Daniel from WingsElectronics repairs ECUs, but has also put up a great explanation of the technical details of the ECU. If you have trouble repairing your own ECU, contact Daniel.
Admin: moved to the Technical Reference Library.
Just in case Daniel's awesome website goes away, here's a copy of the technical part:
"The controller is divided essentially into two distinct functions which share some inputs;
The glow timer portion looks at coolant temperature, ignition, cranking, alternator field, and glow plug temperature. The coolant temperature input reads an NTC thermistor installed in the cylinder head to check coolant temperature. Ignition, crank, and alternator signals come directly from their respective sources (ignition switch and alternator). To measure glow plug resistance, on application of ignition power, the timer outputs a calibrated current on the glow plug rail and measures the voltage drop. Derived from voltage and current is the plug's resistance and thus its deduced temperature.
Once the timer has learned of coolant and plug temperature it calculates the appropriate pre-heat time to achieve the desired plug temperature. During cranking and after startup the timer applies reduced power to the plugs for a calculated duration. The timer checks whether the engine is running by watching the alternator field output.
The ability to apply varying power levels to the plugs is achieved through the use of two relays. One relay connects full battery voltage to the plug rail while the other has a series resistor. 6V glow plugs are used such that pre-heat is accomplished very quickly (12V electrical system). There is the potential to burn out glow plugs if the full power relay gets stuck on for some reason but this rarely happens.
The EGR portion of the controller shares the coolant temperature sensor and ignition power input of the glow time. Its main inputs are engine speed and throttle lever position. A potentiometer on the throttle lever relays position information to the controller while a reluctor pickup in the diesel injection pump relays engine speed information. RPM and throttle are used to decide when and how much to open the EGR valve. To control the EGR valve, the controller operates two solenoid vacuum valves."
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