900 L4-1985cc 2.0L DOHC Turbo EFI (1985)
Air Flow Meter/Sensor: Description and Operation
Mass air flow sensor
The mass air flow sensor is fitted between the air filter and the intake pipe.
It consists of an aluminium body (plastic with effect- from 1989) enclosing an air duct. Centered inside the air duct is an inner tube incorporating a
platinum filament (hot-wire sensor). On both the inlet and outlet sides the hot wire is protected by a fine-mesh filter.
The temperature of the hot wire is maintained at a constant level of approximately 100°C (212°F) above the temperature of the inlet air, regardless of
the composition of the air or the air flow.
Since the hot wire is sited inside the inlet duct it can become coated with dirt which reduces its sensitivity and affects the results of measurement. To
keep the hot wire free from dirt, it is heated to a temperature of about 1000°C (1830°F) for one second. This takes place four seconds after the engine
has been switched off.
The main relay releases after filament burn-off.
The device for controlling the current required to maintain the hot wire at a constant temperature consists of a bridge circuit and a detector resistor, the
voltage variation across which is directly proportional to the magnitude of the inlet air mass.
The electrical components are integrated in the mass air flow sensor housing which also contains a potentiometer, accessible from the outside, for
adjusting the CO content at idling speed. This adjusting screw was discontinued on the LH 2.4 system with effect from 1989 since the function is
incorporated in the new LH control module. The plug which previously covered the hole for the CO adjusting screw has therefore also been
discontinued.
In the event of a break in the signal from the mass air flow sensor caused by a broken hot wire, for instance, an emergency system built into the control
module and known as the "Limp-Home" mode will take over, enabling the car to be driven but with impaired performance.
