System functions
The following system functions of the single-axle electronic height control are described
- Electronic height control Ride height is controlled by air being fed into or withdrawn from the air springs. The EHC control unit receives information about the ride height on the left and right-hand sides of the vehicle from the level sensors. If the ride height is outside defined tolerances, the system will automatically activate the air supply unit to regulate to the specified level. The system is activated and starts to regulate whenever a door or the tailgate is opened. The system is deactivated after a certain time (approx. 20 minutes). The signals from the level sensors are filtered to distinguish between changes in load and other disturbances such as an uneven road surface. For correct use, two signals filtered at different frequencies are computed for each signal (low-pass filter with fast and slow filtering). Fast filtering is always employed when regulation is in progress. This prevents the regulated height from exceeding the specified level. Slow filtering is employed when the vehicle is in motion. This mode of filtering eliminates oscillations induced by the road surface. Both sides of the vehicle are regulated independently. In other words, the comparison of specification to actual value is performed separately for each side (exception: minimum height check in pre-mode and in "twist" control function. In these control functions, the mean value of the left and right-hand sides of the vehicle are considered). The EHC control unit recognizes the different operating conditions from the incoming signals and messages. Depending on operating conditions, appropriately adapted control functions are activated. The following control functions of the single-axle electronic height control are described: Sleep mode Pre-mode (temporary start/run-on) Post-mode (readjustment) Normal operation Drive mode Cornering Twist Level-up function (only E61 All-wheel drive until 09/2006) Inclination detection after tire pressure loss (only E70, E71 with 8-cylinder engine or option 4UB "3rd row of seats") Lifting platform Transport mode Production mode Transport and production modes can be set and cancelled with the BMW diagnosis system. If transport or production mode is set, "power economy mode active" will be stored in the fault memory. NOTE: The following times and dimensions all serve as examples. The actual encoding data depend on the model series and vehicle equipment.
Sleep mode
No regulation is performed in sleep mode. Control functions that were started in other control modes are cancelled when sleep mode is activated.
Pre-mode
In pre-mode (temporary start/run-on), changes in load are equalized even as the vehicle occupants are getting in and then again for a certain time (approx. 20 minutes) after the vehicle is shut down.
In pre-mode, the ride height is only regulated to the specified height if it differs significantly from this height. The tolerance range for regulation is approx. 40 millimeters under the mean value (the tolerance range stipulates when the electronic circuitry in the EHC control unit performs regulation).
This tolerance ensures that regulation is only initiated for large loads, to increase ground clearance before pulling away. Small loads produce smaller amounts of spring travel, which are not equalized until the engine is started.
These control characteristics reduce load on the battery.
In pre-mode, only the mean value of the two level sensor signals is considered.
Post-mode (readjustment)
Post-mode is used to equalize any inclination before sleep mode is initiated (fast filtering).
Control function is time-limited (approx. 1 minute).
In post-mode, regulation is restricted to a defined tolerance range (+7/-10 mm).
Normal mode
Normal mode refers to the normal operating conditions for the vehicle. Normal mode is initiated by the "engine running" signal. Regulation is performed within a defined tolerance range on each side (±10 mm, fast filtering).
Drive mode
If a roadspeed signal greater than 1 km/h is detected, the EHC control unit will switch to drive mode.
In drive mode, regulation employs slow filtering. For example, in drive mode, changes in ride height caused by the reduction in the vehicle's weight due to fuel consumption are equalized.
Cornering
The "cornering" control function prevents adjustments being made when cornering is detected. Slow filtering is suspended and ongoing regulation sequences suspended. Cornering is detected from an incoming transverse acceleration signal (e.g. cornering greater than 2 m/s2, less than 1.4 m/s2 when leaving bend). The DSC control unit transmits the transverse acceleration signal via the K-CAN.
Twist
"Twist" mode prevents equalization of an inclination caused by driving over an obstacle with one wheel. Equalization would cause a new inclination after the vehicle is driven off the obstacle, making a new regulation sequence necessary.
If the vehicle is driven onto the curb and stopped, the EHC control unit will switch from drive mode to normal mode. In this control function, the signals from the left and right level sensors are evaluated.
"Twist" control function is activated if the difference in height between the left and right-hand sides exceeds a certain value (e.g. 28 mm).
In this control function, the vehicle's ride height in the middle of the axle is evaluated by the EHC control unit.
If the vehicle is loaded or unloaded, the degree of spring deflection is considered. If this value is outside the defined tolerance range (e.g. ±10 millimeters), regulation is initiated.
The left and right-hand sides of the vehicle are raised or lowered in parallel.
The difference in height between the two sides is maintained.
When the EHC control unit goes into drive mode, control function
"Twist" is terminated. An ongoing regulation sequence will be cancelled. The "Twist" control function is ended if the height differential between the left and right sides of the vehicle drops below a predefined value.
Level-up function (only E61 All-wheel drive until 09/2006)
The level-up function is used to automatically raise the rear of the vehicle after stopping and getting out of the vehicle.
After stopping, the vehicle is automatically raised as follows
- Vehicle left and vehicle "secured" with central locking. The height is automatically adjusted after a short delay.
Alternatively
- Vehicle left and vehicle not "secured" with central locking. The height is automatically adjusted after a longer delay.
When the EHC control unit goes into drive mode, the level-up function is ended The vehicle is lowered to its normal height.
Inclination detection after tire pressure loss (only E70, E71 with 8-cylinder engine or option 4UB "3rd row of seats")
If the Run Flat Indicator (RPA) or tire pressure control (RDC) detects a loss of pressure at one wheel, the control function will relieve the load on the affected tire.
When the tire pressure loss has been rectified and the Run Flat Indicator/tire pressure control no longer indicates a pressure loss, this control function will automatically be ended after about 5 minutes with the engine running.
Lifting platform
In the "Lifting platform" control function, any adjustment is prevented during a wheel change or other work carried out on the lifting platform.
The following conditions trigger the "Lifting platform" control function
- Permissible spring extension exceeded (e.g. more than 50 millimeters, recorded by left and/or right-hand level sensor)
- Vehicle raised slightly (spring extension e.g. less than 55 millimeters), regulation starts but vehicle is not lowered
The height is stored in the EHC control unit.
A reset is performed when the vehicle is again e.g. 10 millimeters under this stored height or when drive mode is detected.
Transport mode
The specified height is raised in transport mode (e.g. 30 mm). The increased ground clearance guarantees risk-free transportation of the vehicle.