The credit wheel you can manually advance and reverse by pushing on the two coil plungers. There should be a pin on the wheel that rotates and opens up a switch when the credit reaches 0. Chances are that switch is either bent closed or jumpered to always be closed. Or another way is some people remove wires from the advance coil and reverse coil on the credit wheel to keep the credit wheel in a certain spot which keeps that switch closed.
If you do take it off free play than you need to make sure coin mechs work along with all the relays in the game that are associated with coining up the game.