1 2 WH START SWITC.H START RELAP W X 3 4 RD/WH MTR BAT STARTER MOTOR 5 BATTERY Typical 12 Volt Starting Circuit (1) Test point. (2) Test point. (3) Test Point. (4) Test Point. (5) Test Point. (X) Hold-in coil. (W) Pull-in coil. Diagnosis Procedure The following simplified procedure is intended to help the serviceman determine if a starting motor needs to be removed and replaced or repaired. It is not intended to cover all possible problems and conditions, but to serve only as a guide. The most common 12 volt circuit is shown and discussed. 2. Is a high current switch rated about 1000 amps that actually turns on the starting motor. The starting motor solenoid has two coils.
Pull-in coil (W) draws about 40 amps and hold-in coil (X) requires about 5 amps. The instant the start relay closes, both coils (W) and (X) receive power. Battery voltage is applied to the high end of both coils, at test point (3) which is the start (S) terminal. The low end of hold-in coil (X) is permanently grounded to the ground post or motor housing of the starting motor. Grounding for the low end, test point (4), of pull-in coil (W) is momentary, and takes place through the DC resistance of the starting motor. As soon as magnetic force builds in both coils, the pinion moves toward the flywheel ring gear. The pinion will stop short of engagement of the flywheel ring gear.
Only then will the solenoid contacts close to power the starting motor. This temporarily removes the ground from pull-in coil (W), and puts battery voltage on both ends of it while the starting motor cranks. During this period, the pull-in coil is out of the circuit. Cranking continues until power to the solenoid is broken by releasing the ignition switch. General Information All starting systems are made up of four elements. They are the ignition switch, start relay, the starting motor solenoid and starting motor. Start switches are relatively low current devices. They are rated to switch approximately 5 to 20 amps.
Because the coil of a start relay [between test point (1) and (2)] draws about 1 amp, the start switch can easily turn on the start relay and have long life. The switch contacts of a typical start relay are rated to switch 30 amps. Because the solenoid requires 5 to 20 amps the start relay can easily switch this load. The starting motor solenoid has two functions: 1. Engages the pinion with flywheel. G424 Service Manual Starting System 33