1999 LAND ROVER DISCOVERY ECU

FRONT SUSPENSION
60-18 DESCRIPTION AND OPERATION
ACE Electronic control unit (ECU)
1Body Control Unit (Ref. only)
2Bracket3ACE Electronic control unit
4Attachment nuts 3 off
The ACE ECU is mounted on a bracket behind the passenger glove box and is identified from the other ECU's by its
single electrical harness connector. The single 36 pin connector is located on the lower face of the ECU and mates
with a connector from the main harness. The connector supplies power, ground, signal and sensor information to/from
the ECU for control of the ACE system.
The ACE ECU receives a battery power supply from fuse 15 in the engine compartment fusebox via the ACE relay,
also located in the engine compartment fusebox. The ECU provides an earth path for the relay coil, energising the
coil and supplying power to the ECU.
An ignition on signal is supplied from the ignition switch via fuse 29 in the passenger compartment fusebox. The
ignition on signal provides an input to the ECU which tells the ECU that ignition has been selected on and initiates a
250 ms start time. The start time is used to prevent functions operating when software routines are being initialised.
The ignition on signal, when removed, tells the ECU that the ignition is off. The ECU will remain powered for thirty
seconds after the ignition is turned off. The thirty second period allows the ECU to store values and fault flags in the
memory. These values are read by the ECU when the next ignition on signal is received.
An engine speed signal is transmitted to the ECU from the Engine Control Module (ECM) as a pulsed digital signal.
The engine speed signal is used by the ACE ECU to detect that the engine is running and hydraulic pressure for ACE
system operation is available.
A road speed signal is transmitted to the ACE ECU as a pulsed digital signal from the Self-levelling/Anti-lock Braking
System (SLABS) ECU. The road speed signal is used by the ACE ECU for on and off-road roll compensation.
When reverse gear is selected, an input is received from the reverse lamp switch. When the ACE ECU detects that
reverse gear has been selected, the ACE system reverts to a 'locked bars' condition until reverse gear is disengaged.
The diagnostic connection allows diagnostic interrogation of the ACE ECU. The diagnostic socket allows diagnostic
equipment to be connected to interrogate the ACE ECU for fault codes.

FRONT SUSPENSION
DESCRIPTION AND OPERATION 60-19
When system faults are detected by the ECU, the ACE warning lamp in the instrument pack is illuminated by the ECU
continuously in amber for minor faults or flashing red with an audible warning for faults which require the driver to stop
the vehicle immediately.
The ACE ECU supplies a control current to the pressure control valve in the valve block. The current supplied by the
ECU is determined by a number of input signals from the upper and lower accelerometers, road speed etc.. The
pressure control valve controls the hydraulic pressure supplied to the actuators proportional to the current supplied
by the ECU.
Power is supplied to the two solenoid operated directional control valves (DCV's) in the valve block by the ECU.
Together, the DCV's control the direction of flow of hydraulic fluid to the actuators. When the ECU supplies power to
the solenoids the valves open allowing hydraulic fluid to flow to the actuators. When power is removed the valves
close.
The pressure transducer in the valve block receives a 5 V supply from the ECU. The pressure transducer measures
hydraulic pressures in the range of 0 to 180 bar (0 to 2610 lbf.in
2) and returns a linear output voltage to the ECU
dependent on hydraulic pressure.
The ECU supplies a 5 V current to each of the accelerometers. Each accelerometer is capable of measuring lateral
acceleration in the range of ± 1.10 g. An analogue input to the ECU of between 0.5 and 4.5 V relative to the lateral
acceleration sensed is returned by each accelerometer. The ECU processes the two signals received to produce a
'pure' lateral acceleration signal which is then used as the main control signal for the ACE system.
ACE ECU connector pin details
Pin No. Description Input/Output
1 Not used -
2 Not used -
3 Spare Input
4 Not used -
5 Road speed Input
6ARC relay Output
7 to 9 Not used -
10 K line (diagnostics) -
11 Ignition switch Input
12 Accelerometer - lower (supply) Output
13 Pressure transducer (supply) Output
14 Reverse switch Input
15 Accelerometer - lower (signal) Input
16 Pressure transducer (signal) Input
17 Accelerometer - upper (signal) Input
18 Accelerometer - upper (supply) Output
19 Engine speed Input
20 Main earth 1 -

