The starter motor is a special type of electrical motor designed for intermittent use only. During testing, it should never be operated for more than 30 seconds without resting for 2 minutes in between operation cycles to allow it to cool.
Cranking Voltage Test (Area Test)
The cranking voltage test measures the available voltage to the starter during cranking. To perform the test, disable the ignition or use a remote starter switch to bypass the ignition switch. Normally, the remote starter switch leads are connected to the positive terminal of the battery and the starter terminal of the solenoid or relay.
Using a remote starter switch to bypass the control circuit and igniton system. Courtesy of Ford Motor Company.
Refer to the service manual for specific instructions on the model car being tested. Connect the voltmeter's negative lead to a good chassis ground. Connect the positive lead to the starter motor feed at the starter relay or solenoid. Activate the starter motor and observe the voltage reading. Compare the reading to the specifications given in the service manual. Normally, 9.6 volts is the minimum required.
If the reading is above specifications but the starter motor still cranks poorly, the starter motor is faulty. If the voltage reading is lower than specifications, a cranking current test and circuit resistance test should be performed to determine if the problem is caused by high resistance in the starter circuit or an engine problem.
Cranking Current Test (Area Test)
The cranking current test measures the amount of current the starter circuit draws to crank the engine. Knowing the amount of current draw helps to identify the cause of starter system problems.
Nearly all starter current testers use an inductive pick-up measure the current draw. However, some earlier models were equipped with an ammeter that needed to be connected in series with the battery.
Connecting the test leads of a typical CSB tester.
To conduct the cranking current test, connect a remote starter switch or disable the ignition prior to testing. Follow the instructions given with the tester when connecting the test leads. Crank the engine for no more than 15 seconds. Observe the voltmeter. If the voltage drops below 9.6 volts, a problem is indicated. Also, watch the ammeter and compare the reading to specifications.
The most probable causes of low and high current draw within a starter motor circuit are summarized in the table below. If the problem appears to be caused by excessive resistance in the system, conduct an insulated circuit resistance test.
Starter Relay By-Pass Test
The starter relay by-pass test is a simple way to determine if the relay is operational. First, disable the ignition. Connect a heavy jumper cable between the battery's positive (+) terminal and the starter relay's starter terminal. This bypasses the relay. When the connection is made, the engine should crank.
If the engine cranks with the jumper installed and did not before the relay was by-passed, the starter relay is defective and should be replaced.
Free Speed or No Load Test
- Clamp the starter firmly in a bench vise.
- Connect an ammeter to the battery cable and the starter to a battery. This should cause the motor to run.
- Check current draw and motor speed and compare them to specifications. If they meet specs when the battery has at least 11.5 volts, the starter is working properly.
If the current draw was excessive or the motor speed too low, there is excessive physical resistance, which can be caused by worn bushings or bearings, a shorted armature, shorted field windings, or a bent armature.
If there was no current draw and the starter did not rotate, the problem could be caused by open field windings, open armature coils, broken brushes, or broken brush springs.
Low armature speed with low current draw indicates excessive resistance. There may be a poor connection between the commutator and the brushes, or the connections to the starter are bad. If the speed and current draw are both high, check for a shorted field winding.
Starter Motor Component Tests
With the starter motor disassembled, tests can be conducted to determine the reason for failure. The armature and field coils should be checked for shorts and opens first. Normally, if the armature or coils are bad, the entire starter is replaced.
Field Coil Tests
The field coil and frame assembly can be wired in a number of different ways. Accurate testing of the coils can only be done if you follow the specific guidelines of the manufacturer or if you know how the coils are wired. To do this, look at the wiring diagram and figure out where the coils get their power and where they ground. When you have this information, you will know if the coils are wired in series or parallel.
The usual way to check the field coils for opens is to connect an ohmmeter between the coils' power feed wire and the field coil brush lead. If there is no continuity, the field is open. To check the field coil for a short to ground, connect the ohmmeter from the field coil brush lead and the starter (field frame) housing. If there is continuity, the field coil is shorted to the housing.
Checking a field coil for an open. Reprinted with permission.
The armature should be inspected for wear or damage caused by contact with the permanent magnets or field windings. If there is wear or damage, check the pole shoes for looseness and repair as necessary. A damaged armature must be replaced.
Check the armature for wear and damage. Courtesy of American Honda Motor Co., Inc.
Next, check the commutator of the armature. If the surface is dirty or burnt, clean it with emery cloth or cut it down with an armature lathe. Measure the diameter of the commutator with an outside micrometer or vernier caliper. If the diameter is less than specifications require, replace the armature.
Measure commutator runout by mounting the armature in V-blocks. Position a dial indicator so that it rides on the center of the commutator. If the runout is within specs, check the commutator for carbon dust or brass chips between the segments. If the commutator runout is beyond specs, replace the armature.
Check the depth of the insulating material (mica) between the commutator segments.
Check the depth of the mica between the commutator segments. Courtesy of American Honda Motor Co., Inc.
Check each one and compare the depth with specifications. If necessary, undercut the mica with the proper tool or a hacksaw to achieve the proper depth. If the proper depth cannot be achieved, replace the armature.
Check for continuity between the segments of the commutator. If an open circuit exists between any segments, replace the armature.
Checking the armature for an open. Courtesy of Isuzu Motors Limited.
Place the armature in an armature tester, commonly called a growler. Hold a hacksaw blade on the armature core. If the blade is attracted to the armature's core or vibrates while the core is turned, the armature is shorted and must be replaced.
Testing an armature on a growler. Courtesy of American Honda Motor Co., Inc.
With an ohmmeter, check the armature windings for a short to ground. Hold one meter lead to a commutator segment and the other on the armature core. Also check between the armature shaft and the commutator. If there is continuity at either of these two test points, the armature needs to be replaced.
Checking an armature for a short. Courtesy of Ford Motor Company.