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EFI-Terminoly
Engine Management System Terminology/Description.
Electronic Control
Unit (ECU). There are an 8, 16 or 32 bit
microprocessor which has a predetermined memory map which is configured to match
the engine in terms of valve timing and capacity etc. There are five main input
sensors which allow the fuel feed and ignition timing to be adjusted to match
engine load and cold starting.
Crankshaft/RPM sensor. This is mounted at the crankshaft pulley and
senses 24-2, 32-2, 60-2 or 4 teeth fly wheel on the pulley. The resultant pulses
indicate the Top Dead Centre times for the four pistons and allow the computer
to know how quickly the crankshaft is turning.
Manifold Air Pressure sensor. The air flow into the cylinders is
dependant on how hard the engine is working and so the air pressure (vacuum) in
the manifold is an indication of how much fuel (air and petrol mixture) is
required. Also the ignition timing is adjusted to suit
Manifold Air Temperature sensor. The mixing of petrol and air varies at
different temperatures, so this sensor allows adjustment for ambient
temperature.
Camshaft Position sensor. Mounted within the distributor body this lets
the computer know when piston number one is at top dead centre of the firing
stroke
Water Temperature sensor. Allows adjustment for cold start and cold
running.
Stepper Motor. This is a solenoid operated which allows an alternative
air flow path when the throttle is closed. The computer opens and closes this
throttle to keep the engine running at idle speeds.
Fuel Injectors. The injectors are mounted at the end of the inlet
manifold feeds into the cylinder head. They are individually driven in
synchronization with the valve timing as determined by the predetermined
mapping. The injectors are solenoid valves which open to allow a spray of petrol
into the inlet ports, to mix with the air which is drawn in by the intake
stroke.
Throttle Position
Sensor (TPS). Mounted on the end of the throttle
spindle, the ECU (Electronic Control Unit) reads this value to determine the
need for extra fuel when accelerating and to cut the fuel supply when the engine
is in overrun.
Fuel Pump. Electrically driven, situated near the tank, this supplies
fuel under pressure as long as the crankshaft is turning. The pump is switched
off after a few seconds if the crankshaft stops turning. The fuel is fed via a
filter to pipe (rail) which is the common feed to all injectors.
Ignition Amplifier and Coil. An integrated unit which is driven by pulses
from the computer and generates a spark in the normal way. There are one or two
amplifier/coil units and each one feeds two cylinders simultaneously (a spark
occurs in the exhaust stroke but does not do any harm).
Electrical Supply Relays. The fuel injection system has a separate wiring
loom with just one connector to the rest of the car wiring plus a fused
connection to the battery and earths. Two relays are used, the first is
controlled by a feed from the ignition switch and supplies power to the
computer, injectors, and the second relay. The second relay is controlled by the
computer and supplies power to the petrol pump, idle by-pass valve and over
boost valve (if fitted). The ignition coil and amplifier are supplied by a
separate feed from the ignition switch. Another, 3 way connector exists (beside
the computer) for connection of the W/MM diagnostic computer.
Diagnostic Information
As with any internal combustion engine, smooth running is dependant upon the
three basics, fuel supply, fuel compression and fuel ignition. The Weber/Magneti
Marelli system controls the supply and ignition. The comments below are intended
as a guide to checking the "electronics" but cannot cover all situations (e.g.
timing belt broken, or piston holed).
Poor engine
performance. The system will function without the
water temperature sensor and the idle by-pass valve, but the engine will not run
cleanly. If one injector is not operating it can be traced by removing spark
plug leads one by one to note the difference in idling (this presupposes that a
spark is being generated at all plugs and that there is sufficient compression
in each cylinder).
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