Air Suspension and typical faults – Discovery 3 / 4

Air Suspension and typical faults – Discovery 3 / 4

8. August 2020 Off By Kenny

The Land Rover 3 and 4’s aerodynamic suspension provides impressive off-road capability of the vehicle and at the same time a very high level of driving comfort. In addition, due to the air suspension it is possible to raise the vehicle (standard) by 55mm (off-road height) or lower it by 50mm (entry height).

When the vehicle is set up, the vehicle additionally activates the extended mode, which cannot be activated manually, and lifts the vehicle by another 30mm. In this extended mode a manual lifting of the vehicle is possible by long (min. 3s) pressing the “lift up” button while the brake is activated. The vehicle now moves to the maximum height that can be reached.

The extended mode is exited when the vehicle speed becomes higher than walking speed.

Off-road mode is automatically exited when the vehicle speed becomes higher than 50km/h.

Access altitude is automatically exited when vehicle speed exceeds 10km/h.

At a vehicle speed of more than 160km/h (for min. 5s) the high speed height is automatically activated, lowering the vehicle from normal height by 20mm. This mode is terminated if the vehicle is moved below 130km/h for more than 30s.

Note: Opening a door pauses the height change of the landing gear. This will continue if the door is closed again within 90s.

Unfortunately, occasionally there are errors in the landing gear which are indicated by a yellow or red flashing suspension warning light (car symbol with three arrows) and by various error messages (suspension errors, elevation only when system has cooled down, only normal altitude available, etc.).

These errors have different causes, although we will go through some of these causes here. But first a few lines of theory about the Land Rover Discovery’s landing gear.


Structure of the air suspension system

The air suspension consists essentially of an air compressor which builds up the air pressure and an air pipe system which distributes the air from the compressor to the 4 air bellows on the 4 wheels. On the front and rear axle there is a valve block each, which can switch and lock the air supply to the respective axle. Between the front and rear axle there is an additional air reservoir, which is supplied with air via a third valve block which is installed directly on the compressor. The compressor is also directly connected to the front and rear valve block via a line so that all systems have the same pressure available (the so-called gallery).
In this gallery there is a pressure sensor which can be read out with a diagnostic device, for example. The valve blocks on the front and rear axle have 4 connections each. One connection is directly connected to the compressor, the second to the air reservoir. The two outlets are connected to the air bellows on the wheels.
In addition, there is a sensor on each wheel to detect the height of the respective wheel, which is connected to the wheel suspension with a coupling rod. The compressor is located in front of the rear wheel on the left side of the vehicle, the electronic control unit of the air suspension is located behind the A-pillar on the driver’s side.

During normal operation the pressure in the gallery should be about 15bar – 18bar. The maximum pressure that should be applied in the gallery is approx. 23bar, whereby the gallery can mechanically withstand pressures of up to 35bar.

If the engine is switched off, the compressor cannot start anymore. Lifting the landing gear is therefore not possible with the engine stopped (to save battery power). In addition, when the engine is switched off and the doors are locked, the vehicle height is levelled so that the vehicle is as level as possible again. This is particularly noticeable when heavy persons/objects are ejected/removed from the vehicle and the vehicle then sinks visibly.

When the vehicle has levelled, a slurping noise is often heard. This noise – often referred to as disco fart – is caused by the release of pressure in the gallery of the system and is completely normal. When parked, the system pressure is only present behind the valve blocks to the wheels and in the pressure reservoir. The gallery and the compressor are pressureless.

The Discovery’s air suspension automatically levels every 6 hours, even when the vehicle is stationary (except when the battery is disconnected). However, since the compressor does not start, this correction can only be made by releasing the pressure, i.e. downward.


Chassis height change

If the desired height of the suspension system is changed, a new setpoint value for the height of each wheel is given. The control unit thus determines that the current actual value of the wheel no longer corresponds to the target value of the wheel and either increases or decreases the pressure in the bellows. If there is a high pressure in the gallery, the system can either open the valve to the gallery for this purpose, or alternatively the system can start the compressor and open the valve for direct connection to the compressor. By using the reservoir, small pressure changes can be implemented very quickly without having to start the compressor each time.


