Griff temp gauge inaccurate?
Discussion
he gauge is not calibrated so shows only an indication. To have a better understanding carry out this procedure: Remove the sender from the pipe/engine, reconnect the gauge wire and add a wire to earth the threaded part of the sender. Have an electric kettle (connected via a suitable extension lead) and place in the engine bay so the sender can be placed so the end is in the water. Turn the kettle on as well as the car ignition. When the kettle is boiling, note the position of the gauge needle. That will be very close to 100DegC, still within the 'normal' range for the engine as the pressurised system will be 15psi (1bar) and boils at 120degC. Of course normal running will be (just) above the thermostat opening temperature and higher with stationary traffic or hard (track day) running.
There are three separate and independent systems involved here.
Fans are controlled by a mechanical otter switch in the top hose which sees the temperature of the coolant allowed past the thermostat. This will keep working regardless of anything the other two systems do.
The ECU sees the coolant temperature via a thermister in the water jacket at the top of the engine on the 'hot' side of the thermostat. This controls engine enrichment during cold running.
The gauge is driven by a separate thermister in the water jacket at the top of the engine. This works on the same principle as the ECU sensor but has different electrical properties. The characteristic of these thermisters is that a higher temperature produces a lower resistance. If your gauge is showing a lower temperature it may be that the temperature has actually reduced ie the thermostat isn't closing fully at the correct temperature, or it could be that the resistance of the sender / circuit has increased or the regulated voltage has reduced. For example corrosion in an electrical connector or a mechanical fracture inside the thermister could increase the resistance.
The starting point for any investigation should be to find whether the gauge is displaying the coolant temperature accurately. You can do that by measuring the actual temperature of the coolant at the same place in the cooling system, or by putting the sensor in boiling water and seeing whether the gauge shows the correct temperature. That will tell you whether youre looking for qa cooling system problem or a sender / electrical problem.
Fans are controlled by a mechanical otter switch in the top hose which sees the temperature of the coolant allowed past the thermostat. This will keep working regardless of anything the other two systems do.
The ECU sees the coolant temperature via a thermister in the water jacket at the top of the engine on the 'hot' side of the thermostat. This controls engine enrichment during cold running.
The gauge is driven by a separate thermister in the water jacket at the top of the engine. This works on the same principle as the ECU sensor but has different electrical properties. The characteristic of these thermisters is that a higher temperature produces a lower resistance. If your gauge is showing a lower temperature it may be that the temperature has actually reduced ie the thermostat isn't closing fully at the correct temperature, or it could be that the resistance of the sender / circuit has increased or the regulated voltage has reduced. For example corrosion in an electrical connector or a mechanical fracture inside the thermister could increase the resistance.
The starting point for any investigation should be to find whether the gauge is displaying the coolant temperature accurately. You can do that by measuring the actual temperature of the coolant at the same place in the cooling system, or by putting the sensor in boiling water and seeing whether the gauge shows the correct temperature. That will tell you whether youre looking for qa cooling system problem or a sender / electrical problem.
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