ALTERNATOR TEST 2: Checking The Continuity Of The Alternator Fusible Link

How To Test The Alternator (Ford 4.6L, 5.4L)

The voltage and Amperage that the alternator delivers to the battery and the rest of the vehicle is delivered across one single cable. Before this juice reaches the battery, it has to cross an inline Fusible Link. The newer Ford vehicles use an inline fuse that goes by the name of Mega Fuse.

So, before we can condemn the alternator as BAD, we need to make sure that the Fusible Link or Mega Fuse between the alternator and the battery is not blown, cause if it's blown... the alternator could be working OK, but unable to deliver its output to the battery.

Verifying if the Fusible LInk or Mega fuse is blown or not can be done with a simple multimeter resistance test (Ohms).

  1. 1

    It's important that you first disconnect the negative battery cable from the negative battery Post. Disconnect only the negative cable. The battery positive cable MUST remain connected for this test.

  2. 2

    Once you've disconnected the negative battery cable, place your multimeter in Ohms mode and place the red multimeter test lead on the center of the battery positive terminal.

  3. 3

    Now, with the black multimeter test lead... touch the center of the alternator stud to which the battery cable is attached to. The orange arrow in the photo points to this stud.

  4. 4

    If the Mega Fuse is not blown, your multimeter will show an Ohms value of about 0.5 Ohms.

    If the 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 Mega Fuse is OK and not blown.

Now, there's just one more thing to check and you're done! The next test is to make sure that the alternator's voltage regulator is getting power. For this test, go to ALTERNATOR TEST 3.

CASE 2:  Your multimeter DID NOT registered .5 Ohms, it registered OL: This result tells you that the Mega fuse is blown.

Replace the Fusible Link or Mega Fuse and retest. More than likely your problem will be solved and the alternator will begin charging again.

A blown Fusible Link or Mega Fuse doesn't happen very often. But if it does, you'll have to find out the reason for this, since they don't get blown for no apparent reason.

Now, in case this is the condition that your specific Ford vehicle is experiencing, you'll need to consult a repair manual to find the exact location of this Fusible Link or Mega Fuse. But, it's general location is in the battery cable that connects to the alternator to the battery or Engine Fuse Box. If you follow this cable to its final destination... you'll find the Inline Fusible Link or Mega Fuse.

ALTERNATOR TEST 3: Checking The Alternator Fuse

How To Test The Alternator (Ford 4.6L, 5.4L)

The alternator's voltage regulator needs an injection of 12 Volts to get the alternator to start producing a charge.

This voltage comes from a fuse and is provided at all times, even when the engine is turned Off.

In this test, you're gonna' verify that there is indeed Power (12 Volts) available in this circuit.

This is what you'll need to do:

  1. 1

    Reconnect the battery negative terminal, that you disconnected in the previous test step.

  2. 2

    Locate the Engine Compartment Fuse Box. This is where you'll find the alternator fuse.

    The exact location of this ALT fuse will vary from one vehicle to another. You'll need to consult your Owner's Manual or repair manual to find its exact location.

  3. 3

    Once you have located the ALT fuse, pull it out and verify that it is not blown.

Let's interpret your test result:


CASE 1: The fuse is NOT blown This means that the alternator's voltage regulator is getting power.

Now, this test result, in conjunction with ALTERNATOR TEST 1 and 2, let's you now that the alternator on your Ford 4.6L or 5.4L is equipped car, pick up or SUV is BAD. Replace the alternator. Replacing the alternator will solve the No Charge condition on your vehicle.

CASE 2: The fuse IS blown this tells you that the voltage regulator is not getting power.

Without this voltage (12 Volts), the alternator will NOT produce a charge. Replace the fuse and re-test.