How To Test The Starter Motor (4.0L Ford Explorer, Aerostar, And Mercury Mountaineer)

TEST 2: Verifying The 12 Volt Start Signal

The starter motor needs an activation signal to activate and crank the engine.

This activation signal is delivered to the starter motor by the wire that connects to the 'S' terminal on the starter motor's solenoid.

I've identified the 'S' terminal (of the starter motor solenoid) with the letter S in the illustration above.

What you'll do in this test section is check that the 'S' terminal wire is delivering an activation signal when your helper turns the key to crank the engine.

Let's get testing:

1. 1

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

2. 2

   Grab and set your multimeter on 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? This is 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 interpret your test results:

CASE 1: Your multimeter registered 10 to 12 Volts. This is the correct and expected test result and it confirms that the starter motor is receiving its activation signal.

Now, in the majority of the cases, you could stop testing here and replace the starter motor and be done. But, I suggest one more test so that you can be absolutely sure there isn't another issue to deal with.

Your next test is to voltage drop test the battery positive (+) cable. This is a very easy and simple test to do. Go to: TEST 3: Voltage Drop Testing The Battery (+) Cable.

CASE 2: Your multimeter DID NOT register 10 to 12 Volts. This test result lets you know that the starter motor isn't receiving an activation signal.

Although finding out why this activation signal is missing is beyond the scope of this tutorial, the most likely causes are:

• A bad starter motor relay.
• A bad park-neutral safety switch.
• A bad clutch pedal switch.
• A bad ignition switch.

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

The battery positive (+) wire is the wire that delivers the voltage and amperage starter motor needs to crank the engine.

This wire has to be corrosion-free and securely attached on one end to the battery positive (+) post and the other end to the starter motor solenoid. It won't deliver the battery's total voltage and amperage output if it isn't.

In this test section, you'll ensure that this battery wire delivers all of the battery's output. You'll accomplish this test by doing a simple voltage drop test on the wire.

This is what you need to do:

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 (+) terminal bolt 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 with the plus (+) sign, 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 take a look at what your results mean:

CASE 1: Your multimeter indicated NO voltage drop (which is 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 (TEST 1).
• Confirmed that the starter motor is receiving an activation signal (TEST 2).
• 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 one more important thing:

• Turn the engine manually (using a 1/2 ratchet and the appropriate socket on the crankshaft pulley bolt). This will check to see if the engine is mechanically locked up (or not).

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) (at: easyautodiagnostics.com).

CASE 2: Your multimeter registered 5 Volts or more. This result tells you that a voltage drop does exist in the wire (and it's not a good thing).

This voltage drop will prevent the battery's total amperage output from reaching the starter motor.

You'll need to thoroughly clean both ends of the battery positive (+) cable to solve this problem.

Cleaning the end that attaches to the starter motor solenoid should be done with a small piece of sandpaper. Once both ends of the battery positive cable are clean, reconnect everything and try cranking the engine. If the voltage drop was the cause of the no-crank condition, the starter motor will now crank the engine.

More 4.0L Ford Explorer, Aerostar, And Mercury Mountaineer

You can find more 4.0L V6 Ford Explorer diagnostic tutorials here:

• Ford 4.0L Index Of Articles.

Here's a sample of the tutorials you'll find in the index:

• How To Test For A Blown Head Gasket (Ford 4.0L Explorer, Aerostar And Mercury Mountaineer).
• How To Test The Fuel Pump (Ford 4.0L Explorer, Aerostar And Mercury Mountaineer).
• How To Test Engine Compression (1991-2010 4.0L Ford Explorer, Aerostar, And Mercury Mountaineer).