Suspension: Description and Operation

Single-Axle Self-Levelling Suspension

Single-axle self-levelling suspension
The single-axle self-levelling suspension has the task of keeping the vehicle height at the rear axle at the specified nominal height setting independently of the load status.

Brief description of components
The single-axle self-levelling suspension consists essentially of the following components:
- EHC control unit
The EHC control unit (EHC = Electronic Height Control) decides on a case-by-case basis whether there is a need for adjustment to compensate for changes in the load. The EHC control unit prevents adjustments caused any other way. This makes it possible to gear the intervention to each situation as regards optimum frequency, target height, tolerance thresholds and battery load.
In addition to handling the self-levelling suspension, the EHC control unit has the task of monitoring the system components, saving and displaying faults.
The EHC control unit (EHC = Electronic Height Control) uses various signals and messages to detect the different driving conditions. Depending on the drive status, the EHC control unit switches into various control functions.
The EHC control unit processes the following signals and K-CAN (body CAN) messages:
- Ride-level height of vehicle from height-level sensors located at the rear right and rear left.
- Terminal 15 ON/OFF (K-CAN message terminal status from the CAS)
- Driving speed (CAN message dynamism from the DSC)
- Lateral acceleration (CAN message dynamism from the DSC)
- 'Engine running' signal (CAN message, engine running indicator from the DME or DDE)
- Information regarding the status of the doors and luggage compartment lid (CAN message lid/door status from CAS, KGM, KBM, FRM, JBE)
- Kilometer reading (CAN message from the instrument cluster)
- Status terminal R (CAN message terminal status from the CAS)
- Trailer detection (CAN message from the trailer module, AHM, control unit)
- Status of the Tire Pressure Monitor or Run Flat Indicator (CAN message from TPM control unit or RPA control unit).
The status of the Tire Pressure Monitor or Run Flat Indicator is not processed in all vehicles, rather depends on the model series and engine version or model series and optional extras (e.g. only E70, E71 with 8-cylinder engine or optional extra 4UB "3rd row seats").

- Air supply system
The functions of the air supply unit are implemented by activating the compressor and the solenoid valves. The EHC control unit controls the functions.
The air supply unit contains the following components:
- Electric motor for drive of the compressor
- Compressor with air drier
Drain valve with pressure retention and pressure limitation function
- Electr. control valve
- Solenoid valve block with 2 solenoid valves (2/2-way valves)







Compressor
The compressor is driven by an electric motor. Power is supplied to the electric motor via the compressor relay. The compressor is activated by the EHC control unit until the specified nominal height is reached.
The compressor temperature is monitored by a time model (stored in the EHC control unit). If the maximum run time of the compressor is exceeded, other adjustments are suppressed. If the value falls short of the threshold value for reactivation, adjustment is possible again.
A non-return valve is integrated in the compressor. The check valve prevents uncontrolled pressure decreases via the compressor.

Air drier
The air taken in by the compressor passes over a water-absorbing filter in the air drier. This withdraws moisture from the air. As long as the air is more moist than the filter, the filter absorbs the moisture. On flowing back, the air is drier than the filter. This returns the moisture to the air and is escapes.

Valve block
The valve block contains the following valves:
- Control valve (electrically controlled)
- Drain valve (pneumatically controlled) with pressure retention and pressure limitation function

Control valve and drain valve
The electrical control valve functions as a type of "relay" for the drain valve. The drain valve is pneumatically controlled with a small controlled air flow from the control valve. This enables large volumes of air to escape quickly in the drain valve. This combined arrangement of the valves means that a solenoid valve with high current consumption can be eliminated.
The EHC control unit controls the control valve.
The pressure retention and pressure limitation function of the drain valve limits the pressure in the air supply unit as follows:
Minimum pressure: 1 bar
Maximum pressure: 11 + 2 bar

- 2 air springs
The air springs are fitted in place of the steel springs. The air springs form the airtight and moving connection between the body and the wheel carrier. The air pressure in the air springs bears the vehicle load in each case. The air springs and the shock absorber are separate.

- 2 height-level sensors
The height sensors provide the EHC control unit with information on the heights of the left-hand and right-hand sides of the vehicle. The height-level sensors are installed on the rear axle.

- Check Control
A fault in the single-axle self-levelling suspension is shown in the LC display by means of a Check- Control symbol (yellow). The text for the Check Control message can be called up in the Central Information Display (CID).

