How To Test The Alternator (1997-2002 4.2L Ford F150)

How To Test The Alternator (1997-2002 4.2L Ford F150)

You can test the alternator on your 4.2L V6 Ford F150 with just a multimeter. That's right, you can do it yourself and without any expenses diagnostic test equipment.

The alternator test is pretty easy to do and you'll be able to accurately diagnose the it as good or bad. In this tutorial, I'll show you how in a step-by-step way.

In Spanish You can find this tutorial in Spanish here: Cómo Probar El Alternador (1997-2002 4.2L Ford F150) (at:

Symptoms Of A Bad Alternator

Every time you start the engine on your Ford F150 the battery gets discharged. If the battery weren't recharged, eventually you wouldn't be able to restart the engine.

So it's the alternators job to recharge the battery so that you're able to crank up the engine every time you need it to. Besides that, the alternator also provides for all of the electrical needs of the accessories that'll need power while your F150 is running.

By accessories I mean things like: the headlights, the radio, the air conditioning system, the fuel pump, the ignition coils, 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.

Where To Buy The Alternator And Save

The alternator on your Ford F150 can get quite expensive. Check out the following links and comparison shop. I think you might be able to save yourself a few bucks:

Not sure if the above alternator and alternator belt fit you particular Ford F150? Don't worry, once you get to the site, they'll make sure the parts fit and if they don't they'll ask you about your F150's specifics to find you the right parts.

TEST 1: Battery Voltage Test With Engine Running

Battery Voltage Test With Engine Running. How To Test The Alternator (1997-2002 4.2L Ford F150)

The first order of business is to see what the alternator is doing. We need to know if it's actually charging the battery (or not) when the engine is running.

This simply involves starting the engine and letting it run while we check the battery's voltage with a multimeter. 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.

If the alternator is doing its job and charging the battery, you'll see a battery voltage of a round 13.5 to 14.5 Volts DC. If it isn't doing its job, then you're going to see a voltage around 12.5 Volts which will decrease the longer the engine is running.

NOTE: The battery needs to be fully charged before you attempt this test. If the battery is discharged, charge it up so that you're able to start the engine and keep it running for at least 5 to 10 minutes.

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 test 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 is good, since it tells you that the alternator is OK. In other words the alternator is charging the battery and this is NOT defective.

Now, if you're having to jump-start the pick up to get it going then 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 vehicle, 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 test result tells you that the alternator isn't doing its job.

Now before we conclude that the alternator is bad, we need to do a few more simple tests. Your next step is to make sure that the alternator's battery output circuit is OK for this test go to: TEST 2: Checking Alternator's Battery Output Circuit..

Ford Vehicles:

  • Ford F150
    • 1997,