BASIC PRINCIPLE OF FAULT DIAGNOSIS FOR ELECTRONIC
FUEL INJECTION SYSTEM
·RECORD OF MALFUNCTION INFORMATION
The electronic control unit consistently monitors the operations of sensors, actuators,
related circuits, malfunction alarm lights, voltage of battery and so on, even the
operation of the electronic control unit itself, as well as carries out the examination on
reliabilities of the signals output by the sensors, driving signals of actuators, and
internal signals (such as oxygen closed loop control, knock control, idle speed control,
battery voltage control and etc.). Once it is found that there is a malfunction in some
chain or some signal is not reliable, the electronic control unit will set the record of
malfunction information in the RAM of malfunction memory. The record of
malfunction information is stored as diagnostic trouble code and displayed in the
same order as the occurrence of the malfunctions.
Based on their frequency of occurrence, the malfunctions can be classified as “steady
state malfunction” and “random malfunction” (such as the malfunctions caused by
temporary circuit open of wiring harness or defective contact of connectors).
·MALFUNCTION STATUS
If the duration period of the identified malfunction exceeds its setting stabilizing time
at the first time, ECU will regard this malfunction as a stable malfunction and store it
in the memory of “steady state malfunctions”. If the malfunction disappears within its
setting stabilizing time, it will be stored as “random malfunction” or “non existence”.
If this malfunction is identified again, it will still be regarded as “random
malfunction”, but the “existence” of historic malfunction will not influence normal
operation of the engine.
·MALFUNCTION TYPES
Short to positive grid of power supply;
Short to ground;
Circuit open (if there is pull up resistance or pull down resistance in input stage, the
ECU will regard the malfunction of circuit open on input terminal as the malfunction
that the input terminal is short to power supply or to ground.);
Unreliable signals.
·FOUR TYPES OF MALFUNCTIONS
Maximum malfunction, the signal exceeds the upper limit of the rated range.
Minimum malfunction, the signal exceeds the lower limit of the rated range.
Signal malfunction, no signal.