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Electronic / Engine Control Unit – ECU



First of all the ECU (Electronics Control Unit or sometimes referred to as Engine Control Unit, I prefer the latter, it makes more sense, all control units nowadays would be electronic in nature) is a sophisticated beast.  Reading up on ECU systems and failures on the many forums I was staggered to see the lack of knowledge and dangerous opinions and suggestions.  One person had it right though – if you don’t know, don’t touch, leave it to the professionals. Most avid DIY’ers have a multimeter but most do not know how to apply Ohm’s Law i.e. Current = Voltage / Resistance.  It’s no use ducking and diving under the bonnet not knowing what to search for – a multi-meter set to a current setting is going to cause untold damage (short-circuits) and on diode test sensitive electronic devices can be damaged due to the current generated through this test function.

The ECU is essentially a computer – it has inputs (e.g. mouse, keyboard etc) and it has numerous outputs (e.g. graphics, modem, printers etc).  The inputs are in the form of sensors and the outputs drive low impedance/resistance actuators.  The ECU can take it’s input from a potentiometer (variable resistor), thermistor (temperature sensitive resistor), magnetic pickups (possibly a pulse generator), voltage generator and switches (open and/or closed).  The variable instances of a potentiometer or voltage is analoguous whilst switching is digital, either a low or high. The throttle feedback mechanism in the engine management system is analoguous whilst the radiator fan is switched on by a sensor which kicks in or out at a predetermined temperature setting.  All the inputs in an ECU are processed which in turn monitors and controls fuel mixture and firing angle.

Mazda RX-7 Engine Control Unit
Engine Control Unit – Mazda RX-7

This diagram comes courtesy of Auto-Wiring-Diagram.com – in our endeavours to move images over from our previous CMS certain files were lost. In hindsite there are other great websites offering fantastic material – auto-wiring-diagram.com is one of them.

ECU Inputs of a fuel injected vehicle.

Engine Air Flow Sensing
Vane type air flow sensor (Vs) – situated in the air induction system between air filter and throttle. A potentiometer is set by the air flow which in turn has a variable voltage output.

Vane Air Flow Sensor
Vane Airflow Sensor – courtesy autopartsnetwork

Karman Vortex Air Flow Meter (Ks) – measures the frequency of vortices generated as the air flow increases, using a vortex generator, oscillating mirror and photocoupling device. Output is a low to high frequency digitised train. (constant voltage).

Hot Wire Mass Air Flow Meter – A very simple but effective way of measuring the mass of air flowing through the induction process is by putting resistance wire across the air inlet and applying a regulated voltage to it – thereby passing current through the wire. Air passing over the wire will cool it, causing current to increase.  Dense air is cooler as well which also causes an increase in current. Although simpler than the above two sensors they are easier to modify and are cheaper.

Hot wire mass air flow meter
Hot wire mass air flow meter – courtesy Autozone.com
  • Manifold Absolute Pressure Sensing (MAP) – this sensor is usually a piezzo device (bending a piezzo chip changes the resistance) with a vacuum pipe leading to the intake manifold.
    Engine speed and cranshaft angle sensing – along with the engine air flow sensing, the engine speed and crankshaft angle sensors determine angine load.  Commonly known as the Ne (number of engine revolutions per minute) and G1 (crank angle) signals, G1 determines the injection and ignition timing relative to Top Dead Center (TDC).  Both the G1 and Ne sensors work on magentic pickups, situated in the distributor housing.
    Ignitor pulse – the ECU feeds a regulated DC voltage to the spark ignitor circuit through a pull-up resistor, when ignition is sensed a negative going pulse is returned to the ECU – if these pulses are not sensed by the ECU the injectors are shut down preventing flooding and possible damage to the catalytic converter.
    Water Temperature Sensing (THW) – a thermistor, here a negative temperature coeffiecient sensor, (as temperature rises, resistance drops) in the block near the coolant return to the radiator. This sensor allows the ECU to determine fuel enrichment – cold starting requires a richer air/fuel mxture.
    Air Temperature Sensing (THA) – also a thermistor (also negative coefficient) situated normally in the air intake region or air flow metering system.  With air being more dense at lower temperatures the ECU requires an accurate assessment of inlet air temperture to process the correct air/fuel mixture.
    Throttle Angle – this is usually a linear function, showing various stage of throttle opening as well as when either full or at idle. The throttle angle sensor is connected to the throttle valve.
    Exhaust Oxygen Content – the oxygen sensor is located near the manifold collector / exhaust valves to allow for the best possible fuel/air mixes = 1:14.7 which is usually at about 400 degrees Celsius for optimal results (measured manifold temperature).  A 1:14.7 ratio is only applicable under normal running conditions and not when accelerating, loading, engine starting, cold temperatures.  The exhaust oxygen sensor is heated through a circuit driven by the ECU when temperatures are below the optimum 400 degrees Celsius to prevent erratic processing of fuel/air mixtures – i.e. low exhaust gas temperatures.
    Engine Cranking Signal – the ECU allows for a richer fuel/air mixture when it senses that the engine is being cranked. This is an electrical connection made to the ignition switch.
    Engine Knock Sensor – there are quite a few variants of the knock sensor,  the chief result required is to retard timing if and when timing is too advanced.  A typical sensor would be a piezoelectric device mounted on the cylinder head which picks up unusual detonation vibration.
    Altitude Sensing (HAC, High Altitude) – this is an atmospheric pressure sensor which reduces injector firing when at higher altitudes where air is thinner. This prevents mixture from running too rich.

High Altitude Sensor – Barometric Pressure
High Altitude Sensor – courtesy MechToday

Stop Light sense (STP) – when braking, fuel flow is minimised to cut-off or near cut-off.

Next:  Part Two – electronic ignition – 10th September 2011

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