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

Testing to see if the alternator is BAD or not, on your Ford (or Lincoln or Mercury) 4.6L or 5.4L equipped car, pick up or SUV, is a simple test that can be done with just a multimeter. This is a test that you can do with the alternator still in the vehicle and in this article I'll show you just how to do it.

To see if this alternator test article covers your specific Ford vehicle and alternator, you can take a look at the list of applications on the box labeled ‘Applies To:’ on the column on the right and scroll with the 'prev' and 'next' arrows.

Here are the contents of this article at a quick glance:

  1. ALTERNATOR TEST 1: Battery Voltage Test With Engine Running.
  2. ALTERNATOR TEST 2: Checking The Continuity Of The Alternator Fusible Link.
  3. ALTERNATOR TEST 3: Checking The Alternator Fuse.
  4. Symptoms Of A BAD Alternator.
  5. Related Test Articles.

ALTERNATOR TEST 1: Battery Voltage Test With Engine Running

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

The battery on your Ford (or Lincoln or Mercury) vehicle must be fully charged for this test... since you'll need to have the vehicle with the engine running.

Also, you can use a digital or analog multimeter. Both types of multimeters will do the job! One last thing, you'll be working around a running engine... so be careful and take all necessary safety precautions.

OK, here's the first test:

  1. 1

    Start the vehicle and let it stay running. Turn your multimeter On and set it on the Volts DC mode.

  2. 2

    Test the voltage of the battery by placing the RED multimeter lead on the battery positive terminal and the black multimeter lead on the negative battery terminal.

  3. 3

    Record the voltage reading. You'll be using this Volts DC reading that your multimeter is registering to compare it against the next test steps.

  4. 4

    Now, turn On all electrical accessories that your vehicle has... for example: headlights, blower motor on high, windshield wipers, radio, etc.

  5. 5

    Now, eyeball your multimeter display, after you have turned all those things On.

  6. 6

    One of two things is going to happen, after the accessories have been turned On... either the multimeter will display a steady voltage that will not drop. Or the multimeter will register a voltage that will drop down to 9 Volts.

    To be more specific: If the multimeter on your car (or pick up or SUV) is working correctly... the voltage will drop momentarily a bit every time you turn something On. But will regain or maintain a DC voltage of about 14 to 14.5 Volts the entire time that the engine is running and the accessories are On.

    OK, let's interpret your multimeter test results in the next page...

OK, let's make sense of the readings that your multimeter recorded in the test:

CASE 1: If your multimeter registered 14 to 14.5 Volts from the start of the test to the end of the test: This test result let's you know that the alternator on your 4.6L or 5.4L Ford car, pick up, or SUV is working like it should. No further testing is required.

CASE 2: If your multimeter DID NOT register 14 to 14.5 Volts from start to end and the voltage dropped to 9 Volts DC: This test result let's you know that the alternator is probably BAD.

Probably BAD? What I mean is this... around 90% of the time, you could stop here, replace the alternator and the problem is solved. To be 100% sure the alternator is BAD... there are 2 more tests to do and I'll show you how to do them. Go to ALTERNATOR TEST 2.