Typical error messages and possible causes

Suspension failure during a speedy off-road ride:

During a speedy off-road ride, it can happen that the height sensors on the wheels send so many strongly changing signals to the electronics of the air suspension that they permanently and very quickly try to equalize pressure by opening and closing the valves. At some point, the electronics can no longer keep up and switch off. The display shows the error message with the aim of informing the driver to reduce the load.

Restarting the vehicle or clearing the error usually corrects this error if no damage has occurred.


Vehicle dropping due to pressure loss
A gradual loss of air pressure in the system is noticeable, for example, when the vehicle has sunk after a long period of standing still. Due to the fact that the air suspension levels the vehicle level every 6 hours, even when the engine is switched off, it is often not possible to tell at which wheel the pressure is escaping, but the vehicle is lower overall. However, this automatic level adjustment can be deactivated by disconnecting the battery, so that it can be determined at which point the pressure is most likely to escape.

In this case, troubleshooting should begin in the area between the lowered wheel and the corresponding valve block. In particular, the valve block should also be checked for leaks, a leak detection spray is helpful in this case.


Compressor overheated – increase only possible after system has cooled down
The compressor is equipped with a temperature monitor. If the temperature of the compressor becomes too high, the display will show this error message and the function of the compressor will be interrupted until the temperature has dropped again. The error often occurs when a large number of altitude changes are requested in a short time or when the compressor does not build up pressure in the usual time due to leaks and therefore runs for a very long time without cooling down.

To eliminate the error, the leaks should be eliminated. Maintenance of the compressor and the seals inside it is also a possible remedy. Special compressor maintenance kits are available for this purpose. But not every compressor is suitable for maintenance, sometimes a new one is needed.

If there are leaks in the system this is of course another reason for the compressor to overheat. The compressor tries to build up the target pressure, but some air is continuously escaping from the system. So the compressor works much longer than normal, which causes the system to overheat.

Compressor exchange – AMK and Hitachi
Until about 2012 Land Rover has installed an air compressor from Hitachi in the Discovery. With good care and maintenance and little extreme off-road use, this can last quite a long time, but problems with it are often reported. Therefore Land Rover has installed a more powerful compressor from AMK in the 2012 models, which should also have a longer service life. It should be noted that the temperature sensors in the two compressors are different (one has a PTC, the other an NTC) which means that after a replacement a software update is necessary. Alternatively you can keep the temperature sensor from the Hitachi when exchanging to the AMK compressor.

Suspension fault directly after engine start
When the vehicle is locked out and especially when the ignition is activated (by ignition key), the vehicle goes through a self-check. If this check is not waited for and the ignition key is turned all the way through too quickly, an error message is often displayed. In this case a restart with a little more patience helps to avoid the problem.

Suspension error due to electronic vehicle elevation
An electronic vehicle height adjustment (IID Tool, XLifter, Anitas) changes the value that the height sensors transmit to the vehicle electronics, so that the vehicle electronics assume that the vehicle is too low (in the case of an electronic lowering, of course, the reverse is true). Consequently, the vehicle electronics will continue to raise the vehicle, which results in the elevation. However, the sensors for determining the vehicle height only have a certain range in which they function properly. Unfortunately, the sensors are also often not installed and calibrated in such a way that they have the same clearance in both directions (up and down).

Assume a sensor has a measuring range of -100 to +100 in the initial situation and is already at a value of +30 in the standard position due to a non-optimal installation and calibration.

Now an electronic elevation of +20mm is set, whereby the sensor rises to a value of +50. Now the driver activates the terrain height, which again causes an increase of +55. The value of the sensor would now go to +105. But its value range already ends at +100, so the sensor goes to the end stop –> the result is a suspension error. The same can of course happen during lowering.

If the suspension errors occur after an electronic raising, this can also be the cause.

Search for leakage in air suspension
Leaks in the air suspension are usually manifested by the fact that the vehicle is lowered after a longer period of standing. To find out exactly where in the air system a leakage is located, the following trick can be used: The vehicle is parked in a good leveled area. Then the battery is disconnected and the vehicle is left standing. Normally the vehicle adjusts the height every 15min, so that the vehicle can not stand crooked. If the battery is disconnected, the vehicle has no voltage and can therefore not control any valves or register the height change (control units have no current). So the vehicle will only sink at the wheel (or axle) where the leakage is. All other valves are closed. In this way it is very easy to find out where to look for the leakage.


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All information as always to the best of our knowledge and belief, but without guarantee!