Engine coolant temperature sensor - Simtec (GM) type

The purpose of this test is to evaluate the correct operation of an Electronic Coolant Temperature (ECT) sensor responding to a change in coolant temperature during the engine warming phase. 

How to perform the test

View connection guidance notes.

1. Connect PicoScope Channel A. to the temperature sensor 5 V supply circuit.
2. A good earth is required connecting to the battery negative terminal will check out the earth circuit.
3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
4. Start the scope to see live data.
5. Start the engine.
6. Observe the waveform as the engine coolant warms up.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

The sensor is a Negative Temperature Coefficient type (NTC), where the output voltage falls as the coolant temperature increases.

At a voltage in the range 40-50°C the Engine Control Module (ECM) increases the sensor supply voltage. The voltage then continues to fall as coolant temperature rises.

Further guidance

The ECT is a small two pin sensor its function is to provide a voltage relative to coolant temperature back to the ECM. It is this signal that determines the engine warm up, enrichment and fast idle duration.

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 manufacturer states that this gives a finer control.

GT016-EN