The first thing we'll do, to troubleshoot the starter motor, is to manually apply 12 volts to the S terminal of the starter motor solenoid (using a remote starter switch).
This is is a pretty simple and straight-forward test... the hard part is connecting one of the two remote starter switch's leads onto the solenoid's S terminal (the other lead you have to connect to the battery positive (+) post).
What I do is use a wire piercing probe to pierce the S terminal wire and then I connect my remote starter switch to it and proceed from there... I suggest you do the same (to see what this tool looks like, go here: Wire Piercing Probe Tool Review (Power Probe PWPPPPP01)).
IMPORTANT: Remove the key from the ignition switch for this test.
OK, this is what you'll need to do:
Raise your Ford Explorer (Ranger, Aerostar, or Mountaineer) and place on it jack stands. Remember, to only way to gain access to the starter motor is from underneath the vehicle.
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 a remote starter switch to the S terminal of the starter motor solenoid.
This is easier said than done... so take your time and make sure the connection is on the S terminal of the starter motor solenoid.
Also, in case you're wondering... you can leave the starter motor solenoid's S terminal wire connected to the engine's wiring harness connector or not, the test will work either way.
Reconnect the negative battery cable to the battery negative post.
Now, apply 12 volts to the S terminal wire of the starter motor starter solenoid with your remote starter switch. As you apply these 12 Volts (to the S terminal wire of the starter motor solenoid), you'll get one of two results:
Let's take a look at what your test results mean:
CASE 1: The starter motor cranked the engine. This test result let's you know that the starter motor is OK and functioning. It also tells you that the probable cause, of it not working when you turn the key to start the engine, is a lack of the 12 Volt Start signal on the S terminal of the starter motor solenoid.
The next step is to go to TEST 2 and see if the starter motor is getting the 12 Volt Start signal on the S terminal wire (circuit). Go to TEST 2: Verifying the 12 Volt Start Signal.
CASE 2: The starter motor DID NOT crank the engine. This usually means that your starter motor is BAD and needs to be rebuilt or replaced.
I suggest 2 more tests and these are to make sure that the starter motor is getting its 12 Volt Start 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.
In this second starter motor test, you'll gonna' check to see if the starter motor is receiving a 12 Volt crank signal on the wire that connects to the solenoid's S terminal.
For this test, you can use an automotive 12 Volt test light... or, as I prefer to do, use a multimeter (the following test steps assume you're using a multimeter).
As you may already know, this 12 Volt signal originates from the ignition switch.
This is what you'll do:
Raise and place the front of your pick up (or SUV or mini-van) on jack stands (if it isn't already).
Grab and set your multimeter on Volts DC mode.
From underneath the pick up (or mini-van or SUV), located the wire that connects to the s terminal of the starter motor solenoid and connect the the RED multimeter test lead to this wire using an appropriate tool (like a Wire Piercing Probe).
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 via the neutral safety switch on the transmission (if automatic transmsission equipped) or on the clutch pedal (if equipped with a Manual Transmission).
Ground the multimeter's BLACK 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.
When everything is ready, have your helper turn the key to crank the engine.
As your helper is cranking the engine (altough the starter motor really isn't cranking the engine), your multimeter should read one of two results: Either 10 to 12 Volts DC or 0 Volts.
Alright, let's jump to the next page to interpret the results of this test...
OK, let's make sense of the readings that your multimeter recorded in the test:
CASE 1: If your multimeter registered 10 to 12 Volts: This is good, since it let's you know that the starter motor is getting a crank signal from the ignition switch.
This test result also confirms that the neutral safety switch is good and doing its job. Now, in the majority of the cases, you could stop testing here, 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.
The next test would be to voltage drop test the battery positive cable that connects to the starter motor solenoid. This is a very easy and simple test to do, and in STARTER TEST 2 I'll show you how to do it. Go to: TEST 3: Voltage Drop Testing the Batt (+) cable.
CASE 2: If your multimeter DID NOT register 10 to 12 Volts: This result let's you know that the reason the starter motor is not cranking up the engine is through a lack of a Crank (Start) Signal.
Here's why: Without the ignition switch's 12 Volt crank signal, the starter motor will not crank the engine. This 12 Volt signal could be missing because either the neutral safety switch is BAD or because ignition switch itself is fried. Testing these two components is beyond the scope of this article... but you have now eliminated the starter motor as BAD.
“A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.”