My temp gauge goes all the way to hot when I connect the wire to the sending unit(new and supposed to be for rally gauges) when I disconnect, it falls down to the cold side. I tested the gauge itself with a size D battery checking with the power both ways to make sure the needle operated correctly. Before I go and buy another sender, is there some thing else I should be looking at?

68 442 4 speed convertible

 

Sounds like the sending unit you got is for a light & not a gauge there is a difference

 

Most sending unit works by presenting a resistance in the path of the gauges ground (ie ground = the frame/engine block/neg battery terminal). The sending unit is supposed to have a high resistance when cold and lower as it warms up. Less resistance = More current which means needle is pushed higher. The behavior you describe indicates that the resistance is already low, even though the engine is cold.

About the only thing I can think to try would be removing the sending unit from the socket, but leave the Gauge wire connected. Make sure its not touching anything metal on the car then use an extra wire to make the Ground connection. That would verifiy that something isn't physically shorted some where around the socket.

If you have a multimeter you can measure the resistance of the sending unit and heat it up with a lighter or a torch to see if the resistance changes. It should have a sizeable resistance when cold then start to drop as it heats up.

If the resistance lowers properly with the rising temp, you could "jerry-rig-it" to work by adding a resistor "inline" with the gauge wire that plugs into the unit which would make it read "cooler" for any given temperature. You would have to play with the value of the inline resistor and see what value will get the gauge to read in the middle when the engine is at optimum temperature. Not to mention be careful that the Current*Voltage (Watts) remains with in the resistors rated tolerances.

If neither test shows the unit to be functioning then its for sure the wrong sending unit or the sending unit has shorted out internally.

Another thought, If the temp gauge needle ever jumps when its not connected to the sending unit then there could be a short else where and I would start tracing wires looking for exposed copper or cracks in the insulation allowing copper to touch anything metal.


Good luck!