When testing indicates that a starter malfunction is the cause of the no-start or hard-start condition, the starter must be removed from the vehicle for additional testing, the first of which should be a no-load test. The no-load test is used to identify specific defects in the starter that can be verified with tests when disassembled. Also, the no-load test can identify open or shorted fields, which are difficult to check when the starter is disassembled. The no-load test also can be used to indicate normal operation of a repaired motor before installation.
To perform a no-load test, first clamp the starter motor in a bench vise. Then, connect the test equipment as shown in Figure 9–22. Connect a voltmeter from the motor terminal to the motor frame and use an rpm indicator to measure armature speed. Connect the motor and an ammeter in series with a fully charged battery of the specified voltage and a switch in the open position from the solenoid battery terminal to the solenoid switch terminal. Close the switch and compare the rpm, current, and voltage reading with specifications. It is not necessary to obtain the exact voltage specified, because an accurate interpretation can be made by recognizing that if the voltage is slightly higher, the rpm will be proportionately higher, with the current remaining essentially unchanged. However, if the exact voltage is desired, a carbon pile connected across the battery can be used to reduce the voltage to the specified value. If more than one 12V battery is used, connect the carbon pile to only one of the 12V batteries. If the specified current draw does not include the solenoid, deduct from the ammeter reading the specified current draw of the solenoid hold-in winding. Disconnect only with the switch open. Interpret the test results as follows:
1. Rated current draw and no-load speed indicate normal condition of the cranking motor.
2. Low free speed and high-current draw indicate:
• Too much friction. Tight, dirty, worn bearings, bent armature shaft, or loose pole shoes, allowing the armature to drag.
• Shorted armature. This can be checked further after disassembly.
• Grounded armature or fields. Check further after disassembly.
3. Failure to operate with high-current draw indicates:
• A direct ground in the terminal or fields.
• Seized bearings. This can be determined by turning the armature by hand.
4. Failure to operate with no current draw indicates:
• Open field circuit. This can be checked after disassembly by inspecting terminal connections and tracing the circuit with a test lamp.
• Open armature coils. Inspect the commutator for burned bars after disassembly.
• Broken brush springs, worn brushes, high insulation between the commutator bars, or other causes that would prevent good contact between the brushes and commutator.
5. Low no-load speed and low-current draw indicate high internal resistance due to poor connections, defective leads, dirty commutator, and the causes listed under step 4.
6. High free speed and high current draw indicate a shorted field. If shorted fields are suspected, replace the field coil assembly and check for improved performance.