Is there a way to measure the voltage coming from the sending unit to ensure that it is accurate?
Meaning if the gauge is reading 200ºF and sending out a 6.43 volt signal (just pulling these random numbers out of the air), is that the correct voltage output? Then how do you ensure your dash gauge (not idiot light) is accurate?
About the only thing I have seen is to ground the signal wire and ensure the gauge needle spikes at full temp reading. This is a SBC '81 Chevy truck btw.
The sensor does not send out voltage. It changes resistance as the the coolant temperature changes. These resistance changes vary the current flow through the gauge causing the needle to deflect. The hotter the coolant the lower the resistance the more current through the gauge.
A quick way to assess the accuracy of the dash gauge is to install a new thermostat in the engine. This will control the engine water temperature at say 180* or whatever rating is selected.
Remember that there can be differences in coolant temperature in various parts of an engine. So if the gauge sensor is near the thermostat, the comparison is a good indicator of gauge accuracy.
The above method assumes that the cooling system is working properly.
Another method is to remove the sensor and test it against another thermometer in pan of water. Heat up the water and compare the dash gauge against the thermometer.
Remember that the sensor will need to grounded to the car with a jumper.
It seems that using the GM units, lots of people find that their gauge is more accurate at ~180* on their Corvettes by adding a small value resistor (~20-30 ohm) is series with the sensor. Alternatively, you could use a relatively low value potentiometer and dial this in, assuming the unit has less than the the desired resistance.
Know that accuracy at one reading does not guarentee accuracy across the band.
The gm maunals I have say, for stock sensors, it should be:
1365 ohm @100F
55 ohm @260F
those ranges right there should tell you how non-linear the system is on these things
They dont mention anything about bias or offset, but like anything that is manufactured, they certainly will. Those numbers above are a good starting point for fine adjustment with resitors/reostats/potentometers (if it matters to you that much).
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