How to Test the Alternator (Ford 4.0L)

How to Test the Alternator (Ford 1.9L, 2.0L)

Testing the alternator on your 4.0L Ford Explorer, Aerostar, or Ranger or 4.0L Mercury Mountaineer isn't hard. The best part is that you don't need any expensive testing equipment.

In this tutorial, I'm gonna' show you how in a step-by-step way.

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

  1. Important Tips.
  2. ALTERNATOR TEST 1: Checking Alternator Voltage Output With a Multimeter.
  3. ALTERNATOR TEST 2: Testing the Continuity of the Alternator's Battery Circuit.
  4. ALTERNATOR TEST 3: Checking the Alternator Fuse.
  5. Symptoms of a BAD Alternator.
  6. Related Test Articles.

Important Tips

To successfully accomplish the tests in this article, I suggest you follow these tips:

TIP 1:   The battery's has to be fully charged since you'll have to crank and start the engine to test the alternator.

TIP 2:   Read the entire article first, before you jump into the actual testing on your Ford Ranger (Explorer, Aerostar, or Mountaineer).

TIP 3:   You can use a digital multimeter or an analog multimeter.

TIP 4:   Take all necessary safety precautions. Be alert and think safety all of the time since you'll be working around a running engine.

Checking Alternator Voltage Output With a Multimeter

How to Test the Alternator (Ford 4.0L)

OK, the very first thing that you're gonna' do is to check, with your multimeter, the voltage of the battery while the engine is running.

If the alternator on your Ford Explorer (Mountaineer, Aerostar, Ranger) is working and charging the battery, the battery will read 13.5 to 14.5 Volts DC.

Remember, the battery has to be fully charged and these are the steps:

  1. 1

    Start the engine and probe the battery terminals with your multimeter in Volts DC mode.

  2. 2

    If the alternator is charging your Ford Explorer (Aerostar, Ranger, or Mountaineer) you'll see 13.5 to 14.5 Volts on your multimeter. If it doesn't... don't worry about this just yet, continue to the next step.

  3. 3

    Now, have a helper turn on the headlights, the A/C or heater on High, the windshield wipers, the radio, the rear window defroster, etc.

    Each time something is turned On, observe the voltage reading on your multimeter.

  4. 4

    As each accessory comes on, they'll place a load on the Charging System (alternator) and your multimeter will do one of two things:

    1.) The multimeter's DC voltage will decrease slightly and then stabilize around 13.5 to 14.5 Volts DC when something comes on or...

    2.) The DC voltage reading will start to decrease till it reaches 10 Volts DC and your vehicle may stall.

  5. 5

    OK, now that the test is done, turn to the next page to interpret your test results...

Take a look at the following test interpretations to find out which one best fits your multimeter test results:

CASE 1:   The multimeter maintained a 13.5 to 14.5 Volts value thru' out the whole test: This result confirms that the alternator is charging the battery and providing enough electrical power for any electrical load that is applied to it.

Here's why: The battery's voltage, at rest, is 12.5 Volts. So one of the best indicators that the alternator is charging it, is verifying that battery voltage should be at 2 Volts over battery voltage (at rest) when the engine is running.

CASE 2:   The multimeter DID NOT maintain a 13.5 to 14.5 Volts value: This usually means that the alternator is BAD.

Here's why: Since the alternator is not charging the vehicle or providing power for all the stuff you turned On, your car or mini-van is running off of the battery only. As the battery's juice starts to deplete... your multimeter shows a decreasing voltage reading.

You could stop here and replace the alternator. Around 90% of the time... you would hit the nail right on the head and solve the No Charging Condition. But to be absolutely sure, I suggest doing two more very simple tests. For this first one, Go to ALTERNATOR TEST 2.

Ford Vehicles:

  • Aerostar 4.0L
    • 1993, 1994, 1995, 1996, 1997
  • Explorer 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003
  • Ranger 4.0L
    • 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003

Mercury Vehicles:

  • Mountaineer 4.0L
    • 1997, 1998, 1999, 2000, 2001, 2002, 2003

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