Engine 850: B5234T/2.3/20v/T5: High temperature reading

[Derek UK]

I would expect the 850 to have a compensator circuit with gauge to stop it wandering up and down. When it is at a quarter past 3 it is in the safe zone but doesn't actually give true indication of the coolant temp. It only rises from there when the temp is much higher. It's possible that there is a weak electrical component or a poor solder joint/loose connection. Do they attach with a couple of captive screws?. Perhaps find another gauge or that part of the cluster? Robert DIY talks about it here but he doesn't mention the needle staying at 10 past 3.
https://www.youtube.com/watch?v=OZUvOOTkFv4

[Clan]

Originally Posted by Derek UK

I would expect the 850 to have a compensator circuit with gauge to stop it wandering up and down. When it is at a quarter past 3 it is in the safe zone but doesn't actually give true indication of the coolant temp. It only rises from there when the temp is much higher. It's possible that there is a weak electrical component or a poor solder joint/loose connection. Do they attach with a couple of captive screws?. Perhaps find another gauge or that part of the cluster? Robert DIY talks about it here but he doesn't mention the needle staying at 10 past 3.
https://www.youtube.com/watch?v=OZUvOOTkFv4

The gauge is a high precision moving coil instrument which reacts exactly to the resistance of the temperature sensor . first thing is to read out the actual coolant temperature from the ECU to see if appears in the right area around 90C .. next suspect the sensor in the thermostat housing , they have been known to fail . Best to measure its resistance when hot to compare to what it is suppose to be,

[Luxobarge]

Originally Posted by Clan

The gauge is a high precision moving coil instrument which reacts exactly to the resistance of the temperature sensor .

Not quite, the temperature sensor (varying resistance) is sensed by the ECU, which uses this information for a variety of purposes, one of which is to turn this information into a voltage which drives the temperature guage. There's no real direct feed from sensor to guage.

However, in essence the principle is the same, and your suggestion of using a scanner or similar to read the actual temperature that the ECU is seeing is a good one - I'd want to compare that with an independent measurement of coolant temperature though, to see if there's a difference. My bet will be that the actual true temperature is fine, and that the ECU is seeing (and therefore displaying) a high temperature due to a faulty sensor.

[Clan]

Originally Posted by Luxobarge

Not quite, the temperature sensor (varying resistance) is sensed by the ECU, which uses this information for a variety of purposes, one of which is to turn this information into a voltage which drives the temperature guage. There's no real direct feed from sensor to guage.

However, in essence the principle is the same, and your suggestion of using a scanner or similar to read the actual temperature that the ECU is seeing is a good one - I'd want to compare that with an independent measurement of coolant temperature though, to see if there's a difference. My bet will be that the actual true temperature is fine, and that the ECU is seeing (and therefore displaying) a high temperature due to a faulty sensor.

Yes probably the sensor as i said they are not 100% they have been known to fail I have changed about 5 or 6 when they cars were current .