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Anyone know how to test a Temp sending unit without taking it off?
If the fuel gauge reads correctly then you can assume the instrument regulator is OK and the problem is with the temp gauge or sender. If both gauges are off then the regulator is suspect. Reading too low usually indicates the regulator voltage out is low and is not at the specified 5v. Reading too high and the regulator is not cutting the voltage at all and is putting out 6 or more volts. 6v car gauges are somewhat forgiving of higher voltage but on a 12v car usually the gauge heaters will burn open if they see the full 12v longer than a few seconds.
There is no really good way to test the temp sender in the car but if the engine is cold, disconnect the wire and measure the resistance thru the sender to ground. To have the needle on C the sender resistance needs to be somewhere around 75 ohms.
Testing the response of the sender is the hard part on the car. To have the needle on H the resistance needs to be around 10 ohms but to get close to that point either the engine has to be quite hot or the sender needs to be in some water that is at the boiling point. I don't know exactly there the top end of the Packard sender was but frequently 220 or 240 degrees is where the gauge is supposed to have reached the Hot indication.
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