I have a 1970 and am having trouble with the compressor. The clutch will not engage when I turn on the A/C control on the shifter console.

I have used a voltmeter and determined that I am getting 12V to the compressor when the A/C switch is on. I can watch the needle rise and fall when I cycle the switch on and off. So I know I am getting juice to the compressor. The puzzle is that I can remove the connector to the compressor, hook up two direct leads from a spare battery directly to the compressor, and the clutch will come on.

Why will the spare battery cause the clutch to come on but the 12V that I am getting through the wires will not??

It must be a fundamental electrical property that is beyond my knowledge.

Help!

Your low pressure switch could be bad or you are low on freon is one reason the compressor won't engage. Jump the low pressure switch & if it works it's one of the above.
Good luck

Do you know where the low pressure switch is? DOes the 1970 Vette have one?

All auto A/C systems have a low pressure cutoff. Some are in form of a temp switch. With the relation of freon and temp, it works well. But if the system is empty the temp won't operate correctly.
The low pressure cutoff can be located near the evaporator, along the low pressure line, or in the back of the compressor.