Air Flow Meter & Potentiometer
The air flow meter measures the amount of air entering from air cleaner to the throttle housing. The air flow meter is basically a tunnel with a similarly shaped measuring flap and dampening flap (offset 90° on same casting). The measuring flap swings on an axis in air stream against reverse pressure of a spiral spring. The measuring flap is linked to a variable potentiometer which regulates the voltage sent to ECU, depending upon angular displacement of flap as it opens and closes in relation to air flow. See Fig 1.
In addition to monitoring air flow, the air flow meter also controls the fuel pump (except Datsun models) and idling. Within the potentiometer is an electrical contact for fuel pump relay. With engine stopped, no air flow is present, fuel pump contact is open and fuel pump is inoperable. While engine is being started, fuel pump is supplied power from starter relay via the combination relay. Once engine starts, fuel pump contact closes and fuel pump operation is controlled by open position of air flow measuring flap. If flap closes, fuel pump contacts open and power is cut to fuel pump. At idle, the measuring flap is almost closed due to spiral spring pressure. An idle air by-pass channel receives air from main air flow through a small hole, the size of which is controlled by the idle speed adjusting screw. The idle speed adjusting screw also controls quantity of air and exhaust emissions during idle speeds.