Typical 24 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
The following troubleshooting procedures are intended to help the technician find the cause of the starting motor problem. The procedures are not intended to cover all possible problems and conditions. A common 24 volt circuit is shown. This 24 volt circuit will be discussed. If the troubleshooting procedures do not determine the cause of the malfunction, use Testing and Adjusting, “No Load – Test”. On machines that are equipped with a diagnostic connector, use the 6V-2150 Starting/Charging Analyzer in order to diagnose common problems. The Special Instruction, SEHS7768, “Using the 6V-2150 Starting/Charging Analyzer Group” is provided with the analyzer.
All starting systems are made up of four elements:
- Start switch
- Start relay
- Starting motor solenoid
- Starting motor
Note: On some small engines, the start relay may not be required. In this case, the start switch powers the starting motor solenoid directly.
Start switches are relatively low current devices. These switches are rated for 5 to 20 Amperes. Because the coil of a start relay draws about 1 Ampere, the start switch can easily turn on the start relay. The coil of a start relay is between test points (1) and (2) on Illustration 1.
The switch contacts of a typical start relay are rated for 100 to 300 Amperes. Because a starter solenoid requires 5 to 50 Amperes, the start relay can easily switch this load.
The starting motor solenoid is a high current switch that is rated at about 1000 Amperes. The starting motor solenoid engages the pinion with the flywheel and turns on the starting motor. The starting motor solenoid has two coils:
- Pull-in coil (W)
- Hold-in coil (X)
Pull-in coil (W) draws about 40 Amperes. Hold-in coil (X) requires about 5 Amperes. When the start relay closes, both coils receive power. Battery voltage is applied to the high end of both coils at test point (3) which is the start terminal (S). The low end of hold-in coil is permanently grounded to the ground post of the starting motor. Grounding for the low end of pull-in coil (test point (4) ) is momentary. Grounding takes place through the DC resistance of the starting motor. When the magnetic force builds up in both coils, the pinion moves toward the flywheel ring gear. On 37-MT models, the pinion will stop short of engagement. On 41-MT and 42-MT models, the pinion will engage the flywheel ring gear. Then, the solenoid contacts will close in order to power the starting motor. Closing the contacts will temporarily remove the ground from the pull-in coil. The battery voltage will be applied to both ends of the pull-in coil while the motor cranks. During this period, the pull-in coil is out of the circuit. Cranking continues until the start switch is released.
The above switches and relays permit a 5 Amp dash-mounted switch to turn on a 500 to 1000 Amp motor that is used to crank an engine.