How To Test The Alternator With A Multimeter (1994-1995 3.8L Ford Mustang, 1994-1995 3.8L Ford Thunderbird, 1994-1995 3.8L Mercury Cougar)

This simple tutorial will help you to diagnose the alternator, to find out if it's defective or not, on your 1994 through 1995 3.8L Ford Mustang.

You don't need more than a multimeter to be able to test the alternator and all the test steps are explained in a step-by-step manner.

NOTE: This tutorial applies to: 1994-1995 3.8L Ford Mustang, 1994-1995 3.8L Ford Thunderbird, and 1994-1995 3.8L Mercury Cougar.

Contents of this tutorial at a glance:

  1. Symptoms Of A Defective Alternator.
  2. TEST 1: Testing Battery Voltage With The Engine Running.
  3. TEST 2: Making Sure The Alternator's Battery Circuit Has Continuity.
  4. TEST 3: Making Sure The Alternator's Voltage Regulator Is Getting 12 Volts.
  5. Where To Buy The Alternator And Save.
  6. More 3.8L Ford Tutorials.

En Español You can find this tutorial in Spanish here: Cómo Probar El Alternador (1994-1995 3.8L Ford Mustang) (at: autotecnico-online.com).

Symptoms Of A Defective Alternator

Your Ford Mustang's alternator has two important functions. The first one is to recharge the battery so that it's ready to crank the engine the next time you need it to.

The second is to provide all of the current that the electric and electronic components need to be able to maintain your Ford Mustang running once the engine has been started.

So if the alternator fails (and therefore isn't charging the battery), you'll usually always see the battery light lit on your Ford Mustang's instrument cluster.

You're also going to see one or more of the following symptoms (when the alternator has failed and isn't charging the battery):

  1. The battery light (also known as the charge indicator light) will be eliminated 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 vehicle with another vehicle's battery.
  4. Your Ford Mustang starts only if you charge the battery first.

Alright, with this info let's go ahead and start with our first test.

TEST 1: Testing Battery Voltage With The Engine Running

How To Test The Alternator With A Multimeter (1994-1995 3.8L Ford Mustang, 1994-1995 3.8L Ford Thunderbird, 1994-1995 3.8L Mercury Cougar)

To successfully diagnose a defective alternator, the very first thing that we need to do is confirm if the alternator is charging the battery or not.

This is a super easy test that consists of reading the battery's voltage with the engine running. Your multimeter should register 13 - 14.5 Volts DC If the alternator is functioning correctly.

If the alternator isn't charging the battery, then your multimeter will generally read 12.5 Volts DC. These 12.5 Volts DC will gradually decrease the longer the engine stays running.

These are the test steps:

  1. 1

    Crank and start your 3.8L Ford Mustang 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 the battery on your Ford Mustang.

  3. 3

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

    1.) A steady 13.0 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 13.0 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: Your multimeter registered 13 to 14.5 Volts DC. This is the correct test result. You can now conclude that the alternator is charging the battery and therefore it's not defective.

CASE 2: Your multimeter registered a voltage of 12 Volts and this voltage decreased the longer the engine stayed running. This test result tells you that the alternator is not charging the battery.

In about 90% of the cases replacing the alternator will solve the problem. But I suggest you continue to the next test to make sure: TEST 2: Making Sure The Alternator's Battery Circuit Has Continuity.