FRONT SUSPENSION
60-20 DESCRIPTION AND OPERATION
Failure modes
Failures where the vehicle can still be driven safely are indicated by the ACE warning lamp illuminating continuously
with an amber colour. The amber warning lamp will remain illuminated until the ignition is turned off. For all faults the
warning lamp will only illuminate again if the fault is still present. Failures which require the driver to stop the vehicle
immediately are indicated by the ACE warning lamp flashing with a red colour and an audible warning. All faults are
recorded by the ACE ECU and can be retrieved with diagnostic equipment.
The following tables show the type of system failures and their effects on the system operation. Torsion bar 'floppy'
means that fluid is allowed to circulate freely through the system. With no pressure in the actuators the torsion bar will
have no effect on vehicle roll. 'Locked bars' means that all pump flow is directed through the valve block and returns
to the reservoir. Both DCV's close and fluid is trapped in the actuators but can flow from one actuator to the other via
the valve block. In this condition the torsion bar will perform similar to a conventional anti-roll bar, resisting roll but still
allowing the axles to articulate.
Acceleration sensors
Pressure transducer
Road speed signal
21 Pressure transducer (earth) Input
22 DCV 2 (earth) Input
23 DCV 1 (earth) Input
24 DCV 1 & 2 (supply) Output
25 Pressure control valve (earth) Input
26 Not used -
27 Pressure control valve (supply) Output
28 Main supply (+ V Batt) Input
29 to 31 Not used -
32 Main earth 2 -
33 Accelerometer - lower (signal) Input
34 Accelerometer - upper (signal) Input
35 Not used -
36 Warning lamp Output
Failure Effect
Valve stuck closed No ACE control
Short circuit - Ground No ACE control
Short circuit - VBatt No ACE control
Loose sensor Erractic ACE activity when driving in straight line
Failure Effect
Short circuit - VBatt Large sensor dead band - possible random
movements
Failure Effect
Open circuit No ACE control - 'Locked bars' condition
Short circuit - Ground No ACE control - 'Locked bars' condition
Short circuit - VBatt No ACE control - 'Locked bars' conditionPin No. Description Input/Output

FRONT SUSPENSION
DESCRIPTION AND OPERATION 60-21
Engine speed signal
Reverse gear signal
Ignition ON signal
Pressure control valve failure
Directional control valves
Failure Effect
Open circuit No ACE control - 'Locked bars' condition
Short circuit - Ground No ACE control - 'Locked bars' condition
Short circuit - VBatt No ACE control - 'Locked bars' condition
Failure Effect
Open circuit No reverse signal to ECU. ACE active in reverse,
may give abnormal handling when reversing
Short circuit - Ground No reverse signal to ECU. ACE active in reverse,
may give abnormal handling when reversing
Short circuit - VBatt Permanent reverse signal to ECU. Permanent
'Locked bars' condition
Failure Effect
Open circuit ECU does not receive ignition ON signal. No ARC
control, 'Locked bars' condition
Short circuit - Ground ECU does not receive ignition ON signal. No ARC
control, 'Locked bars' condition
Short circuit - VBatt Permanent ignition ON signal to ECU. Possibility of
flat battery
Failure Effect
Open circuit No ACE control
Short circuit - Ground No ACE control
Short circuit - VBatt No ACE control
Valve stuck open No ACE control
Valve stuck closed Maximum system pressure - no proportional control.
Pressure relief valve operating at 185 bar (2683
lbf.in
2)
Failure Effect
DCV 1 DCV 2
Valve open or stuck open Valve open or stuck open No ACE control - Anti-roll bars floppy
Valve closed or stuck
closedValve closed stuck
closedNo ACE control - 'Locked bars' condition (default)
Valve open or stuck open Valve closed or stuck
closedVehicle leans to left when pressure is applied to
actuators
Valve closed or stuck
closedValve open or stuck open Vehicle leans to right when pressure is applied to
actuators

