Sensors and adaptations

James Dillon looks at sensor faults and diagnostic conundrums

Published:  19 October, 2015

It's interesting to consider the developments that have been made in the petrol engine management system, concerning its strategies, control and data stream. In the 'good old days', an open circuit sensor, such as the coolant temperature sensor, would provide default data of -40ºC (and around 5 volts on the sensing wire).

The engine management system would use this default value and calculate a very rich mixture (excess fuel to suit a very cold temperature). This is despite only a few moments prior to going open circuit, and in the same drive cycle, the sensor had been measuring a coolant temperature of 99ºC.

The drop in temperature in such a short period of time is physically impossible and, therefore, illogical. In addition to this, other temperature sensor readings from around the engine bay, such as Intake Air Temperature (which on a hot engine may be reading 55ºC), would highlight the illogical nature of the Coolant Temperature Reading.

Low-spec

Sensor development

Under these circumstances, the computer can substitute internally the 'wrong' coolant temperature reading with a substitute value. The system will set a diagnostic trouble code, as the physical sensor voltage input is above the maximum threshold (below), and at 5.0 volts, is operating in the defective sensor (or wiring) region.

Confusing calculations

Hopefully, the technician will recognise the lack of correlation using the just the scan tool data stream, identifying a difference between the sensor data and the sensor voltage.

Power increase

Modern engine control units have another feature that has been made possible by the increase in processing power, programming and memory. It is known as adaption. A popular and well-known adaption feature is the diesel fuel quantity adjustment, which is also known as smooth running control (SRC). This is a cylinder, by cylinder adaptive value, which is used by the engine at idle and fast idle speeds to equalise the engine speed. Many technicians use these values to diagnose fault diesel injectors. Understanding the detail behind adaptive values (which is based on sensor inputs) can help prevent misdiagnosis.

Skewed data

This data is a good hint at what may be occurring within the fuel system, but it should be acknowledged that a range of factors could cause smooth running data to be skewed. These data stream values alone are not good enough to condemn any single part, without secondary verification tests being performed. Factors affecting the rotation of the crankshaft can affect the adaption data, and so the cause of any anomalies could be one of the following; dual mass flywheel, front pulley (vibration damper), bent con rod, poor sealing valves, piston rings, injectors, injector control, worn camshaft, jammed EGR valve (rough idle), etc.

It can be seen that the scan tool live data stream can provide many clues as to what may or may not be wrong with the system under test. Scan tool data should, WITHOUT EXCEPTION; be critically evaluated by a skilled technician in search of the root cause. Remembering that the scan tool provides data, and that the technician provides the diagnostics. Take responsibility, boss your scan tool, use your brain, and be more successful.

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