Coolant temperature sensor - GM/Vauxhall/Opel Simtec

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. 

How to perform the test

Plug a BNC test lead into Channel A on the PicoScope, place a large black clip on the test lead with the black moulding (negative) and a Back-pinning Probe onto the test lead with the coloured moulding (positive). Place the black clip onto the battery negative terminal and probe the coolant temperature sensor with the Back-pinning Probe.

The two connections are the voltage supply at about 5 volts and the earth return. It is the first of the two connections that needs to be made.

Alternative connections can be made using the TA012 two-pin test lead adapter. Connect as above, but without the Back-pinning Probe, and monitor both sides of the shrouded plugs to identify which wire is the sensor return, as illustrated in Figure 1.

Example waveform

Waveform notes

The Coolant Temperature Sensor (CTS) on this vehicle has a different voltage characteristic to that on an a conventional system. On the conventional CTS, the voltage falls continuously as the engine temperature rises. When cold the voltage is about 3 to 4 volts, and once normal operating temperature is achieved it is around 1 volt. The quoted voltages are, however, manufacturer-specific. Most temperature sensors have a Negative Temperature Coefficient (NTC) so their resistance, and therefore the voltage output, falls with increasing engine temperature. Positive Temperature Coefficient (PTC) sensors have a resistance and voltage output that rise with increasing temperature.

The CTS used in the Multec system on the GM/Vauxhall/Opel Vectra 1.6 litre engine has a distinctive waveform when viewed on the oscilloscope. The voltage seen at the CTS displays a conventional drop until the engine reaches 40 to 50 C, at which point the voltage rises suddenly due to switching inside the Electronic Control Module (ECM). This is caused by the ECM increasing the voltage at higher operating temperatures (above 50 C) so that it can offer finer control.

coolant temperature sensor

Figure 2 - A typical coolant temperature sensor

Technical information

The Coolant Temperature Sensor (CTS) is a small two-connection device whose job is to report the engine's temperature back to the Engine Control Module (ECM). It is this signal that determines the engine's warm-up enrichment and the engine's fast idle speed.

This sensor normally has a Negative Temperature Coefficient (NTC) which means that its resistance drops as the temperature increases. A Positive Temperature Coefficient (PTC) sensor is not as common as the NTC and its resistance reacts to temperature in the opposite way.

To increase the drivability and performance of pre-1992 non-cat cars, the resistance can be altered by inserting a resistor in series with the coolant temperature sensor.

However, this resistance has to be calculated before its insertion. This modification cannot be implemented on engines equipped with a catalytic converter as the extra fuelling would upset the corrective action of the lambda or oxygen sensor.

The sensors are manufacturer-specific and the outputs vary dramatically, although the units may look identical. Any poor connections on this circuit will introduce an extra resistance in series and will falsify the readings that the ECM sees. Reading the resistance at the ECM's multi-plug will confirm this.


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.

Suitable accessories

  • 2 Pin AMP connector breakout lead


  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • Multimeter Probes


  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


  • Premium Test Lead: BNC to 4 mm, 3 m


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


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Guided test: Vauxhall Simtec