How To Test The Starter Motor (1997-2008 4.2L V6 Ford F150)

TEST 2: Verifying The 12 Volt Start Signal

So far, you've confirmed that the starter motor is OK since it activates and cranks the engine when you applied 12 Volts directly to it in TEST 1.

Now it's time to see if the ignition switch is sending this 12-Volt signal to the starter while a helper turns the key to the START position.

To check this, you'll use a multimeter to measure the voltage at the solenoid's S terminal.

Remember, there should ONLY be 12 Volts present at the S terminal when you turn the key to the START position.

NOTE: You can perform this test with the S wire connected or disconnected to the S terminal.

These are the test steps:

1. 1

   Raise and place the front of the vehicle on jack stands (if it isn't already).

2. 2

   Place your multimeter in Volts DC mode.

3. 3

   Ground the black multimeter test lead directly on the negative (-) battery terminal using a jump start cable.
   
   You can also Ground it on the engine, if you can find a clean, unpainted and rust-free spot of metal.

4. 4

   Connect the red multimeter test lead to the S terminal wire using an appropriate tool.
   
   Don't know which wire is the S terminal wire? It's the one that connects to the smaller of the three studs on the starter motor solenoid.
   
   The S terminal wire (circuit) is the one that delivers the Start (Crank) signal from the ignition switch.

5. 5

   Have your helper turn the key to crank the engine when the test is setup.

6. 6

   Your multimeter should read 10 to 12 Volts DC.

OK, let's take a look at your test result:

CASE 1: The wire is delivering 10 to 12 Volts when you turn the key to crank the engine. This is the correct test result and confirms that the starter motor is receiving an activation signal.

Your next and final test is: TEST 3: Voltage Drop Testing The Battery (+) Cable.

CASE 2: The wire IS NOT delivering 10 to 12 Volts when you turn the key to crank the engine. This test result confirms the starter motor isn't receiving an activation signal. Without it, the starter motor won't activate and crank the engine.

The missing starter motor activation signal is most likely caused by one of the following issues:

• A bad starter motor relay.
• A bad park-neutral safety switch (automatic transmission).
• A bad clutch pedal switch (manual transmission).
• A bad ignition switch.
• An anti-theft system fault (if equipped).

Although it's beyond the scope of this tutorial, your next step is to find the cause of the missing activation signal and resolve the issue.

TEST 3: Voltage Drop Testing The Battery (+) Cable

The starter motor needs all the available current the battery can provide to crank the engine. This high amount of current travels through a thick cable connected to the starter's BAT+ terminal (see image above).

Over time, the connection points of this cable (at the battery and starter) can corrode. This corrosion creates resistance in the cable, which can block some or a lot of the current trying to pass through it.

This type of problem, where resistance reduces the voltage reaching the starter motor, is known as a voltage drop and is a very common issue.

If there's less than the full amount of current/voltage delivered to the starter motor, it won't have enough power to crank the engine effectively.

To check if a voltage drop in the cable is causing the starter motor failure, we'll use a multimeter to perform a voltage drop test.

These are the test steps:

1. 1

   Place your multimeter in Volts DC mode.

2. 2

   Attach the red multimeter test lead to the positive (+) battery post. The positive (+) battery post must be clean and corrosion-free.
   
   You may need two helpers for this test step, since someone will have to hold the red multimeter test lead onto the battery positive (+) post and someone else inside the vehicle (to crank it when everything is set up).

3. 3

   Place the black multimeter test lead on the starter motor solenoid stud that connects to the battery positive (+) cable (see illustration above).
   
   The orange arrow labeled with the "BAT +", in the illustration above, points to this stud.
   
   Maintain the black multimeter test lead in this position throughout the next step.

4. 4

   When everything is ready, have your helper turn the key to crank the engine.
   
   Although the starter motor won't crank the engine, your helper has to turn the ignition switch to start the engine for the voltage drop test to work.

5. 5

   The multimeter should register 0.5 Volts or less (0.5 V = 0 Volts).
   
   If there's a problem in the wire, your multimeter will register some voltage, usually 5 Volts or more.

Let's interpret your test result:

CASE 1: Your multimeter indicated NO voltage drop (0.5 Volts or less). This is the correct test result, and it confirms the starter motor is receiving all of the battery voltage and amperage it needs to crank the engine.

You can conclude that the starter motor is bad and needs replacement if you have:

• Confirmed that the starter motor does not crank the engine when you directly apply 12 Volts to the starter's S terminal (TEST 1).
• Confirmed, in this test section, that the battery positive (+) cable does not have a voltage drop issue.

Now, before you remove the starter motor, do two more important things:

• Confirm the starter motor is receiving an activation signal (if you haven't already). TEST 2 will help you with this.
• Check that the engine is NOT mechanically locked up by manually turning it via its crankshaft pulley bolt (using a 1/2" ratchet and the appropriate size socket).

If you'd like to bench-test the starter motor (after removing it). You can find the step-by-step instructions here:

• How To Bench Test A Starter Motor (Step By Step).

CASE 2: Your multimeter registered 5 Volts or more. This result confirms that the starter's battery (+) cable has corrosion on one or both terminal ends.

The issue causing the voltage drop in the cable will prevent the battery's total amperage output from reaching the starter motor.

Your next step is thoroughly cleaning both ends of the battery positive (+) cable with a small piece of sandpaper.

Eliminating the cause of the voltage drop should get the starter motor to crank the engine.

More 4.2L Ford F150 Tutorials

There are quite a few 4.2L V6 F-150 pickup 'how to' tutorials and wiring diagrams you can find in this index:

• 4.2L Ford F150 And F250 Index of Articles.

F-Series Vans: You can find all of the available tutorials and wiring diagrams for the 4.2L V6 E-Series vans in this index:

• 4.2L Ford E150 And E250 Index of Articles (at: troubleshootmyvehicle.com).