Coolant Temperature Sensor Troubleshooting

The Coolant Temperature Sensor (CTS) is fitted to the underside of the inlet manifold, this sensor reads the temperature of the coolant.
Normal operating temperature is usually from 80C to 100C and is a critical value in the calculation of ignition timing and duration. Faults with this sensor can manifest themselves as issues that may not appear to be directly attributed to coolant.

How the Coolant Temperature Sensor works

As the coolant heats up the resistance of the CTS starts to drop. Since the CTS is a negative temperature coefficient (NTC) thermistor it is highly unlikely that it would fail with a short circuit but more likely to open circuit or start to read high over time. For example, at say 80°C instead of reporting 200 ohms it may read 600-1000 ohms. This will cause the ECU to extend the injector timing.

As a guide, the approximate resistance at a given temperature should be as follows

0°C 6K
20°C 2.4K – 2.6K
40°C 1.1K – 1.3K
60°C 500 Ohms
80°C 300 Ohms
100°C 100 Ohms

These values may vary based on the condition of the connectors and wiring in the loom.

Fault Diagnosis

The CTS is an expensive sensor and difficult to obtain access for replacement, after checking the temperature gauge, it is worth changing the Thermostat, which is cheap and easy to replace if the engine isn't reaching operating temperature. An 88C should be fitted to the SPi mini. It should be noted, that the ECU determines correct operating temperature has been reached once the coolant temperature is 80°C or above.

The CTS is a 2 wire system, the pink / green wire goes direct to ECU pin 33 and the pink / black wire goes pin 30. The pink / black wire is a common with other sensors, including the fault diagnostic connector pin 3. Because of this, it is possible that issues with other sensors could have an impact on the resistance measurements leading to a chasing problems and poor performance such as misfiring, unstable idle speed, poor fuelling etc.

When diagnosing a suspected issue with the CTS, first check that the temperature gauge is working correctly by activating it in the Web Memsfcr application when the engine is not running. Then run the engine up to operating temperature and check that the temperature measures approximately mid-way on the gauge.

Assuming that the thermostat is working and the coolant readings are not as expected, then the resistance of the sensor can be measured to determine if the sensor is faulty.