How To Test The Alternator (1997-2003 4.2L V6 Ford F150, F250)

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.

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

APPLIES TO: This tutorial applies to the following vehicles:

• 4.2L V6 Ford F150: 1997, 1998, 1999, 2000, 2001, 2002, 2003.
• 4.2L V6 Ford F250 Light Duty: 1998.

Wiring Diagrams: The following wiring diagrams are available:

• Charging System Circuit Wiring Diagram (1997-1998 4.2L V6 Ford F150, F250 Light Duty).
• Charging System Circuit Wiring Diagram (1999-2003 4.2L V6 Ford F150, F250).

E-SERIES VANS: The alternator test tutorial for the 1997-2003 4.2L V6 Ford E150/E250 vans can be found here:

• How To Test The Alternator (1997-2003 4.2L V6 Ford E150, E250) (at: troubleshootmyvehicle.com).

Symptoms Of A Bad Alternator

Every time you start the engine on your Ford pickup 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 pickup 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 pickup's instrument cluster.
2. Whenever you turn on the headlights (night driving), they glow very dim.
3. The engine won't crank. It'll only crank and start if you jump start the engine.
4. The only way the engine 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 pickup 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 pickup? 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 pickup's specifics to find you the right parts.

TEST 1: Battery Voltage Test With Engine Running

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 the engine and let it idle.

2. 2

   Select Volts DC mode on your multimeter.

3. 3

   Check the battery's voltage with your multimeter.

4. 4

   The multimeter should register 13.5 to 14.5 Volts.
   
   If it doesn't, don't worry about this just yet, continue to the next step.

5. 5

   Put an electrical load on the alternator.
   
   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.

6. 6

   The multimeter's voltage reading will decrease slightly and then stabilize around 13.5 to 14.5 Volts DC as each accessory comes on.

7. 7

   Let the engine run for about 5 minutes with all of the accessories on.

8. 8

   The battery voltage should remain between 13.5 to 14.5 Volts DC with all of the accessories on during the whole 5 minute duration.

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 pickup 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.

TEST 2: Checking Alternator's Battery Output Circuit

As you're probably already aware, the charge that the alternator creates reaches the battery across a cable. This cable is connected to a stud on the back of the alternator via a nut. In the illustration above the red multimeter test lead points to this stud.

We need to make sure that the fuse that protects this circuit isn't blown. So what we're going to do in this test section, is make sure that this circuit has continuity.

If the fuse is blown, your multimeter will show no continuity (usually by showing you the letters OL). If the fuse is OK you'll see continuity.

This is what you'll need to do:

1. 1

   Disconnect the battery negative (-) cable from the battery but leave the positive (+) cable connected to the positive (+) post.
   
   IMPORTANT: Do not proceed to the next steps until you have done this first.

2. 2

   Set your multimeter to Ohms mode.

3. 3

   Connect the red multimeter test lead to the stud shown in the photo above.
   
   A BLK/ORG cable connects to the stud indicated by the arrow (in the photo above).
   
   NOTE: Don't disconnect the BLK/ORG cable from the alternator.

4. 4

   Connect the black multimeter test lead on the battery positive (+) terminal (at the battery).
   
   The battery negative (-) wire must remain disconnected from the battery.

5. 5

   Your multimeter will register one of two values:
   
   1.) Continuity (usually an Ohms value of about 0.5 Ohms).
   
   2.) No continuity (an infinite Ohms reading (OL)).

Let's take a look at what your test results mean:

CASE 1: Your multimeter confirms that the circuit has continuity. This test result tells you that the mega fuse or inline fusible links are OK.

In other words, if the alternator were working and charging, all of its charge would actually reach the battery.

Your next step is to check that the alternator's voltage regulator is getting power. For this test go to: TEST 3: Making Sure The Voltage Regulator Is Getting Power.

CASE 2: Your multimeter confirms that the circuit DOES NOT HAVE continuity. This test results let you know that the alternator's charge is not reaching its destination -the battery.

Your next step is to check the the 175 amp mega fuse (1997-1998 pickups) or the 12 gauge in-line fusible links (1999-2002 pickups) and replace if necessary to resolve the alternator's no charge problem.

On the 1997-1998 pickups, you can see the location of the 175 Amp mega-fuse here: Location Of The Alternator 175 Amp Mega-Fuse (1997-1998 Pickups).

On the 1999-2002 pickups, the 12 Gauge grey inline fusible links are located at the starter motor relay.