Single-Axle Air Suspension
Single-Axle Air Suspension
Single-axle air suspension
The self-levelling suspension on the rear axle ensures a vehicle height and ground clearance that are independent of the load. To do so, the height of the vehicle body is raises by 2 air springs under all load conditions to a specified nominal ride-level height setting.
Brief component description
The single-axle air suspension comprises the following components:
EHC control unit
The EHC control unit (EHC = Electronic Height Control) decides whether there is a need for adjustment to compensate for changes in the load. The vehicle heights and vehicle conditions are taken into account here. 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 and saving faults.
The EHC control unit recognizes the different operating conditions from the incoming signals and messages.
Depending on the drive status, the EHC control unit switches into the corresponding control functions.
The EHC control unit processes the following signals and bus signals:
- Vehicle ride height from ride height sensors located at the rear right and rear left.
- Terminal 15 ON or OFF or terminal R (bus signal with terminal status from CAS)
- Driving speed (bus signal driving dynamics from Integrated Chassis Management ICM)
- Lateral acceleration (bus signal driving dynamics from Integrated Chassis Management ICM)
- 'Engine running' signal (bus signal from DME or DDE)
- Information on the status of the doors and luggage compartment lid (bus signal with flap status from CAS, FRM, JBE)
- Kilometer reading (bus signal from the instrument panel)
- Trailer detection (bus signal from the AHM control unit)
- Status of the Tire Pressure Monitor or Run Flat Indicator (bus signal from the RDC control unit or DSC control unit).
Air supply system
The functions of the air supply system are implemented by compressor activation and activation of the solenoid valves. The EHC control unit controls the functions.
The air supply system contains the following components:
- Electric motor for drive of the compressor
- Compressor with air drier
- Discharge valve with pressure limitation function
- Solenoid valve block with 2 solenoid valves (2/2 way valves)
Compressor
The compressor is driven by an electric motor. Voltage is supplied to the electric motor via a relay. The relay 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 operating period for the compressor is exceeded, further adjustments are suppressed. Once the value drops below the threshold value for switching on again, adjustments can be made again.
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 directed outside.
Solenoid valve block
The solenoid valve block contains the following valves:
- Solenoid valve for controlling the rear-left air spring
- Solenoid valve for controlling the rear-right air spring
Discharge valve
The discharge valve is activated by the EHC control unit. The air escapes from the solenoid valve block via the discharge valve through the air drier.
The pressure limitation function of the discharge valve limits the pressure in the air supply system as follows:
Maximum pressure 13.5 bar (temperature-dependent tolerance +5 to + 6.5 bar = breakaway moment if discharge valve not used for a longer period)
2 air springs
The air springs are integrated in the spring struts. The air springs are surrounded by an aluminium sleeve. The air springs form the airtight and moving connection between the body and the wheel carrier. The air pressure in the air spring supports the vehicle's weight.
2 ride height sensors
The ride height sensors provide the EHC control unit with information on the heights of the left and right sides of the vehicle. The ride height sensors relevant to EHC are fitted to the rear axle.
NOTICE: no direct connection to EHC control unit.
As of 09/2010 direct connection of the ride height sensors to the EHC control unit has been discontinued. The EHC control unit receives the signals via the bus system.
The coupling bar converts the wheel lift into a rotational movement at the ride height sensor. As the spring compresses and rebounds, the output voltage of the ride height sensor changes linearly. On compression at the rear left, the output voltage rises. On rebound, the output voltage falls (rear right exactly vice versa, determined by the installation position).
Check Control
A fault in the single-axle air suspension is shown in the instrument panel 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 ride height sensors provide the EHC control unit with information on the heights of the left and right sides of the vehicle. If the ride heights are outside of the specified tolerance (depending on vehicle condition, e.g. engine running), the system regulates to the nominal ride-level height setting via the air supply system.
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 luggage compartment / tailgate have to be opened. The system is activated when a door or the luggage compartment lid is opened. The system only switches off several minutes after the driver leaves the vehicle.
