Freelander System Description and Operation
ENGINE MANAGEMENT SYSTEM - SIEMENS
18-4-26 DESCRIPTION AND OPERATION
Knock Sensors
The knock sensors enable the ECM to operate the engine at the limits of ignition advance, for optimum efficiency,
without combustion knock damaging the engine. The ECM uses two knock sensors, one for each cylinder bank,
located between the cylinder banks on cylinders 3 and 4.
The knock sensors consist of piezo ceramic crystals that oscillate to create a voltage signal. During combustion knock,
the frequency of crystal oscillation increases, which alters the signal output to the ECM. The ECM compares the signal
to known signal profiles in its memory. If the onset of combustion knock is detected the ECM retards the ignition timing
for a number of cycles. When the combustion knock stops, the ignition timing is gradually advanced to the original
setting.
The knock sensor leads are of different lengths to prevent incorrect installation.
M19 2840