FRONT SUSPENSION
60-22 DESCRIPTION AND OPERATION
Operation
Hydraulic circuit diagram
1Pressure transducer
2Directional control valve 2
3Front torsion bar assembly
4Actuator
5Actuator
6Rear torsion bar assembly
7Directional control valve 18Valve block
9Pressure control valve
10Reservoir
11Filter
12High pressure filter
13Hydraulic pump
14Attenuator hose
Vehicle not moving
When the engine is running and the vehicle is not moving, both DCV's are closed, locking fluid in each side of the
actuator pistons. The hydraulic pump draws fluid from the reservoir and passes it at very low pressure to the valve
block. Because both DCV's are closed, after the fluid passes through the high pressure filter, it is directed through the
pressure control valve to the reservoir. The pressure control valve is open fully to allow the full flow to pass to the
reservoir. The DCV's will remain closed until the ECU detects a need to operate.

FRONT SUSPENSION
DESCRIPTION AND OPERATION 60-23
Vehicle moving and turning left
When the vehicle is turning left, the accelerometers detect the cornering forces applied and transmit signals to the
ECU. The ECU determines that an opposing force must be applied to the torsion bars to counter the cornering forces.
The ECU supplies a current to the solenoid of the DCV2. Simultaneously, a current is sent from the ECU to the
pressure control valve which operates to restrict the flow of fluid returning to the reservoir.
The restriction causes the hydraulic pressure in the system to rise and the pressure is sensed by the pressure
transducer which sends a signal to the ECU. The ECU determines from the inputs it receives what pressure is required
and adjusts the pressure control valve accordingly.
The pressure in the system is applied to the annulus of each actuator, applying an opposing force to the torsion bar
and minimising the cornering effect on the vehicle and maintaining the vehicle attitude. The fluid displaced from the
full area of the actuator is returned to the reservoir via the valve block.
As the cornering force is removed when the vehicle straightens up, the ECU opens the pressure control valve to
reduce the pressure in the system. The fluid bleeds from the actuator back into the system as the cornering force is
reduced, removing the force from the torsion bar. When the vehicle is moving in a straight line DCV 2 closes.
Vehicle moving and turning right
When the vehicle is turning right, the accelerometers detect the cornering forces applied and transmit signals to the
ECU. The ECU determines that an opposing force must be applied to the torsion bars to counter the cornering forces.
The ECU supplies a current to the solenoid of the DCV1. Simultaneously, a current is sent from the ECU to the
pressure control valve which operates to restrict the flow of fluid through the by-pass gallery.
The restriction causes the hydraulic pressure in the system to rise and the pressure is sensed by the pressure
transducer which sends a signal corresponding to the pressure to the ECU. The ECU determines from the inputs it
receives what pressure is required and adjusts the pressure control valve accordingly.
The pressure in the system is applied to the full area of each actuator, applying an opposing force to the torsion bar
and minimising the cornering effect on the vehicle and maintaining the vehicle attitude. The fluid displaced from the
annulus of the actuator is returned to the reservoir via the valve block.
As the cornering force is removed when the vehicle straightens up, the ECU opens the pressure control valve to
reduce the pressure in the system. The fluid bleeds from the actuator back into the system as the cornering force is
reduced, removing the force from the torsion bar. When the vehicle is moving in a straight line the DCV 1 closes.
Vehicle moving in a straight line
The ECU is constantly monitoring the signals received from the accelerometers and operates the DCV's and pressure
control valve to maintain the vehicle attitude when the vehicle is moving.
Off-road driving
Off-road detection is achieved by the ECU by monitoring the signals from the upper and lower accelerometers for
varying degrees of body movement. Off-road driving generates differing signals to the accelerometers which in turn
produce differing outputs due to their vertical separation and the location of the roll centre of the vehicle. The two
signals are passed through a filter to remove any offset caused by the vehicle leaning or the terrain. The ECU then
uses this signal to calculate the percentage of road roughness.
Below 25 mph (40 km/h) the percentage of road roughness calculated is used by the ECU to limit the operation of the
ACE system. The system is completely inoperative at speeds below 2 mph (3 km/h). At speeds above 25 mph (40
km/h) the system disables the percentage road roughness signal and full ACE system assistance is restored.
Side slope detection
The ECU uses side slope detection when the upper and lower accelerometers detect an average acceleration of more
than ± 0.2 g and a road speed of less than 25 mph (40 km/h).
When side slope is detected both DCV's close to provide a 'locked bars' condition. This condition increases stability
and gives a consistent vehicle response. As the road speed increases up to 25 mph (40 km/h), the level of average
lateral acceleration must also increase and be maintained for the system to recognise that the vehicle is on a side
slope. If the side slope angle is steep and the road speed is low, the ECU will detect the side slope in a short time.

