Air Suspension ECU Operation &
Reaction to Faults
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
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. (Photo: Murray James-Wallace's RR at Off-Road height).
Modes of Normal ECU Operation
The air suspension ECU has 3 modes of operation -- normal, periodic wakeup and transportation.
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.
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.
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.
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.
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.
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 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
As of this writing little is known on this subject. On previous models, once the ECU enters fault mode we are sunk, even if we repair the problem cause (eg replace a punctured spring). What we would like to see is 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, we know this does happen in the case of loss of the speed signal (one of the more unlikely faults), which is commendable. In the case of other faults, it is not yet clear whether repairing them will restore normal function or not. The likelihood is that at least in the case of "major" faults, since operation is "frozen", and the ECU is not even trying to level the vehicle, all bets are off and a TestBook or equivalent is needed to reset the fault condition. However we reserve judgement on this until more user experience is accumulated.
Replacing the ECU
If you think the ECU is suffering from mental strain or senility, its replacement 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.
If you have corrections, comments or suggestions, email us.
Page revised February 2, 2012