Air Suspension ECU Operation &
Reaction to Faults
Overview
Modes of Normal ECU Operation
Normal Compressor Operation Characteristics
Normal Inhibition of EAS Operation
Fault Detection by the ECU
ECU Reaction to Faults
ECU Response After Repairing Faults
Replacing the ECU
More Information
Overview
Knowing how the ECU is supposed to act
normally makes diagnosing problems easier. In particular, while testing
out components of the EAS you might think some aspect of it (such
as the compressor) is
not operating; this can easily be because the ECU has disabled it as
part of its normal operatiing conditions. THis page attempts to
summarize the ECU's modus operandi as an aid to our diagnostic
effforts.
Modes of Normal ECU Operation
The air suspension ECU has 3 modes of
operation -- normal, periodic wakeup and transportation.
Normal
Mode
Whenever a door is opened, the vehicle is unlocked
remotely, or the the ignition is turned on, the air suspension ECU
will get a wake-up signal from the Body Control Unit (BCU), and enter
Normal Mode. The BCU continues to supply this signal, keeping the EAS
ECU in normal mode until 15 minutes after the ignition is switched off.
After that, the ECU remains active for another 15 seconds while it
saves its settings to its EEPROM memory.
During operation in the normal mode, the ECU
constantly monitors the corner sensor
height signals, which are filtered in different ways depending on
operating conditions. In normal driving, a "slow" filter is used to
filter out changes due to rapid road surface undulations to produce a
long term average. At rest and during height changes, a "fast" filter
is used to give an accurate real-time height signal. No height
adjustments are made unless the sensor is more than ±10 mm from
the
target height, in which case the ECU operates the compressor and/or the
valves to make the needed changes. The
ECU monitors the rate of
change of height of the corner signals to predict when to close the
valve so that the target height is not overshot.
While the ECU is in normal mode,
full operation of all functions will only occur while the engine is
running and all the doors are closed. If a wake-up signal has been
received but the ignition is off, the manual EAS controls will be
ignored, and the control LEDs will not illuminate. However the ECU will
monitor vehicle height. When
the engine is not running, target height tolerance is increased to +20
mm and -25 mm. If outside these tolerances (eg
due to leaks or changes in load), the ECU will try to restore it using
air from the reservoir, but the compressor will not be allowed to
refill it. If reservoir pressure falls below 9 bar and more air is
needed, the compressor will come on and supply the air directly (ie the
reservoir solenoid valve will be closed).
If the ignition is on but the
engine is not running, operation is as above but with the LED lights
illuminated and the acceptance of rotary switch commands to lower (but
not raise) the vehicle.
Periodic
Wakeup Mode
When the BCU removes the wakeup signal, the EAS ECU
powers down and periodically (about every 6 hours) wakes up to check on
vehicle height. It tries to determine which corner of the vehiclee is
lowest, and lowers the other corners to the same level. (During
'wake-up', the tolerance band is ±20 mm). This
is the brilliant feature we have come to know and love on previous
models that results in finding the vehicle on the bump stops after
leaving it overnight on an uneven surface. Note that in this mode the
height is never raised, only lowered.
If the ECU is unable to level
the vehicle for any reason (eg if you are camping off road and the
brainy operation of the ECU has lowered the vehicle so it hits a rock
underneath it), periodic wakeup mode is suspended until the ECU
receives another wake-up signal that puts it into normal mode.
Transportation
Mode
This can only be selected by Testbook or equivalent. It keeps the
suspension at standard height ("Transportation High") with all four
LEDs illuminated while the engine is running, and lowers it to
"Transportation Low" (20 mm below access height) with all four LEDs
flashing when the engine is stopped. This effectively rests the vehicle
on its bump stops and allows it to be chained down without danger of
the chains loosening if the system leaks or lowers itself.
Normal
Compressor Operation Characteristics
When the engine is
started, and in "normal mode" (see above) with the engine running, the
ECU runs the compressor subject to the overheating and duty cycle
constraints listed under "Normal Inhibition
of EAS Operation" to try and keep the system pressure at 13.7 bar
(200 psi). When the engine is not running, and if height changes are
approved by the ECU (see above), they are made without operating the
compressor unless pressure drops below 9 bar (130 psi). In this case,
any height increases will be made using the compressor only, with the
reservoir valve inhibited so that air cannot be drawn from the air tank
.Normal Inhibition of EAS Operation
The shop manual lists
numerous conditions under which one or other aspect of normal operation
is inhibited. For
diagnostic
purposes, one should keep in mind that the inhibition (ie failure to
operate) could be due to the ECU merely "thinking" one of the following
conditions exists when it is actually caused by failures in one or more
of the system components. For example, the ECU thinking a door is open
could be due to a faulty door switch, etc.
