Step motor strategy

Figure 1 The step motor is characterized by high precision and resolution (about 20 revs). Pulses sent by the ECU to the engine are transformed from rotary motion into linear translation movements (about 0.04 mm/step) through a screw/worm screw type mechanism. This operates a plunger whose movements alter the cross section of the by-pass duct. The constant minimum air flow Qo is due to leakage under the throttle valve. This is set during manufacture and safeguarded by a protective plug. Maximum output Q₂ is ensured by the plunger in its position of maximum retraction (about 200 steps corresponding to 8 mm). Between these two values, the air flow follows the rules shown in the graph alongside.

Step motor strategy

The number of working steps is determined by engine conditions:

• Start-up stage
  When the key is inserted, the step motor acts upon a command from the ECU and takes up position according to coolant temperature and battery voltage.
• Warming up stage
  The number of revs is corrected mainly as a function of coolant temperature.
• Engine warm:
  Idle control depends on a signal from the engine rpm sensor. When external loads are activated, the ECU controls idle to restore it to the established rpm level.
• During over-run:
  The ECU recognises the over-run stage from the position of the throttle potentiometer.
  It controls step motor position through the idle output law ( DASH-POT law), i.e. slows the return of plunger (3) toward its seat. In this way, a quantity of air by-passing hole (2) reaches the engine and reduces the levels of pollutants in the exhaust gas.

C - ELECTRIC/ELECTRONIC CIRCUIT

Connects all injection/ignition system components and provides them with electricity. This consists mainly of an ECU and the following components.

• Double system supply relay
• Throttle valve position sensor
• Coolant temperature sensor
• Fuel pump submerged in tank
• Injector
• Absolute pressure sensor
• Intake air temperature sensor
• Engine idle speed actuator (step motor)
• Rpm and TDC sensor
• Two ignition coils
• Lambda probe
• Fuel vapour control solenoid
• Four spark plugs
• Inertia safety switch
• Diagnostic socket for Fiat/Lancia Tester