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Illustration 1 g00318603
Starting circuit for 12 volt system (typical example)
(1) Test point
(2) Test point
(3) Test point
(4) Test point
(5) Test point
(X) Hold-in coil
(W) Pull-in coil Procedures for Diagnosis of the Electric Starting Motor
The procedures for diagnosing the starting motor are intended to help the serviceman determine if a starting motor needs to be replaced or repaired. The procedures are not intended to cover all possible problems and conditions. The procedures serve only as a guide. The most common 12 volt circuit is shown in Illustration 1.General Information
All electrical starting systems have four elements:
Ignition switch
Start relay
Starting motor solenoid
Starting motorStart switches have a capacity of 5 to 20 amperes. The coil of a start relay draws about 1 ampere between test points (1) and (2). The switch contacts of the start relay for the starting motor are rated between 100 and 300 amperes. The start relay can easily switch the load of 5 to 50 amperes for the starting motor solenoid.The starting motor solenoid is a switch with a capacity of about 1000 amperes. The starting motor solenoid has two functions:
The pinion gear is engaged with the ring gear of the flywheel.
The starting motor is rotated.The starting motor solenoid has two coils. Pull-in coil (W) draws about 40 amperes. Hold-in coil (X) requires about 5 amperes.When the start relay closes, coils (W) and (X) receive power. Battery voltage is applied to test point (3), which is the start terminal (S). Terminal "G" of coil (X) is permanently grounded to the ground post or the housing of the starting motor. Grounding for test point (4) of pull-in coil (W) is momentary. This ground takes place through the DC resistance of the starting motor.When the magnetic force increases in both coils, the pinion gear moves toward the ring gear of the flywheel. Then, the solenoid contacts close in order to provide power to the starting motor. When the solenoid contacts close, the ground is temporarily removed from pull-in coil (W). Battery voltage is supplied on both ends of the pull-in coil while the starting motor cranks. During this period, the pull-in coil is out of the circuit.Cranking of the engine continues until current to the solenoid is stopped by releasing the ignition switch.Available power during cranking varies according to the temperature and the condition of the batteries. The following chart shows the voltages which are expected from a battery at the various temperature ranges.
Table 1
Typical Voltage Of Electrical System During Cranking At Various Ambient Temperatures
Temperature 12 Volt System 24 Volt System
−23 to −7 °C (−10 to 20 °F) 6 to 8 volts 12 to 16 volts
−7 to 10 °C (20 to 50 °F) 7 to 9 volts 14 to 18 volts
10 to 27 °C (50 to 80 °F) 8 to 10 volts 16 to 24 volts The following table shows the maximum acceptable loss of voltage in the battery circuit. The battery circuit supplies high current to the starting motor. The values in the table are for engines that have a service life of 2000 hours or more.
Table 2
Maximum Acceptable Voltage Drop In The Starting Motor Circuit During Cranking
Circuit 12 Volt System 24 Volt System
Battery post "-" to starting motor terminal "-" 0.7 volts 1.4 volts
Voltage drop across the disconnect switch 0.5 volts 1.0 volts
Battery post "+" to the terminal of the starting motor solenoid "+" 0.5 volts 1.0 volts
Solenoid terminal "Bat" to the solenoid terminal "Mtr" 0.4 volts 0.8 volts Voltage drops that are greater than the amounts in Table 2 are caused most often by the following conditions:
Loose connections
Corroded connections
Faulty switch contactsDiagnosis Procedure
If equipped with electric start, do not crank the engine for more than 30 seconds. Allow the starter to cool for two minutes before cranking again.Never turn the disconnect switch off while the engine is running. Serious damage to the electrical system can result.
If the starting motor does not crank or cranks slow, perform the following procedure:
Measure the voltage of the battery at the battery posts with the multimeter when you are cranking the engine or attempting to crank the engine. Measure the voltage across the battery posts. Do not measure the voltage across the cable post clamps.
Battery voltage
If the voltage is equal or greater than the voltage in Table 1, then go to Step 3.
The battery voltage is less than the voltage in Table 1. Refer to Special Instruction, SEHS7633 in order to test the battery.A low charge in a battery can be caused by several conditions.
Deterioration of the battery
A shorted starting motor
A faulty alternator
Loose drive belts
Current leakage in another part of the electrical system
Measure the current that is between the positive battery post and the starting motor solenoid. Use the 8T-0900 Ammeter. Refer to the Specifications Module, "Starting Motor" topic for the maximum current that is allowed for no load conditions. Note: If the following conditions exist, do not perform the test in Step 1 because the starting motor has a problem.
The voltage at the battery post is within 2 volts of the lowest value in the applicable temperature range of Table 1.
The large starting motor cables get hot.The current and the voltages that are specified in the Specifications Module are measured at a temperature of 27 °C (80 °F). When the temperature is below 27 °C (80 °F), the voltage will be lower through the starting motor. When the temperature is below 27 °C (80 °F), the current through the starting motor will be higher. If the current is too great, a problem exists in the starting motor. Repair the problem or replace the starting motor.
Use the multimeter in order to measure the voltage of the starting motor. Measure the voltage from test point (4) to test point (5) when you are cranking or attempting to crank the engine.
Voltage across test points (4) and (5)
If the voltage is equal to the