I have trouble getting the gauge to work correctly. Could you point me out where to connect what wires ?
The reason why is that the sender (1/8npt) might not be the correct resistance, is that possible?
Yes that is quite possible.
If things do not seem to be working as you expect the first thing to do is check your installation.
1.turn on your system and check that power is going to your gauge.
2. check that the negative is flowing freely back to the battery. A good negative is more important than a good positive to a gauge.
3. check that the signal wire is connected to the (correct) pole of your sensor.
For a temperature gauge in a cold system the gauge should be reading zero. If your gauge is reading any value disconnect the signal wire from the sensor. It should now be reading zero. If not your gauge is faulty.
4. disconnect the signal wire from the sensor and touch the wire on to the body of the sensor (not the pole the brass part of the sensor). The gauge should jump up to max temp. If this happened your gauge and signal wire are good if not:
4.1 touch the signal wire on the NEGATIVE pole of your battery. if the gauge jumps up to maximum you have a engine earthing problem to fix. If the gauge did not move then you have a signal wire problem to fix.
5 if the gauge and signal wire are working correctly then you have a sensor issue.
I hope that this helps.