One of the members reported a problem with the coolant temperature gauge which moved to it's mid position as soon as the ignition was switched on, even when the engine was completely cold.

Removing the sensor connector made no difference to the display as it stayed in the mid position. He tried a new sensor on the wire without actually fitting it into the engine and still the temperature display remained in the centre position. As soon as the ignition was switched off the needle dropped to the bottom position.

Other members reported similar faults which in at least one occasion had led to major engine failure due to overheating. The problem was that the gauge was not reporting either cold or hot conditions.

Tim then offered the following useful information.

"There are two temperature sensors mounted on the thermostat housing, one for temperature gauge and one used for PCM reporting.

The PCM sender is of a sensible value ranging from 200k cold to 2k ohms (normal hot engine) and does not seem to play up much. (this one starts fans / controls fuelling etc via the PCM) The gauge sender is around 100 ohms (normal hot engine) and is required to go practically short circuit for an overheat condition. This sender only operates the temperature gauge but its analogue output is digitised in the instrument cluster for obd reporting. (this is the temperature signal available in the instrument reset mode).

This sensor has a habit, after a few years, of going intermittent or high in value giving rise to low readings on the gauge. To test the gauge remove the plug from the sensor and push a 100 ohm resistor into the plug. This should give a normal mid scale reading on the gauge. A wire link inserted in the plug should give full scale hot. Obviously left disconnected the gauge should remain at minimum."

However the problem described was not so much sensor failure but a fault in the Instrument Cluster which can be diagnosed by taking the plug off the temperature sensor and checking the temperature display in the instrument reset mode.

Roger had found this problem earlier and offered the solution along with the photos below :-

"There are two solder joints on the circuit board that are very close and if the solder is a bit on the large side the two 'blobs' make contact. You may find it happens more in winter or cold spells when the board contracts. One member had reported that when the lights were on the gauge behaved itself - believe it or not that was due to the dash panel bulbs warming the circuit board so that it expands. Unbelievable but true!

The fix is to remove some of the solder from the joints. but the dash is a bit of a pig to get out."

For advice on dash removal see here