Compressor Disabled
Operation of the compressor is prevented by the ECU in the following
conditions:
1. Temperature sensor
on compressor indicates compressor is overheated. Interestingly the ECU
will allow the comressor to be run up to 110 degrees C for height
adjustments but only up to 100 degrees C for reservoir filling.
2. System pressure
sensor indicates pressure exceeds 13.7 bar (200 psi).
3. Timeouts: The ECU
will only allow the compressor to run continuously for a maximum of 3
minutes regardless of temperature or pressure. If it is attempting to
fill rthe reservoir it remains off for 3 minutes before resuming, but
if it is just regulating system pressure it only takes a 20 second
break.
Height Changes Inhibited
Height changes are inhibited under many different conditions.
1. A Door or Tailgate Open
will inhibit all height changes and vehicle leveling.
2. Cornering: If the
ABS ECU detects a cornering force more than 0.2 g, the air suspension
ECU detects it via signals on the CAN Bus and inhibits height changes
until cornering forces fall below .15 g for half a second.
3. Rapid Acceleration or
Deceleration: The Air Suspnesion ECU monitors vehicle speed
using a signal from the CAN Bus. If it calculates that acceleration or
deceleration exceeds 0.2 g, height changes are inhibited until
acceleration falls below this level for a second.
4. If Axle Articulation
greater than 350 mm is detected the ECU inhibits height changes until
articulatio falls below this level.
5. Vehicle being Lifted:
If all four corner heights are detected as greater than 90 mm adove
standard and releasing air does not lower them, the ECU thinks the
vehicle must be on a lift, and inhibits all height changes until the
corner heights fall below these levels, a command is entered via the
rotory switch, or vehcile speed rises above 25 mph for 3 seconds.
6. Jacking: If the
ECU cannot lower a corner by releasing air it assumes that corner is on
a jack. More precisely it thinks this if it detects a corner lowering
too slowly when air is released. In this case it inhibits height
changes until the corner reverts to the hwight where this was detected,
the rotary switch is moved to the up or down position, or speed rises
above 25 mph for 3 seconds.
7. Trailering: If the
BCU thinks the trailer connector is being used it will inform the air
suspension ECU and the suspension will be restricted to standard height
until the trailer connector is unplugged.
8. Valves Exceed Allowable
Open Time: The solenoid valves in the 5 way valve block have a
limited duty cycle, as operating them heats them up. If any of them
is open for more than a third of the time during any 5 minute
period the ECU will delay its further operation until the duty cycle
limit is satisfiied.
Fault Detection by the ECU
The
ECU thinks it can directly detect certain kinds of faults, termed
hardware faults, including sensor hardware, valve hardware, sensor and
actuator supply, bus failures, ECU hardware errors. The ECU also tries
to second guess other faults indirectly. This activity is called
plausibility checking, and includes the following checklist according
to the workshop manual:
1. Average height does not change correctly, or changes too slowly.
2. Suspension moves in the wrong direction.
3. Reservoir pressure does not increase when reservoir filling
requested, does not decrease when reservoir used to lift vehicle, does
not decrease when reservoir is vented, or varies too much when inactive.
4. Compressor temperature increases when compressor inactive, or does
not increase when compressor active.
5. Energy' used to change height of corner is too much, or long term
filtered height does not reach target.
6. Sensor signals floating, inconsistent (eg signal on one side of axle
is varying but other side remains static), or indicating constant
articulation when moving.
ECU Reaction to Faults
Whenever a fault occurs, if
the ECU can determine the ride height and the vehicle is not above
standard ride height, the driver will be notified via a 'AIR SUSP.
INACTIVE' message in the message centre. If the ECU cannot determine
the height this message is accompanied with an alternating 'MAX 35MPH'
message.
In terms of restricting EAS function, the
reaction of the ECU to faults (although highly
irritating and overly conservative) is at least somewhat more flexible
than on presious models.
The ECU divides faults that it thinks exist into "major" and "minor"
categories, and its reaction in terms of restricting system
functionality varies accordingly. Also, in a MAJOR IMPROVEMENT over the
P38/4.0/4.6 models, the ECU checks the system every few seconds to see
if the fault sitill exists. If you have fixed the cause (eg replaced a
punctured air spring) the system will return to normal. This is a major
benefit to those of us who actually go off road -- on my 4.0 I was
stranded in the boonies on the bump stops more than once, even though I
had (or could) repair the cause of the problem. So this characteristic
of the new system ``
The following sections summarize the ECU reaction to different kinds of
faults.
Minor Faults:
If the ECU decides there is a "minor" fault, it will freeze ride height
but continue to try and level the vehicle at the current height.