In order to distinguish load changes from other influences such as uneven roads, the signals of the ride height sensors are filtered. For correct use, two signals filtered at different frequencies are computed for each signal (low pass filtering with fast and slow filtering). Fast filtering is always employed when regulation is in progress. Slow filtering is used while the vehicle is being driven to monitor the control thresholds. This type of filtering means that vibrations produced by the roadway surface are filtered out.
Both sides of the vehicle are adjusted individually. This means that the nominal/actual value comparison is also run for both sides individually. Exception: Checking the minimum height in the Pre mode and in the control function "transposition". With these control functions, the mean values of the left and right-hand sides of the vehicle are taken into account.
The EHC control unit uses the various signals and messages to detect the various driving conditions. Depending on the driving conditions, the adapted control functions are switched accordingly.
The following control functions of the single-axle air suspension are described:
- Rest state
- Pre mode (temporally controlled pre-run / after-run)
- Post mode (post adjustment)
- Normal mode
- Drive mode
- Cornering
- Transposition
- vehicle hoist
- Transport mode
- Production mode
NOTE: all times and dimensions cited below are examples only.
The actual coding data depend on the model series and vehicle equipment.
Rest state
The Sleep mode is the initial state for interventions. In the rest state, no controls takes place. Adjustments that were started in other intervention modes are cancelled when the Sleep mode takes effect.
Pre mode
In Pre mode (timed initial run / after-run), changes in load are equalized even as the vehicle occupants are getting in and then again for a few minutes after the vehicle is shut down. When the vehicle is locked, the after-run is shortened.
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 band for the control is approx. 60 millimeters below the mean value. The tolerance band stipulates when the electronic circuitry in the EHC control unit performs regulation. This tolerance ensures that the vehicle is only regulated for heavier loads to increase ground clearance before driving off. Smaller payloads only lead to small amounts of compression travel which are only balanced out after the engine has been started. These control characteristics reduce load on the battery. In pre-mode, only the mean value of the two ride height sensor signals is considered.
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 band (± 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 band 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 drive mode, regulation employs 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. Slow filtering is suspended and any ongoing regulation sequences are cancelled. Cornering is detected from an incoming lateral acceleration signal (e.g. cornering greater than 2 m/s2, less than 1.4 m/s2 when leaving bend). The ICM control unit sends the lateral 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. 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 kerb stone and stopped, the EHC control unit will switch from drive mode to normal mode. In this control function, the signals of the ride height sensors on the left and right are evaluated. If the difference in ride height between the left and right sides of the vehicle exceeds a certain amount (e.g. 28 millimeters), switch-over to the control function "skew" takes place. In this control function, the vehicle 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 rebound is considered. If the figure is outside the defined tolerance band (e.g. ±10 millimeters), modulation is initiated. The left and right-hand sides of the vehicle are raised or lowered by the same amount as each other. The difference in ride height between the two sides is retained. When the EHC control unit goes into the Drive mode, the control function "transposition" is terminated. Any ongoing control is cancelled. The "skew" control function is cancelled when the ride height difference between left and right sides of the vehicle is below a specified amount.
vehicle hoist
In the control function "Vehicle hoist", adjustments due to wheel changes or repair work on the vehicle hoist are prevented. The following conditions trigger the "Lifting platform" control function:
- Permissible spring extension exceeded (e.g. more than 50 millimeters, recorded by left and/or righthand ride height 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 ride height is stored in the EHC control unit. A reset is performed when the vehicle is again e.g. 10 millimeters under this stored ride height or when 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.
Notes for Service department
General notes
Coding
After replacement, the EHC control unit must be encoded.
Carrying out ride height calibration.
Ride height adjustment must be performed after the following servicing operations:
- Replacement of the control unit for the single-axle ride height control system (EHC control unit); up to 08/2010 only
- Replacement or repair of one or of both ride height sensors or their coupling bar
- Repair to the lines of the ride height sensor
To calibrate the ride height, the vehicle ride height measured at the rear axle (actual ride height on the left and right) must be entered in the diagnosis system. To do so, select the service function "Ride height calibration" on the diagnosis system. For details on how to proceed, refer to repair instructions.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.