Freelander System Description and Operation
ENGINE MANAGEMENT SYSTEM - SIEMENS
18-4-50 DESCRIPTION AND OPERATION
Suspend
When the RES switch is pressed, the interface ECU outputs the OFF MFL message to the ECM. On receipt of the
OFF MFL message, and when automatically suspending cruise control, the ECM reverts to normal fuelling control
and changes the cruise control message on the CAN bus to inactive. The set speed is retained in memory by the
ECM. On receipt of the cruise control inactive CAN bus message, the instrument pack extinguishes the cruise control
warning lamp and the interface ECU switches off the cruise control engaged signal to the EAT ECU. The EAT ECU
then returns to its previous operating mode.
Resume
While cruise control is suspended, when the RES switch is pressed the interface ECU outputs the MFL RESUME
message to the ECM. Provided the vehicle is in the correct driving configuration, on receipt of the RESUME MFL
message the ECM engages cruise control at the previous set speed and changes the CAN bus cruise control
message back to active. The instrument pack then illuminates the cruise control warning lamp again and the interface
ECU outputs the cruise engaged signal to switch the EAT ECU back to the cruise control mode.
Cancelling
Cruise control is cancelled by pressing the master switch. When cruise control is cancelled, the LED in the master
switch extinguishes and the power feed to the interface ECU is disconnected to disable the system. If cruise control
is engaged when the master switch is pressed, the interface ECU and the ECM respond in the same way as when
cruise control is suspended, except that the interface ECU no longer reacts to inputs from the steering wheel switches.
During the same ignition cycle, if the master switch is pressed again, the interface ECU is re-enabled. Since the output
of the RESUME MFL message is inhibited until after the first engagement of cruise control, the interface ECU will not
react to an input from the RES switch, and the set speed in the ECM memory is effectively lost to the system. Cruise
control only re-engages if the SET+ switch is pressed, when, provided the vehicle is in the correct driving
configuration, the ECM adopts the current vehicle speed as the new set speed.
Diagnostics
The ECM monitors the MFL serial link to check for faults with the interface ECU. The MFL signal contains a toggle bit
which the interface ECU changes to a different state every second to indicate that the interface ECU is operating
correctly and receiving a valid CAN bus signal. If a fault occurs with the CAN bus signal or the interface ECU, the
toggle bit remains unchanged and the ECM interprets the unchanged toggle bit as a fault. If the interface ECU fails to
output the MFL signal, the ECM also interprets the lack of a signal as a fault.
If a fault is detected, the ECM disables cruise control for the remainder of the ignition cycle and stores a related fault
code in memory. The fault codes can be accessed using TestBook/T4, which communicates with the ECM via an ISO
9141-2 K line from the diagnostic socket.
The ECM monitors the two inputs from the brake pedal position sensor and disables cruise control if a fault is detected.
The ECM can detect open circuits and implausible inputs. However, simultaneous short circuits to 0 volt in both inputs
cannot be detected and, if this occurs, cruise control operates but does not suspend operation when the brake pedal
is pressed.
The ECM resets the cruise control system at the beginning of each ignition cycle and operates normally if a previously
detected fault is no longer present.
