How To Test The Alternator (1997-2002 4.6L Ford F150)

If the alternator has failed on your 4.6L Ford F150, then this is the tutorial you need to test it. The alternator diagnostic test is not hard and the best part is that you'll only need a multimeter for it.

That's right, you don't need any expensive diagnostic test equipment to test the alternator. This is something that you can do and in this tutorial I'll show you how.

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

  1. Symptoms Of A BAD Alternator.
  2. TEST 1: Battery Voltage Test with Engine Running.
  3. TEST 2: Checking Alternator's Battery Output Circuit.
  4. TEST 3: Checking The Alternator Inline-Fuse (1997-1998 F150).
  5. TEST 4: Checking The Alternator Mini-Fuse (1999-2002 F150).
  6. Location Of The Alternator 175 Amp Mega-Fuse (1997-1998 F150).
  7. Location Of The Alternator Battery Output Fuse (1999-2002 F150).
  8. Where To Buy The Alternator And Save.
  9. More 4.6L Ford Test Articles.

Symptoms Of A Bad Alternator

The alternator is a key component, of your Ford F150, that is responsible for two important jobs.

The first one is to charge the battery so that you're able to crank the engine every time you need it to start. Why? Because the process of cranking and starting the engine puts a heavy load on the battery. This load discharges the battery. It's the job of the alternator to replenish the battery's charge so that you can have enough current to crank and start the engine again.

The other job is for the alternator to provide all of the vehicle's accessories electrical current needs. By accessories I mean everything that has to be (or will be) running once the engine has started. These accessories include items like: the fuel pump, windshield wipers, the radio, the fuel injection computer, headlights, brake lights, etc.

So when the alternator fails, you'll see one or several of the following symptoms:

  1. The charge light (also known as the battery light) will be shining nice and bright on your F150's instrument cluster.
  2. Whenever you turn on the headlights (night driving), they glow very dim.
  3. The pick up won't crank. It will only crank and start if you jump start your Ford F150.
  4. The only way the pick up cranks and starts is if you charge the battery.
  5. The idle may get high when you come to a stop.

TEST 1: Battery Voltage Test with Engine Running

Battery Voltage Test with Engine Running. How To Test The Alternator (1997-2002 4.6L Ford F150)

To successfully diagnose the alternator, we need to first see what the alternator is doing.

We're going to start off by checking the battery voltage with the engine running. To do this you will need a multimeter that can read volts DC. If you don't have a multimeter or you need to upgrade yours, take a look at my recommendations here: Buying A Digital Multimeter For Automotive Diagnostic Testing.

This test requires that you crank and start the engine and let it run for about 5 to 10 minutes. So the battery has to have enough of a charge to start the engine and let it run for about 5-10 minutes. So if the battery is discharged, charge her up before you start this test.

These are the test steps:

  1. 1

    Crank and start your Ford F150 and select Volts DC mode on your multimeter.

  2. 2

    Probe the positive battery terminal with the RED multimeter test lead.

    With the BLACK multimeter lead, probe the negative battery terminal on your Ford F150's battery.

  3. 3

    Your multimeter is gonna' register one of two possible readings and they are:

    1.) A steady 13.5 to 14.5 Volts DC.

    2.) Or 12.5 Volts that will decrease the longer the engine stays running.

  4. 4

    Put an electrical load on the alternator to further confirm that it's either charging or not charging.

    You can do this by turning on every accessory possible (inside the vehicle). For example: Turn on the A/C or heater on high, turn on the windshield wipers, turn on the headlights, turn on everything and anything that uses electricity inside and outside of the vehicle.

  5. 5

    Your multimeter will show you one of two things (as you turn on all of this stuff):

    1.) The multimeter will register a nice and steady 13.5 to 14.5 Volts DC no matter what gets turned on or...

    2.) It will register 12.5 V DC and this voltage will decrease more and more as you turn on stuff inside your Ford vehicle.

OK, let's interpret your multimeter test results:

CASE 1: Your multimeter registered 13.5 to 14.5 Volts. This test result confirms that the alternator is charging the battery and providing for the electrical needs of your F150's accessories. Since the alternator is good (not defective), no further testing of the alternator is needed.

Now, if you're having to jump-start the pick up to get it going... this test result points to a bad battery or a parasitic drain. A parasitic drain is tech-speak for something staying on (usually inside the pick up, for example: a dome-light) and draining the battery while the engine is off.

CASE 2: Your multimeter registered a voltage that steadily dropped down to 9 Volts: This multimeter voltage test result lets you know, beyond a shadow of a doubt, that the alternator is not charging the battery.

Before we can conclude that the alternator is fried and needs to be replaced, we need to do one more test. For this test go to: TEST 2: Checking Alternator's Battery Output Circuit..