How To Test The Alternator (2.8L S10 Blazer, 2.8L S10 Pick Up, 2.8L S15 Pickup, and 2.8L S15 Jimmy)

Sooner or later, the alternator on your 2.8L equipped S10 Pick-up (S10 Blazer, GMC S15 Pick-up, or GMC S15 Jimmy) is gonna' fail. Thankfully, testing it is a pretty easy and straight-forward process. In this tutorial I'll show you just how easy it is.

Puedes encontrar este tutorial en Español aquí: Cómo Probar El Alternador Con Multímetro (2.8L V6 GM) (en:

Symptoms Of A Bad Alternator

The alternator is one of the most important components your S10 needs to start and keep and stay running. What? Why? Well, it's job pretty much boils down to two important things:

  1. Charge the battery after it has cranked the engine.
  2. Provide all of the current the vehicle needs (think: headlights, fuel pump, ignition coil, radio, wipers, etc.).

So, when the alternator fails in your 2.8L equipped S10 Pick-up (Blazer), you'll notice 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 S10/S15's instrument cluster.
  2. Whenever you turn on the headlights (night driving), they glow very dim.
  3. The engine won't crank. It will only crank and start if you jump start it with another vehicle.
  4. The only way the car cranks and starts is if you charge the battery.
  5. The idle may get high when you come to a stop.

Thankfully, testing the alternator is not hard to do at all and in this tutorial I'll show you how.

TEST 1: Battery Voltage Test With Engine Running

How To Test The Alternator (2.8L S10 Blazer, 2.8L S10 Pick Up, 2.8L S15 Pickup, And 2.8L S15 Jimmy)

The very first thing we need to do is to start the engine and test the battery's voltage with the engine running. So, if the engine doesn't start, due to a discharged/dead battery, you'll need to charge up the battery.

The battery must be charged enough to keep your S10's (or GMC S15's) engine running for at least 10 to 20 minutes otherwise you won't be able to perform TEST 1 or TEST 2 of this tutorial.

OK, what we're looking for, with this battery voltage test while the engine is running, is to see if the voltage value is at 13.5 Volts to 14.5 Volts. If the battery voltage value hovers around these values, then you can conclude the alternator is OK.

If the battery voltage is at 12.5 Volts (or less), with the engine running, then this test result would be a clear indication the alternator is not charging the battery.

These are the test steps:

  1. 1

    Crank and start the engine 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.

  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 S10/S15.

OK, let's take a look at your test results:

CASE 1: Your multimeter registered 13.5 to 14.5 Volts. This means that the alternator is charging the battery and thus it is not defective.

No further testing is required, since this multimeter test result eliminates the alternator on your 2.8L equipped S10 Pick-up (Blazer) as bad.

Now, if the battery is dead every morning, this tells you that you either have 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 is a clear indication that your S10/S15's alternator IS NOT charging the battery.

Replacing the alternator at this point usually solves around 90% of the cases, but I suggest that you go on to TEST 2 just to make sure. Your next step should be: TEST 2: Checking The Continuity of the Bat (+) Cable.

Chevrolet Vehicles:

  • S10 Blazer 2.8L
    • 1988, 1989
  • S10 Pickup 2.8L
    • 1988, 1989, 1990, 1991 1992, 1993

GMC Vehicle:

  • S15 Jimmy 2.8L
    • 1986, 1987, 1988, 1989

GMC Vehicles:

  • S15 Pickup 2.8L
    • 1986, 1987, 1988, 1989, 1990