TEST 2: Adding 12 Volts To The 'S' Terminal Circuit

How To Test The Starter Motor On the Car (Step by Step)

You've reached this Test because you have verified that there aren't 12 Volts (10V - 12V) present at the S terminal of the solenoid when the key is turned to the START position in TEST 1.

Now we're gonna' apply the 12 Volts ourselves (with a jumper wire or a tool like a Power Probe) to verify that the starter motor works.

We're gonna' get only one result from this test. And it's that the starter motor should engage the engine and crank it.

  1. 1

    Raise and support the vehicle on jack stands (if applicable). The starter motor must be connected to all of its cables/wires.

  2. 2

    Make sure the key (ignition switch) is in the OFF position (or the car or truck may start).

  1. 3

    Apply 12 Volts (with a jumper wire or a suitable tool or tools) to the starter solenoid's S terminal or to the wire itself.

  2. 4

    The starter motor should engage the engine and crank it.

Let's take a look at what your test results mean:

CASE 1: The starter motor cranked the engine. This test result tells you that the starter motor itself is not the cause of the ‘does not crank’ condition.

The most likely cause of the missing START voltage signal (TEST 1) will be either a defective ignition switch or a defective neutral safety switch. You'll need to consult a wiring diagram (for your specific vehicle) to continue your troubleshooting efforts.

CASE 2: The starter motor DID NOT crank the engine. This test result usually means that the starter motor is defective.

Since the starter motor solenoid is not getting a START voltage signal (TEST 1) either, then you've got two separate problems on your hands.

I'm gonna' suggest that you remove the starter motor and bench test it. If it fails the bench test, then you've confirmed that the starter motor is defective (and the reason it won't activate when you applied battery power to it in this test).

As for the missing START signal (TEST 1), the most likely cause will be a defective ignition switch or a defective neutral safety switch. You'll need to consult a wiring diagram (for your specific vehicle) to continue your troubleshooting efforts.

TEST 3: Voltage Drop Testing The Power Circuit

How To Test The Starter Motor On the Car (Step by Step)

In this TEST 3 we're gonna' voltage drop test the power circuit of the starter solenoid. This circuit is represented by the number 1 in the photo. This is where the positive battery cable is attached to.

The voltage drop test will tell us if the starter motor is receiving the full amount of voltage and amperage to turn over the engine. If corrosion or anything else is causing high resistance and impeding this flow of juice, this test will tell us.

This test can only be performed with you helper turning the key to the START position and keeping it there for the duration of the test.

  1. 1

    Raise and support the vehicle on jack stands (if applicable). The starter motor must be connected to all of its cables/wires.

  2. 2

    Place your multimeter in Volts DC mode.

  1. 3

    Attach one multimeter lead (either one, the polarity doesn't matter) to a clean spot on or near the center of the battery positive post. Don't make your connection on the positive battery cable's terminal.

  2. 4

    Attach or touch the other lead to the center of the stud to which the battery positive cable is attached to on the starter solenoid. It's important that the center of the stud be probed and not the cable's terminal itself.

    This is the stud labeled by the number 1 on the photo.

  3. 5

    Have your assistant turn the key to the START position while you observe the multimeter.

  4. 6

    The multimeter should read 0 Volts DC if all is OK.

Let's examine your test results:

CASE 1: The multimeter registered 0.9 Volts or less. Then we can consider this a zero voltage drop and this is a good thing. This voltage value tells us that the battery positive cable is delivering the complete amount of voltage and current to the starter motor.

The next step is to make sure that the starter motor is getting plenty of ground. For this test,go to: TEST 4: Voltage Drop Testing The Ground Circuit.

CASE 2: Your multimeter registered 1 to 7+ Volts. This is considered a high voltage drop and this is BAD. This voltage value tells us that the battery positive cable is NOT delivering the complete amount of voltage and current to the starter motor.

This high voltage drop will cause the starter motor to not work or make it crank the engine very slowly.

The most likely cause is a corroded battery positive terminal and or battery positive post. Or the round terminal that attaches the battery positive cable to the battery terminal of the starter solenoid is corroded. Inspect and clean and retest. More than likely the vehicle will now start.

What Is A Voltage Drop?

OK, what's a voltage drop? and in plain English please!, a voltage drop is when the multimeter helps the juice flow from the battery positive terminal to the starter solenoids battery cable terminal because a high resistance is blocking this flow thru' the cable itself.

High Voltage Drop

When there's a sizable voltage drop, the multimeter will register voltage. This voltage could be anywhere in the range of 1 Volt to 7 Volts. And this is BAD because this is a clear indication that the voltage from the battery positive post is finding a path of least resistance thru' the multimeter to get to the starter.

In other words, the voltage and thus current, is unable to completely flow thru' the battery cable to the starter motor. Whatever amount that can't flow thru' the cable, flows thru' the multimeter. This is what's called a voltage drop.

The most common cause of a high voltage drop reading is corrosion on the battery positive cable terminal or the battery positive post. Inspect and clean and retest.

If you do indeed have a high voltage drop, inspecting and cleaning the battery terminal and post will probably result in the starter motor working and the vehicle starting.

No Voltage Drop

So then, if the multimeter registers .9 Volts or less, we can correctly assume that the battery cable or its terminals don't have corrosion or other resistance issues and are letting the full amount of voltage and more importantly current thru' to the starter motor.

This very very low voltage drop reading tells us that the voltage is finding a path of least resistance thru' the battery cable than thru' the multimeter.

With this result (if this is the result you got from this test) we now can move on the next test.