TEST 1: Applying 12 Volts To The Starter Motor S Terminal
The 1st order of business, when testing the starter motor, is to bypass the ignition switch and directly apply power to the starter motor solenoid's S terminal to see if the starter motor cranks the engine.
If when applying battery power directly to the starter motor and it cranks the engine, then you can eliminate the starter motor itself has the defective.
I can tell you from personal experience, that the fastest, easiest, and safest way to apply battery power to the starter motor is by using a remote start switch. You can see an example of this tool (and where to buy it) here: Sunpro Actron CP7853 Remote Starter Switch.
Now, if the starter motor doesn't turn the engine over, when applying battery power to it with the remote start switch, then you can conclude that the starter motor itself is defective or that its not getting enough current from the battery because of a voltage drop on the battery cable (we'll test this in TEST 3).
IMPORTANT: Before you perform this test remove the key from the ignition switch to prevent the engine from accidentally starting. If your Ford Mustang is equipped with a standard transmission, place it in neutral.
IMPORTANT: Place your Ford Mustang on jack stands if you raise it to access the starter motor!
OK, these are the test steps:
Disconnect the battery negative (-) terminal.
You'll reconnect it back in one of the following steps, for now, it's a safety precaution as you set up the test.
Attach one of the alligator-type terminals of the remote starter switch to the S terminal of the starter motor.
If the S terminal wire has a female quick disconnect connector, you can disconnect the wire from the starter solenoid and connect the remote start switch connector directly on the solenoid's male spade terminal.
If the S terminal wire has a ring type connector (and is fastened by a nut to the stud), don't disconnect it from the starter solenoid. You can connect the remote start switch connector onto the stud with the wire still connected to it.
Reconnect the negative (-) battery cable to the battery negative (-) post.
NOTE: Make sure that the battery cables and posts are clean.
Connect the remaining alligator-type terminal of the remote starter switch to the battery positive post.
IMPORTANT: If your Ford Mustang has a standard transmission, make sure it's out of gear before you make this last connection.
Activate the starter motor with your remote starter switch. As you apply these 12 Volts (to the S terminal of the starter motor), you'll get one of two results:
1) The starter will activate and will turn over the engine -OR- 2) The starter motor won't do a thing.
Let's examine your test result:
CASE 1: The starter motor cranked the engine. This test result lets you know that your Ford Mustang's starter motor is OK and functioning.
If the starter motor is NOT cranking the engine when you turn the key to start it, then it's probably not receiving the activation signal from the ignition switch.
To further test this, go to the next test: TEST 2: Verifying The Start Signal.
CASE 2: The starter motor DID NOT crank the engine. This usually means that your Ford Mustang's starter motor is bad and needs to be rebuilt or replaced.
I suggest 2 more tests and these are make sure that the starter motor is getting its 12 Volt signal signal and to test the battery cable (that attaches to the starter motor solenoid) for corrosion. This can be accomplished very easily with a voltage drop test.
If the above two tests confirm that the start signal IS present and there's no voltage drop on the battery cable (feeding battery power to the starter motor), then you can confidently conclude your Ford Mustang's starter motor is bad and needs to be replaced.
TEST 2: Verifying The Start Signal
You've reached this point because the starter motor on your Ford Mustang cranked the engine when you applied battery powered directly to the S terminal of the starter solenoid.
In this test step we're gonna' make sure that the start signal is present in the S wire that connects to the S terminal of the starter solenoid.
This is a very simple test and it's done with a multimeter in Volts DC and simply involves making sure that when you turn the key to crank and start the engine, battery voltage is present on the wire that connects to the S terminal of the starter solenoid.
OK, these are the test steps:
Connect the red multimeter test lead to the S terminal of the starter motor.
If the S wire has a female quick disconnect connector, disconnect the wire from the solenoid and connect the red multimeter test lead to the connector on the wire itself.
If the S wire has a ring type connector and is fastened to the stud via a nut, then connect the red multimeter test lead to the S terminal stud (without removing the S wire from the stud).
Attach the black multimeter test lead to a clean and rust-free spot on the engine or on the vehicle frame.
I recommend that you use a battery jump start cable to ground the black multimeter test lead directly to the battery negative (-) terminal.
Have your helper crank the engine from inside your Ford Mustang.
The engine won't turn over, but the idea is to verify that the starter motor's internal solenoid is getting the 12 Volt start signal from the ignition switch (or not).
Your multimeter is going to register one of two results: Either 9 - 12 Volts DC or no voltage at all.
Let's analyze your test result:
CASE 1: The multimeter registered 9 to 12 Volts. This is the correct test result and tells you that the starter motor is receiving its activation signal.
This test result eliminates the safety neutral switch and the ignition switch as being faulty. The next step is to do a voltage drop test on the starter's battery cable. For this test go to: TEST 3: Voltage Drop Testing The Battery (+) Cable.
CASE 2: If your multimeter DID NOT register 9 to 12 Volts. This result tells you that the starter motor is not getting its activation signal. Without it, the starter motor will not crank the engine when you turn the key to crank and start the engine.
Usually, when the starter's activation signal is not present on the wire that connects to the starter solenoid's S terminal, it's usually because:
- The ignition switch is faulty.
- The starter relay is defective.
- The neutral safety switch is faulty or misaligned.
Although it's beyond the scope of this article to test the neutral safety switch, the starter relay, or the ignition switch, you have eliminated the starter motor as defective. The following wirind diagram will help you track the problem down: