The alternator delivers its output to the battery across one single circuit (wire). This is the wire that attaches to the back of the alternator with a nut.
Depending on your particular 4.0L Ford pick up (SUV, or mini-van), this wire has an inline fusible link or a mega fuse, that protects the alternator in case of an electrical short. Every now and then, this inline fusible link or mega fuse will get blown. When this happens, the alternator could be OK and creating a Charge, but the blown inline fusible link or blown mega fuse keeps this charge from reaching the battery and the rest of the vehicle.
This test will help you to verify if this inline fusible link or mega fuse is blown or not (with a simple multimeter Resistance Test). This is what you'll need to do:
The most important step, in this test, is to disconnect the battery's negative terminal only. Do not proceed to the next steps till you do this first.
Once the negative battery cable is disconnected from the battery, set your multimeter in Ohms mode.
Now, locate the one and only stud behind the alternator (see the photos in the image viewer). This is where the big wire, that supplies the alternator's output to the battery, is attached to with a nut.
Place the RED multimeter lead on this stud.
Now, place the BLACK multimeter lead on the battery positive terminal (at the battery).
If the inline fusible link or mega fuse is not blown, your multimeter will register an Ohms value of about 0.5 Ohms.
If the inline fusible link or mega fuse is blown, your multimeter will register an infinite Ohms reading... which if you're using a digital multimeter, it'll display the letters OL.
OK, now that the testing part is done... let's take a look at what your results mean:
CASE 1: Your multimeter registered .5 Ohms: This result indicates that the inline fusible link or mega fuse is OK and not blown.
So far, it's looking like the alternator is BAD. The last test to make sure it is is also a very simple test and it involves checking a fuse only. For this test, go to ALTERNATOR TEST 3.
CASE 2: Your multimeter DID NOT register .5 Ohms, it registered OL: This result tells you that the inline fusible link or mega fuse is blown.
You'll need to replace the inline fusible link or mega fuse and re-test the battery's voltage with engine running. This should solve your No Charge Condition.
If equipped with the inline fusible link, it's usually located at the starter motor solenoid that’ bolted to the fender apron or connected at the Underhood Fuse Box. If equipped with a mega fuse, this bad boy will be located on the Underhood Fuse Box.
Your Ford 4.0L Ranger (or 4.0L Aerostar, 4.0L Explorer, 4.0L Mountaineer) will have a dedicated alternator fuse (or another inline fusible link) that provides the voltage regulator on the alternator with 12 Volts. This 12 Volt signal will activate the alternator to start charging the vehicle's battery.
This is what you need to do to test this circuit:
Reconnect the battery to its negative battery cable, that you disconnected in the previous test and place your multimeter in Volts DC mode.
Probe the Yellow with White stripe wire on the alternator's with your multimeter's RED Test Lead.
Ground the BLACK multimeter lead directly on the battery negative terminal.
Turn the Key to the On position but do not start the engine. Your multimeter should register one of two things:
1.) 12 Volts DC, if the fuse (or inline fusible link) is good.
2.) 0 Volts DC, if the fuse (or inline fusible link) is blown.
Let's take a look at what your specific test results mean:
CASE 1: The fuse is NOT blown This is good and let's you know that the alternator has all of the power sources it needs to charge, since it isn't... this result also let's you know that the alternator is BAD and needs to be replaced.
CASE 2: The fuse IS blown, then please replace the fuse with another of the same amperage rating and start the car and check to see if the alternator is now charging the battery by repeating ALTERNATOR TEST 1.
This fuse is usually a 15 amp plastic blade type fuse and is located in the Engine Compartment Fuse Box. You may need to consult with your owner's manual once again if the fuses are not labeled.