The purpose of this test is to evaluate the correct operation of a Coolant Temperature Sensor (CTS) responding to a change in coolant temperature during the engine warming phase.
You can see in the Example Waveform that the sensor is a Negative Temperature Coefficient type (NTC), the voltage falls as the coolant temperature increases.
At a point relative to around 40 to 50°C the engine control unit increases the sensor supply voltage. The voltage then continues to fall as coolant temperature rises. The manufacturer states that this gives a finer control.
The Coolant Temperature Sensor (CTS) is a small two pin sensor whose job is to report the engine temperature back to the ECM. It is this signal that determines the engine warm up, enrichment and fast idle duration.
NTC sensors are predominantly made from a sintered semi-conductor material which will decrease electrical resistance in reaction to temperature rise. When installed into an engine cooling system it will provide an accurate voltage change as the coolant heats up.
The ECM is therefore provided with a signal which will allow accurate control throughout the operating temperature range.
Coolant temperature sensors are manufacturer specific and although the units may look identical the outputs vary dramatically.
All temperature sensors are resistance sensitive. Which means they require a clean circuit. Any poor/corroded connections will introduce an extra resistance in series and will falsify the readings that the ECM sees. Even though the sensor input may be within ECM parameters, not recording any faults, the engine will not operate as designed.
The General Motors Simtec Sensor on this vehicle has a different voltage characteristic to that on the majority of vehicle systems.
The GM/Vauxhall/Opel Vectra 1.6 litre engine uses this sensor and therefore has a distinctive waveform when viewed on the oscilloscope. The voltage displays a conventional drop until the engine reaches 40° to 50°C, at which point the voltage rises suddenly due to switching inside the ECM.
The manufacturer states that this gives a finer control.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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