System functions
The self-levelling suspension works by feeding or extracting air into or out of the air springs.
The height sensors provide the EHC control unit with information on the heights of the left-hand and right-hand sides of the vehicle.
If the heights are outside the specified tolerance, the system intervenes using the air supply unit to set the nominal ride-level height.







Changes to the load mainly take place when the vehicle is at a standstill, before or after a journey. In order to load the vehicle, the doors or boot/rear hatch have to be opened. The system is activated when a door or the luggage compartment lid is opened. The system only switches off 20 minutes after the driver leaves the vehicle (consumer unit shutdown signal).
In order to distinguish load changes from disturbances such as uneven roads, the signals of the height-level sensors are filtered. To ensure use in line with requirements, for each signal two filtered signals with different frequencies are calculated (low-pass filtering with fast and slow filtering).
During an adjustment, it is always the fast filtering that is used. This prevents an increase in the adjusted height above the target value.
The slow filtering is used while the vehicle is being driven. This type of filtering means that vibrations produced by the road surface are filtered out.
Both sides of the vehicle are adjusted individually. This means that the target/actual comparison is also run for both sides individually. (Exception: Checking the minimum height in the Pre mode and in the control function "transposition". (In these control functions, the average values of the left-hand and right-hand side of the vehicle are analyzed.)
The EHC control unit uses the various signals and messages to detect the various driving states. Depending on the driving states, the adapted control functions are switched accordingly.
The following control functions of the single-axle self-levelling suspension are described:
- Sleep mode
- Pre mode (temporally controlled pre-run / after-run)
- Post mode (post adjustment)
- Normal mode
- Drive mode
- Cornering
- Transposition
- Level up function (only E61 all wheel drive up to 09/2006)
- Detection of an inclination after tire pressure loss (only E70, E71 with 8-cylinder engine or optional extra 4UB "3rd row seats")
- Lifting platform
- Transport mode
- Production mode

NOTE: All of the following specified times and dimensions are examples.

The actual coding data depends on the model series and vehicle equipment.

Sleep mode
The Sleep mode is the initial state for interventions. In the Sleep mode, no intervention takes place. Adjustments that were started in other intervention modes are cancelled when the Sleep mode takes effect.

Pre mode
In the Pre mode (temporally controlled pre-run / after-run), loads are balanced out on getting into the car as well as for a certain time (approx. 20 minutes) after parking the vehicle.
The consumer unit shutdown signal is set by activities on the vehicle such as operating a door handle or the central locking system and output on the K-CAN.
In the case of ignition key position terminal R or terminal 15 or 'engine on' signal, the signal is set permanently.
If the vehicle is parked, the consumer unit shutdown signal remains set for approx. 20 minutes after the last action (e.g. door opening). If a door status changes within this 20 minutes, the after-run time is restarted. When this after run time expires, the message 'switch off consumers' is sent and the control unit switches for 1 minute into the Post mode before it goes into the Sleep mode.
In the Pre mode, the vehicle can only be raised to its specified height if the level drops significantly below the specified height. The tolerance range for the control is approx. 40 millimeters below the average value. (The tolerance range indicates when the electronics in the EHC control unit make an adjustment.
This tolerance ensures that adjustments are only made in the event of large additional loads to increase ground clearance before driving away). Smaller additional loads only lead to small amounts of spring travel which are only balanced out after the engine has been started. These characteristics reduce the load on the battery. In the Pre mode, only the average value of the two signals of the height-level sensors are analyzed.

Post mode (post adjustment)
The Post mode is used to balance out any inclination before entering the Sleep mode after completion of a journey. The control function has a time limit (approx. 1 minute).
In the Post mode, adjustment is made in a defined tolerance range (+7 / -10 millimeters).

Normal mode
The Normal mode refers to the normal operating condition of the vehicle. The Normal mode is initiated by means of the 'engine running' signal. Adjustment is made in a defined tolerance range per side (± 10 millimeters, rapid filtering).

Drive mode
The Drive mode is activated in the EHC control unit when a speed signal above 1 km/h is detected. In the Drive mode, adjustment is made by means of slow filtering. In the Drive mode, e.g., different ride heights that occur as a result of the reduction in the vehicle mass due to fuel consumption are balanced out.

