

To counteract the increase in engine coolant
temperature that occurs after the engine stops,
at ignition off:
•

if the fans are already on, the ECM keeps
them on for 5 minutes, or until the ECT 
decreases to a pre-determined value,
whichever occurs first

•

if the fans are off, the ECM determines, from
ECT and intake air temperature inputs, if the
fans need to be switched on. If they do, it
switches them on for 5 minutes, or until the
ECT decreases to a pre-determined value,
whichever occurs first.

Radiator Fan Switching Points

Mode

Coolant Temperature, ˚C (˚F)

A/C System Pressure, Bar (psi)

On

Off

On

Off

Slow

90 (194)

86 (187)

12 (174)

8 (116)

Fast

97.5 (207.5)

93.5 (200.5)

22 (319)

17.5 (254)

Diagnostics

The ECM performs self test routines and
monitors engine functions, inputs and outputs to
ensure correct operation of the engine and the
engine management system. Hard wired inputs
and outputs are monitored for short and open
circuits, and sensor inputs are also monitored for
range.
Additional checks are run on the more critical
sensor inputs to ensure their validity. Some of the
more critical inputs have substitute or default
values which the ECM adopts if the input is
diagnosed as faulty.
Any faults detected are logged in the ECM
memory as DTC. The ECM also outputs engine
malfunction messages on the CAN and adopts a
default mode of operation. Most default modes
retain some degree of engine operation (limp
home).
The ECM also stores OBD II related DTC
detected by other control modules on the CAN.
Non-OBD II related DTC are retained in the
memory of the control module that detects the
fault.

To prevent false DTC being logged, the
monitoring of some inputs and engine functions
is inhibited while the vehicle is above a given
altitude:
•

at altitudes of 2438m (8000ft) and
above,the ECM inhibits diagnostics on:
EGR valve and EGR flow; EVAP valve and 
EVAP purge flow; idle speed control; misfire
detection; catalytic converter efficiency.

•

at altitudes of 2652m (8700ft) and 
above, the ECM also inhibits diagnostics on:
HO2S; MAFS; O2S.

AJ-V8/5HP24

64

Engine Management

Additional Input Checks

Input

Additional Strategy

Action, Substitute or DefaultValue

Accelerator pedal

Accelerator pedal position input 1 compared to

If one input fails, the input of the other

position (x 2) and

accelerator pedal position input 2.

two is used.

mechanical guard

Both accelerator pedal position inputs compared to

If two inputs fail, ECM adopts fixed

position

mechanical guard position input.

throttle mode.

Throttle position

1 compared to input 2.

If either input fails, ECM adopts

(x 2)

Input 1 compared to target value (target value

mechanical guard mode of operation.

depends on operating mode, eg. normal running,
cruise control, traction active).

Engine coolant

After engine start, checks that expected

If fault detected, uses substitute value

temperature

temperature increase occurs, then monitors for

derived from transmission oil

excessive temperature decrease.

temperature.

Mass air flow

Compares input with predicted air flow (derived

If fault detected, uses throttle angle as

from a map of throttle angle against engine speed).

load measurement.

Intake air

Monitors for too fast change of input and for fixed

If fault detected, uses a fixed intake air

temperature

input.

temperature of 50˚C (122˚F).

Default Modes

Mode

Effect

General

CHECK

Message

Warning

ENG

Lamp

Lamp (MIL)

Engine shutdown

Activates fuel cut-off to stop engine

Red

On

ENGINE FAULT

Fixed throttle

Fixed throttle valve angle of approximately 2.5˚

Red

On

ENGINE FAULT

set, producing maximum engine speed (unloaded)
of approximately 1200 RPM.

Mechanical guard

Engine speed increases as throttle valve goes to

Red

On

ENGINE FAULT

mechanical guard position; idle speed increases to
approximately 1400 RPM; full throttle available

Engine speed

Engine speed limited to 3000 RPM

Amber

On

ENGINE FAULT

limiting

Redundancy

Substitute or default value adopted; no noticeable

Amber

Off

ENGINE FAULT

difference in performance

Exhaust emission

Substitute or default value adopted; potential

None

On

None

difference in performance

Note:

In mechanical guard mode, fuel intervention smooths the transition from normal to default mode, to prevent sudden
acceleration of the vehicle. Also, fuel intervention limits idle speed. Without fuel intervention idle speed would be
approximately 2000 RPM and cause excessive shock loads on the transmission when shifting out of Park or Neutral.
When engine load increases, idle speed fuel intervention is progressively withdrawn.