Range Rover P38
EMISSION CONTROL
15
DESCRIPTION AND OPERATION
Secondary air injection system - operation
When the engine is started, the engine control module
(ECM) checks the engine coolant temperature and if it
is below 55
°
C (131
°
F), the ECM grounds the electrical
connection to the coil of the SAI pump relay.
The Main and Secondary Air Injection (SAI) pump
relays are located in the engine compartment fusebox.
A 12V battery supply is fed to the contacts of the SAI
pump relay via fuselink 2. When the ECM completes
the earth path, the coil energises and closes the
contacts of the SAI pump relay to supply 12V to the
SAI pump. The SAI pump starts to operate, and will
continue to do so until the ECM switches off the earth
connection to the coil of the SAI pump relay.
An earth connection from the Main relay coil is
connected to the ECM. When the ECM completes the
earth path, the coil energises and closes the contacts
of the Main relay. When the contacts of the Main relay
are closed, a 12V battery supply is fed to the SAI
vacuum solenoid valve via fuse 26 in the engine
compartment fusebox. The ECM grounds the
electrical connection to the SAI vacuum solenoid
valve at the same time as it switches on the SAI pump
motor.
The SAI pump remains operational for a period
determined by the ECM and depends on the starting
temperature of the engine, or for a maximum
operation period determined by the ECM if the target
engine coolant temperature has not been reached in
the usual time.
When the SAI vacuum solenoid valve is energised, a
vacuum is provided to the operation control ports on
both of the vacuum operated SAI control valves at the
exhaust manifolds. The control vacuum is sourced
from the intake manifold depression and routed to the
SAI control valves via a vacuum reservoir and the SAI
vacuum solenoid valve.
The vacuum reservoir is included in the vacuum
supply circuit to prevent vacuum fluctuations caused
by changes in the intake manifold depression affecting
the smooth operation of the SAI control valves.
When a vacuum is applied to the control ports of the
SAI control valves, the valves open to allow
pressurised air from the SAI pump to pass through to
the exhaust ports in the cylinder heads for
combustion.
When the ECM has determined that the SAI pump
has operated for the desired duration, it switches off
the earth paths to the SAI pump relay and the SAI
vacuum solenoid valve. With the SAI vacuum solenoid
valve de-energised, the valve closes, cutting off the
vacuum supply to the SAI control valves. The SAI
control valves close immediately and completely to
prevent any further pressurised air from the SAI pump
entering the exhaust manifolds.
The engine coolant temperature sensor incurs a time
lag in respect of detecting a change in temperature
and the SAI pump automatically enters a ’soak period’
between operations to prevent the SAI pump
overheating. The ECM also compares the switch off
and start up temperatures, to determine whether it is
necessary to operate the SAI pump. This prevents the
pump running repeatedly and overheating on repeat
starts.
Other factors which may prevent or stop SAI pump
operation include the prevailing engine speed/load
conditions.