Cornering
The control function "cornering" prevents adjustment when cornering is detected. The slow filtering is halted and any ongoing adjustment is cancelled.
The cornering is detected via the incoming transverse acceleration signal (e.g. above 2 m/s(2) cornering detected, below 1.4 m/s(2) cornering completed). The DSC control unit sends the transverse acceleration signal via the K-CAN.

Transposition
The operating mode "transposition" prevents balancing of any inclination when only one wheel of the vehicle drives over an obstacle. Balancing would lead to a renewed inclination of the vehicle after coming down from this obstacle and to a new adjustment.
If one wheel of a vehicle drives up onto the kerb and stops, the EHC control unit switches from the Drive mode into the Normal mode. In this control function, the signals of the height-level sensors on the left and right are evaluated.
If the difference in height between the left-hand and right-hand side of the vehicle exceeds a certain value (e. 28 millimeters), a switch is made to the control function "transposition".
In this control function, the EHC control unit analyses the car height in the centre of the axle.
If the vehicle is now loaded or unloaded, the value of the compression or rebound is analyzed. If the value lies outside of the defined tolerance range (e.g. ± 10 millimeters), an adjustment is initiated.
The left-hand and right-hand sides of the vehicle are raised or lowered in parallel. The difference in height between the two sides remains.
When the EHC control unit goes into the Drive mode, the control function "transposition" is terminated. Any ongoing adjustment is cancelled. The control function "transposition" is terminated when the difference in height between the left-hand and right-hand sides of the vehicle falls short of a specified value.

Level up function (only E61 all wheel drive up to 09/2006)
The level up function is used to raise the vehicle rear end automatically after parking and leaving the vehicle.
The vehicle is raised automatically after parking as follows:
- Leaving the vehicle
and
'deadlocking' the vehicle via the central locking system.
The automatic raising is carried out after a short delay period.

or

- Leaving the vehicle
and
not 'deadlocking' the vehicle via the central locking system.
The automatic raising is carried out after a longer delay period.

When the EHC control unit goes into the Drive mode, the level up function is terminated. The vehicle is lowered to normal height.

Detection of an inclination after tire pressure loss (only E70, E71 with 8-cylinder engine or optional extra 4UB "3rd row seats")

If the Run Flat Indicator (RPA) or Tire Pressure Monitor (TPM) detects tire pressure loss on one wheel, the control function reduces the load on the tire concerned.
When the tire pressure loss has been remedied and the Run Flat Indicator or Tire Pressure Monitor no longer indicates tire pressure loss, with the engine running this control function is terminated automatically after approx. 5 minutes.

Lifting platform
In the control function "lifting platform", adjustments due to wheel changes or work on the lifting platform are prevented.
The following conditions trigger the "lifting platform" control function:
- The permitted rebound path is exceeded (e.g. more than 50 millimeters, picked up by the left and/or right-hand height-level sensor)
- The vehicle is raised slightly (rebound of e.g. less than 55 millimeters), the control operation starts and the vehicle is not lowered

The height is stored in the EHC control unit.
A reset is carried out when the vehicle returns to e.g. 10 millimeters below this stored height or the Drive mode is detected.

Transport mode
In the Transport mode, the nominal height is raised (e.g. 30 mm). Increasing the ground clearance ensures safe transport of the vehicle.

Production mode
The Production mode is used for assembly on the line and prevents regulating operations.
New EHC control unit are delivered with the Production mode set. After assembly of the new EHC control unit, the Production mode must be deleted.

Notes for Service department

General information

Performing ride-height-calibration
A ride-height-calibration must be performed after the following servicing operations:

E61
- Replacement of the control unit for the single-axle self-levelling suspension (EHC control unit)
- Replacement or repair of one or both of the height-level sensors
- Replacement of the wiring harness

E70, E71
- Replacement of the control unit for the single-axle self-levelling suspension (EHC control unit)
- Replacement or repair of one or of both height-level sensors or their control rods
- Replacement of the wiring harness
- Replacement of the control unit for vertical dynamics management (VDM control unit)

For the ride-height-calibration, the measured vehicle height at the rear axle (actual height on the left and right) must be entered in the BMW diagnosis system. To do so, select the service function "ride-height-calibration" on the BMW diagnosis system.
The actual height is measured between the lower edge of the rim flange and the wheel-house lower edge.
No liability can be accepted for printing or other errors. Subject to changes of a technical nature