Weber/Marelli Engine Management as used in four cylinder Lancia cars.
Notes compiled by Leo A. Capaldi, the Lancia fanatic from Edinburgh, Scotland.
The system is computer controlled, multi point sequential fuel injection and integral spark timing. The main bits are:
Computer/Control Unit. This is an 8 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 four lugs 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. Impedance 700 ohms.
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. Impedances: A-B 1.5K ohms; A-C 5.9K ohms; B-C 4.3K ohms.
Manifold Air Temperature sensor (brown). The mixing of petrol and air varies at different temperatures, so this sensor allows adjustment for ambient temperature. Impedance 7K ohms.
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. Impedance 700 ohms.
Water Temperature sensor (blue). Allows adjustment for cold start and cold running. Impedance 7K ohms
Throttle By-Pass Valve. This is a solenoid operated valve which allows an alternative air flow path when the throttle is closed. The computer opens and closes this valve to keep the engine running at idle speeds. Effectively it is a computer controlled throttle. Impedance 6.5 ohms.
Fuel Injectors. The injectors are mounted at the end of the inlet manifold feeds into the cylinder head. They are individually driven in synchronisation 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. Impedance 2.5 ohms.
Throttle Position Potentiometer. Mounted on the end of the throttle spindle, this is used to indicate the need for extra fuel when accelerating and to cut the fuel supply when the engine is in overrun. Impedances: A-C 600 ohms; A-B 45 ohms.
Petrol Pump. Electrically driven, situated near the tank, this supplies petrol under pressure as long as the crankshaft is turning. The pump is switched off after a few seconds if the crankshaft stops turning. The petrol is fed via a filter to pipe (rail) which is the common feed to all injectors.
Petrol Pressure Regulator. A mechanical overflow device is used to maintain the petrol supply pressure at approximately 35 psi. Excess petrol is returned to the tank. The regulator is at the "far end" of the pipe which the injectors are fed from.
Anti-knock sensor. Engines which have turbocharged induction have a sonic sensor fitted into the cylinder head which detects "pinking". When "pinking" is detected the engine parameters are adjusted to compensate.
Boost Control/Overboost Valve. Turbocharged 8 Valve engines have a mechanical pressure regulator which controls the turbo pressure. The Overboost Valve overrides the mechanical device and allows a higher pressure. The computer permits this overboost situation for brief periods when the throttle is wide open. With 16 valve engines this valve controls all the turbocharger pressure regulation.
Ignition Amplifier and Coil. An integrated unit which is driven by pulses from the computer and generates a spark in the normal way. For 8 valve and early 16 valve engines the spark is then fed to the plugs by a conventional distributor. Later 16 valve engines there are 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 is controlled by the computer and supplies power to the petrol pump, idle by-pass valve and overboost 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 (oh, to have one of those!).
Diagnostic Information (8 valve engines only)
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 (eg 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).
No Sparks. The operation of the amplifier and coil can be checked by unplugging the computer connector and shorting between connectors 24 and 25. With the ignition switch on this will generate sparks from the coil.
No Fuel. At the feed end of the pipe which feeds the injectors is a joint which can be undone. Petrol under pressure should leak from here if the pump is running. The pump can be forced to run by shorting the connections in its relay socket (with ignition on).
Non Starting. Check the sensor impedances as above.