Use a volt meter so you get it right. 24 volts melts things! Put it on ohms, go from the block or chassis to one of the cables, it should have a good contact, <10 ohms resistance. That goes to the ground stud...the one on the starter motor. The other side should have an open circuit, WAY >10 ohms, this goes to the B+ stud....the one on the solenoid on top of the starter motor. To check that, go from the relay mounted on the block above the starter/below the exhaust pipe, leave it...the VOM...on ohms and one of the cables...the one that goes to the starter solenoid...should have <10 ohms resistance.