You can use an ohm meter to check your sender. If it is a GM
sender, the following two senders are typical:

Standard Ignition # TS-71 ... 1/2" male pipe thread.
Replaces GM8993106, GM8993107, GM25037345.
Resistance at 100 degrees F. ... 1425 OHMS
Resistance at 220 degrees F. ... 147 OHMS
Used with 240 degrees F dash gauge.

Standard Ignition # TS-76 ... 3/8" male pipe thread.
Replaces GM8993146, GM8993164, GM25037346.
Resistance at 100 degrees F. ...1425 OHMS
Resistance at 250 degrees F. ... 147 OHMS
Used with 260 degrees F. dash gauge.

Notice that both have the same upper and lower resistance BUT the corresponding resistance is at different temperatures. This still gives you a quick check of the sender.

Since the gauge is going full scale at low temperature, the sensor is suspect. A broken wire can also take the resistance low. Since a temp gauge is really a volt meter, the lowered resistance is makes the gauge go full scale (less voltage drop).

If the sender reads OK, you can use a bypass wire to check the wiring.

mike in tucson