Help! I got me a 73 240z. I did not have problem starting the car before but recently I when I start the car I only hear a click from the relay.Changed the ignition switch but no improvement. Clean the wires connected to the solenoid no change. But when I jump the solenoid the car starts. Any ideas?see5E94E238-BAAB-4D40-B43E-60EF75304AF1.MOV video.  any help is appreciated. 

It's either a faulty solenoid or a voltage drop in the ignition circuit. I had the same problem and found I only had 9 volts to the solenoid. To solve the problem I wired in a relay. The relay actuates with the 9 volts then switches on 12 volts from the battery to the solenoid. If you have full voltage to the solenoid then the solenoid is probably the culprit.