FRONT SUSPENSION
REPAIRS 60-27
REPAIRS
Bushes - Panhard rod
$% 60.10.07
Remove
1.Raise front of vehicle.
WARNING: Do not work on or under a
vehicle supported only by a jack. Always
support the vehicle on safety stands.
2.Remove 2 nuts and bolts securing Panhard rod
to axle and chassis.
3.Remove Panhard rod from vehicle.
4.Use tool LRT-60-013 fitted with LRT-60-013/1
and LRT-60-013/3 to press out bushes from
Panhard rod. Refit
1.Clean bush locations in Panhard rod.
2.Use tool LRT-60-013 fitted with LRT-60-013/1
and LRT-60-013/2 to press new bushes into
Panhard rod. Ensure pressure is applied to
the outer edge of the bush, not the rubber
inner.
3.Position Panhard rod to axle and chassis.
4.Fit bolts securing Panhard rod but do not
tighten at this stage.
5.Remove stand(s) and lower vehicle.
6.Tighten bolts securing Panhard rod to 230 Nm
(170 lbf.ft).
CAUTION: Nuts and bolts must be tightened
with weight of vehicle on suspension.

FRONT SUSPENSION
60-28 REPAIRS
Ball joint - upper - steering knuckle
$% 60.15.02
CAUTION: Each ball joint can be replaced up to
three times before the axle yoke bore becomes
oversize. Before commencing work, thoroughly
clean surface of joint and check for yellow paint
marks approx. 12 mm (0.5 in) wide. If 3 marks are
found, the axle case must be renewed.
Remove
1.Remove steering knuckle.

+ FRONT SUSPENSION, REPAIRS,
Steering knuckle.
2.Support the chassis on stands.
3.Support the front axle on stands.
4.Remove 2 nuts securing anti-roll bar lower links
to front axle, use a 16 mm spanner to prevent
link joint from turning.
5.Remove 2 bolts securing each front damper to
axle.
6.Remove 8 bolts securing chassis crossmember
and remove crossmember.
7.Remove bolt securing brake hose and ABS
sensor harness bracket to axle.
8.Lower the front axle, release front spring from
damper and remove front spring.9.Fit tool LRT-54-008/22 to tool LRT-54-008.
10.Fit tool LRT-54-008/4 to tool LRT-54-008, and
secure with screw.
11.Fit tool LRT-54-008 with all attachments to
upper ball joint.
12.Fit tool LRT-54-008/5 to the top of the ball
joint.
13.Press upper ball joint from axle. When ram lead
screw reaches the end of its stroke, retract the
lead screw and screw the ram further into the
tool. Repeat the operation until the ball joint is
released from the axle.
14.Dismantle the tools.