How To Test The Alternator With A Multimeter (1998, 1999, 2000 2.5L Ford Ranger -Mazda B2500)

Testing the alternator on the 1998, 1999, and 2000 2.5L Ford Ranger (Mazda B2500) is a pretty simple thing to do.

In this tutorial I'll explain how to test the alternator with a multimeter in a step-by-step way.

You'll be able to easily find out if the alternator is defective or not.

Symptoms Of A Defective Alternator

Your 2.5L Ford Ranger's alternator is tasked with a job of providing the electrical current that your vehicle needs as it runs.

And there are quite a few things that need it like: the air conditioning system (which includes the A/C compressor, blower motor, etc), the radio, wiper motor, headlights, fuel pump, ignition system, etc.

Not too mention that it has to re-charge the battery to get it ready to crank and start the engine.

Being that the alternator is a critical component, here are some of the symptoms you'll see when the alternator fails:

  1. The battery light (also known as the charge indicator light) will be illuminated on the instrument cluster.
  2. The front headlights, when turned on, we'll shine very dimly.
  3. The engine won't crank. It'll only crank up if you jump start your Ford Ranger (Mazda B2500) with another vehicle's battery.
  4. Your 2.5L Ford Ranger (Mazda B2500) starts only if you charge the battery first.

TEST 1: Testing Battery Voltage With The Engine Running

Testing Battery Voltage With The Engine Running. How To Test The Alternator With A Multimeter (1998, 1999, 2000 2.5L Ford Ranger -Mazda B2500)

The first test that we're gonna' do is to check the battery's voltage with the engine running.

This test will let us know, right off the bat, if the alternator is charging your 2.5L Ford Ranger's battery (or not).

What we're looking for is for the battery voltage to be between 13 and 14.5 Volts DC (if the alternator is functioning correctly).

If the alternator is not charging your 2.5L Ford Ranger's battery, then you're gonna' see a voltage around 12.5 Volts DC which will diminish the longer the engine runs.

NOTE: If your 2.5L Ford Ranger doesn't start, then you'll need to charge the battery enought so that the engine can stay running for a minute or two while you check the battery's voltage.

Let's get testing:

  1. 1

    Crank and start your 2.5L Ford Ranger and select Volts DC mode on your multimeter.

  2. 2

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

    Connect the black multimeter lead to the negative battery (-) terminal on the battery.

  3. 3

    You should see 13.0 to 14.5 Volts DC on your multimeter's display (if the alternator's charging).

  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 13.0 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 3.8L Ford Mustang (3.8L Ford Thunderbird or 3.8L Mercury Cougar).

Let's interpret your multimeter voltage test result:

CASE 1: Battery voltage is between 13 to 14.5 volts DC with the engine running. This is the correct test result and let you know that the alternator charging the battery.

CASE 2: Battery voltage IS NOT between 13 to 14.5 volts DC with the engine running. This test result confirms that the alternator is not charging the battery.

The next step is to test the circuit that delivers the alternator's current to the battery is not ‘open’. For this test go to: TEST 2: Making Sure The Alternator's Battery Circuit Has Continuity.