[hatzie]

You need to test it with a meter.
It could be the switch or valve or the wiring.

Power to the valve runs from the fuse panel to the switch and from there to the single terminal on the valve.

Before loading the parts cannon I'd check the connections for corrosion or loose terminals at the valve, switch, and fuse panel in that order.
I'd also make darn sure the valve is getting a good ground at the mounting bolts. If the frame is crusty it might not be making good contact. Set your digital volt meter for DC Volts in the 20V range.
Test the voltage directly on the battery posts then move the ground to the engine and the frame to give yourself a real world look at the battery cable condition. If the engine or frame show more than a .3VDC difference from direct to the battery you probably need to clean up your cables or replace them but that's up to you.
Climb under and clip the positive meter lead to the Starter BAT stud and probe the metal body of the valve for voltage using the negative meter probe. Assuming the battery cables are in good shape it should read darn close to full battery voltage... or at least the reading you got between the battery positive and the frame... within .2 to .3 VDC. If it's more than .3vdc lower clean up around the mounting bolts and add star washers next to the frame and the valve when you re-install the bolts.

The switch is fairly easy to test.
Switch the ignition on.
Ground the meter Ground probe.
Test the Red wire at the switch for volts. It should be close to battery voltage.
Flip the switch to the AUX position and check voltage at the Green wire. Lower voltage or no voltage means the switch is not in good shape.

You also should check the molded connector at the valve for the teltale green puss from corrosion. If the stud on the valve is a #10 machine screw you can clean the molded connector out with a .17 calibre bronze bore brush. If it's a #8 machine screw you'll need to find other means of clearing the corrosion. Crunching a bore brush down slightly by forcing it through a #8 nut on a drill would probably do it. Clean the connector by hand... DO NOT clean the connector with the brush in a drill.