Hydraulic Trunk-Lid Actuation (HKM)
Trunk Lid ModuleAutomatic opening and closing of trunk lid/tailgate
Function
Automatic opening and closing of the trunk lid/tailgate is controlled by the trunk lid/tailgate module and the general module of the ZKE III.
The trunk lid/tailgate is opened and closed by means of a hydraulic system, consisting of a hydraulic pump and two hydraulic cylinders. The hydraulic system is controlled by the trunk lid/tailgate module.
In addition to the hydraulic system, an automatic soft-close facility (fitted as standard on E38) and an electrical rear lid release (fitted as standard on E39) are also fitted on the rear lid. These components are controlled by the general module of the ZKE III.
The general module and trunk lid/tailgate module are interlinked via the K-bus.
Trunk lid opening procedure
Release button is pressed.
The ZKE III general module receives the instruction to open the trunk lid/tailgate. It activates the drive for the automatic soft-close facility and the drive for the rear lid release. The rotary latch of the trunk lid/tailgate is opened and the lock striker is turned upward.
The rear lid contact signals "trunk lid/tailgate open" to the general module. This signal is routed via the K-bus to the trunk lid/tailgate module.
The trunk lid/tailgate module powers the hydraulic valve and the hydraulic pump. The trunk lid/tailgate is opened by means of the hydraulic cylinders.
Trunk lid closing procedure
Release button is pressed.
The trunk lid/tailgate module powers the hydraulic valve and the hydraulic pump. The trunk lid/tailgate is closed by means of the hydraulic cylinders.
The trunk lid/tailgate module recognizes the opening angle of the trunk lid/tailgate by means of the tilt switch mounted on the trunk lid/tailgate.
The hydraulic pump is switched off and the hydraulic valve opened when the opening angle is less than 8o. The trunk lid/tailgate falls closed.
The trunk lid/tailgate contact signals "trunk lid/tailgate closed" to the general module in the ZKE III.
The general module activates the drive for the automatic soft-close facility. The lock striker of the trunk lid/tailgate is turned down thus pulling the rear lid closed.
Operating buttons
The trunk lid/tailgate opening and closing functions are operated by means of three pushbuttons.
Operating button for trunk lid/tailgate closing cylinder
The trunk lid/tailgate can be opened and closed by pressing the lock cylinder in the trunk lid/tailgate.
The pushbutton is connected both to the trunk lid/tailgate module as well as to the general module in the ZKE III.
Operating button for trunk lid/tailgate, inside
The trunk lid/tailgate can be opened and closed by pressing this pushbutton.
The button is only connected to the trunk lid/tailgate module. A closed trunk lid/tailgate cannot be opened by means of this button as the signal from this button is not sent to the general module in the ZKE III, consequently the rotary latch is not released.
Operating button, vehicle interior
The trunk lid/tailgate can be released and opened by pressing this button (located in A-pillar on driver's side). The trunk lid/tailgate cannot be closed with this button.
The pushbutton is only connected to the general module in the ZKE III. Its signal is routed via the K-bus to the trunk lid/tailgate module.
All operating buttons switch to ground.
If the buttons are struck briefly the trunk lid/tailgate is fully opened (all pushbuttons) or closed (only buttons in trunk lid/tailgate).
If the trunk lid/tailgate is moving, it is stopped when a button is pressed once more.
Radio remote control
The trunk lid/tailgate can be opened via the radio remote control.
The trunk lid/tailgate is opened by pressing the key button with the trunk lid/tailgate symbol. The opening procedure is stopped by pressing the button once again.
It is possible to close the trunk lid/tailgate via the remote control only on vehicles with production date after 9/98 (see display in diagnosis system - coding data).
The button on the key-chain transmitter must be pressed and held in order to close the trunk lid/tailgate.
Tilt switches
A tilt sensor with two tilt switches is fitted in the tailgate for the purpose of detecting the opening angle.
The tilt switches are Hall sensors.
