1956 temp. gauge not working diagnoses

[woodeye]

I'm hoping if I list the circumstances of how my temp gauge is working you might be able to help me determine where the problem lies.
! With the key off my gauge reads 3/4 the way to hot
2 With key on gauge still reads 3/4 to hot.
3 With key on and wire disconnected from sending unit it still reads 3/4 to hot.
4 With key on and wire grounded to motor gauge goes all the way past cold. If I turn key off with wire still grounded gauge stays below cold.
5 If I turn key off and un-ground wire gauge goes to 3/4 way to hot.
6 Key on and wire on gauge it reads 3/4 hot.

With key off should gauge read hot or cold or somewhere in between? Where it was when key was turned off?
Sending unit looks relatively new, clean.
Does it sound like a faulty sending unit. Is there a way of checking if the sending unit is working using hot water and a tester. (OHM meter?).
Thanks

 

[74 Harley]

Yes you can test the sending unit with hot water and an ohm meter.
I would suspect a faulty sending unit, or one not calibrated for your car.

 

[Boozeman]

Yes you can test the sending unit with hot water and an ohm meter.
I would suspect a faulty sending unit, or one not calibrated for your car.

On my '57, when the key is off, the needle in my temp. gauge swings all the way to the left.

 

[jack-in-sac]

Just as a matter of course; when did this start happening? Have you made any changes that could affect the reading, like installing a new sender?
New senders are not particularly expensive. If you have the need to install a new sender then do not forget that the sender is to be installed "dry". By that I mean no teflon tape or other item that would insulate the sender from the cylinder head. It does appear your problem may be a poor ground along the way. One other item; how does your fuel gauge read? If it is also mis-behaving the regulator on the back of the instrument panel may have an issue. both gauges power is supplied through this regulator as the gauges are not actually 12 volt gauges but regulated at ~5 volts.

good luck, check the ground for your gauge circuit.