I have a 69 rr with air. When you turn the a/c on, the tach goes dead. It has a new a/c switch and the only way to get the tach on is wiggle the off button on the a/c?

I had time to check a few more things sat. If you run a direct wire from the battery to the tach, turning the a/c on want affect the tach. Could the voltage drop cause the tach not to work? It's a factory tach.