However it will not respond to requests for changes -- except manual or
automatic requests for a return to standard height from some other
height. The
shop manual notes that if the ECU is able to level the
vehicle to the current ride height, the control switch LED's will
display the current ride height. When the vehicle returns to the
standard ride height and further height changes are disabled, the
'HOLD' LED in the control switch will be permanently illuminated.
Minor
faults are defined as follows:
1. Most height sensor hardware and plausibility problems
2. Failure of the reservoir solenoid valve
3. Failure of the Cross Link Valve(s)
If a height sensor fails, some loss in system functionality is
understandable. However in the case of failure of the reservoir valve
there is no good reason why the system should not be allowed to
continue to operate normally, using the compressor to raise the vehicle
height and bypassing the reservoir. If a cross link valve fails, there
is no reason that should affect the functionality of the system other
than disallowing cross-linking. Unfortunately, the designers have again
taken the approach of excessive conservatism which is especially
annoying for the real off road user.
Major Faults:
Major faults are defined in the sho manual as:
1. Compressor faults
2. Plausibility errors – for example:
Average height does not increase
when lifting and the vehicle is moving. This could be caused by a
compressor fault or a fault in the reservoir valve.
Reservoir pressure decreases when
filling requested. This could be caused by a leak in the common gallery
in the valve block or connecting pipe.
No mention is made of faults actually likely to occur during four
wheeling, like having a spring punctured by an off road obstacle(this
happened to me on my 4.0SE). As
on previous models, the ECU has a fairly knee-jerk reaction to these
faults. All the control switch LED's will remain unlit.
The ECU reaction to major faults is to abandon any further attempts to
level the vehicle at the current height and to attempt to return it to
standard height as soon as any manual or automatic height change
request is received. If it can achieve standard ride height, it will
freeze at this height and refuse to respond to further height change
requests.
Loss of Speed Signal:
If the ECU cannot determine vehicle speed, it immediately
forces the system back to standard ride height. It will perform normal
leveling functions at this height but will not respond to requests for
other heights. This fault can be caused by a problem in the CAN Bus or
the ABS system, which in turn can easily happen if the battery is
disconneccted and when reconneccted the steering sensor is not
immediately recalibrated.
Encouragingly, if you can figure out the cause of the fault and repair
it, the system will return to full functionality although an error
message will be stored.
Inability to Lower or Determine
Ride Height
When the vehicle is above "standard" height and the ECU cannot lower it
or cannot determine the vehicle height, all height changes will be
frozen. A maximum advisable speed warning will be displayed in the
message centre of '35MPH'.
Immediate Freeze of Height
In any of the following cases, the ECU will immediately freeze vehicle
height. Once
again a maximum advisable speed warning will be displayed in the
message centre of '35MPH'.
1. Failure of more than one height sensor
2. Implausible articulation symptoms detected
3. Valve or solenoid (other than reservoir valve) failure
4. Stuck corner or corners, diagnosed using plausibility of sensor
inputs
ECU Hardware Faults
If the air suspension ECU thinks it has a hardware fault, it will
disable all air suspension functions. Supposedly, detectable hardware
errors include memory error, ECU failure and calibrations errors.
ECU Response After Repairing Faults
On previous models,
once the ECU enters fault mode we are sunk, even if we repair the
problem cause (eg replace a punctured spring). FORTUNATELY, on the new model this
situation has been greatly improved. Three
Cheers for Land Rover!!! What we hoped to see was the
ability of
the ECU to revert to normal operating mode once
it detects that a fault has been repaired. On the Mk III/L322 Range
Rover, this ability hasfinally been provided, which is a vast
improvement! Early reports informed us that this recovery does happen
if the speed signal is lost and later restored. Subsequently we have
learned from an ex-dealer tech that the EAS system keeps checking
itself every 120 seconds to try to go back to normal operation.
This is a big relief for those of us
who experience failures in the
field, and then make repairs. On my P38 when this happened I had
to
limp home on the bup stopseven after repairing the problem.
If you have more information on this vital subject, please email me.
Replacing the ECU
In some cases when the ECU generates a bunch of fault messages, dealers
have been prone to replace it. The symptoms usually disappear
temporarily after this, but then usually return, as the problem usually
lies elsewhere. For example, note the experience of Soren
Svenningson whoise dealer leapt to this conclusion -- in his case
the problem turned out to be in the ABS system, which can disable the
EAS ECU.
If you think the ECU is suffering from mental strain or senility and
really does need replacing, the operation is simple and
straightforward. However Note that the new ECU has to be
activated by Testbook T4, Rovacom, Autologic or equivalent before it
will operate.
More Information
Range Rover III Common Problems
and Fixes
Range Rover III Air Suspension Details and
Operation
Range
Rover III EAS ECU Operation and Reaction to
Faults
Range Rover II/P38/4.0/4.6 Air Suspension Pages.
Page last updated January 2006