The following opening angles can be recognized:
Angle range in degrees
Tilt switch 1
Tilt switch 2
0o to 8o open
active
inactive
8o to 50o open
inactive
inactive
50o open up to stop
inactive
active
The signal output of the tilt switches can be checked in the diagnostic program (test module tilt switches or diagnosis requests).
Influence of tilt on operation
The direction of movement of the trunk lid/tailgate (open or close) after pressing a button in the trunk lid/tailgate (inside or outside) is controlled dependent on the tilt of the trunk lid/tailgate.
As from an opening angle of greater than 8o the direction of movement changes if the trunk lid/tailgate movement is stopped by pressing a pushbutton and then reactivated by pressing the button again.
Hydraulic system
Components
The hydraulic system consists of the following components:
Hydraulic unit
contains the hydraulic pump, relay valve (on E38 Series only) and the pressure relief valves
Hydraulic lines
Connections between the hydraulic unit and hydraulic cylinders
Hydraulic cylinders
Mounted on left and right of trunk lid/tailgate
The system is operated with a maximum pressure of 100 bar.
The hydraulic system is maintenance-free and self-venting.
Note
The procedures described in the repair instructions must be followed when working on the hydraulic system.
Hydraulic pump
The hydraulic pump is controlled by the trunk lid/tailgate module in the directions of rotation "open trunk lid/tailgate" and "close trunk lid/tailgate".
Power is supplied by means of two relays integrated in the trunk lid/tailgate module.
A repeat lock facility prevents the hydraulic pump overheating. The pump is switched off for approx. 6 minutes if it was in operation for longer than 2 minutes within a short space of time.
Relay valve
The relay valve controls the hydraulic pressure between the hydraulic pump and hydraulic cylinders. It is powered and thus opened and closed by the trunk lid/tailgate module.
The maximum possible hydraulic pressure is built up in the hydraulic cylinders when power is applied to the relay valve (valve closed).
Only a low pressure is built up in the hydraulic cylinders when no power is applied to the relay valve (valve open).
The relay valve is closed over the entire opening range when the trunk lid/tailgate is opened. The trunk lid/tailgate is opened with the maximum possible hydraulic pressure.
On E38 vehicles, when the trunk lid/tailgate is closed, the relay valve and the hydraulic pump are controlled by the trunk lid/tailgate module dependent on the opening angle of the trunk lid/tailgate.
Trunk lid open up to 50o:
The hydraulic pump operates in the "close trunk lid/tailgate" direction of rotation.
Power is applied to the relay valve (valve closed).
The trunk lid/tailgate is closed with the maximum possible hydraulic pressure.
Trunk lid open 50o to 8o :
The hydraulic pump operates in the "close trunk lid/tailgate" direction of rotation.
Power is not applied to the relay valve (valve open).
Only a low pressure is built up in the hydraulic cylinders. The trunk lid/tailgate is closed without force.
Trunk lid open less than 8o:
The hydraulic pump is witched off.
Power is not applied to the relay valve (valve open).
The trunk lid/tailgate falls closed under its own weight.
The opening angle of the trunk lid/tailgate is recognized by means of the tilt switches.
Note on E39 Series
The hydraulic unit of the E39 Series does not contain a relay valve. When closing the tailgate, the hydraulic pump continues to operate until the tailgate contact signals that the tailgate is closed.
Pressure relief valves
The pressure relief valves are integrated in the hydraulic unit and limit the maximum possible pressure in the hydraulic cylinders.
The pressure is limited to maximum 100 bar to open the trunk lid/tailgate and to maximum 60 bar to close the trunk lid/tailgate.
Emergency mode
In the event of system failure, the trunk lid/tailgate can be opened and closed manually by exerting little pressure.
An emergency valve is additionally integrated in the hydraulic unit which, in the event of system failure, maintains a system pressure of 40 bar thus ensuring the trunk lid/tailgate movement is dampened.