You can very accurately test the alternator on your 1994-1997 2.2L Chevy S10 (GMC Sonoma, Isuzu Hombre) with a simple multimeter.
In this tutorial, I'll show you three specific tests that will tell you if the ‘no charge' problem on your S10 (Sonoma, Hombre) is due to a bad alternator.
Contents of this tutorial:
Symptoms Of A Bad Alternator
Your S10's (Sonoma's, Isuzu's) alternator is tasked with two specific tasks. The first is to charge the battery, as the engine runs, so that the next time you crank and start your S10 (Sonoma, Hombre) pickup, the battery has the capacity stored in it to get the engine going.
The other is to provide the electrical current for everything that requires it (examples: fuel pump, ignition coils, A/C compressor, radio, blower motor, headlights, etc.), as you're driving down the road.
You'll usually notice one or several of the following symptoms, when your S10's (Sonoma's, Hombre's) alternator fails:
- The charge light (also known as the battery light) will be shining nice and bright on your pickup's instrument cluster.
- Whenever you turn on the headlights (night driving), they glow very dim.
- The pickup won't crank. It will only crank and start if you jump start your pickup.
- The only way the pickup cranks and starts is if you charge the battery.
- The pickup stalls and won't crank (unless you jump-start it).
With this info under our belts, let's get our first test under-way.
TEST 1: Battery Voltage Test With Engine Running
The first thing we'll do is to whip out a multimeter and see what the battery voltage is with the S10's (Sonoma's, Hombre's) engine running.
The battery voltage, with the engine running and a good alternator, should be 13.5 Volts to 14.5 Volts.
If the battery voltage (with your S10's engine running) is at 12.5 Volts or below, then you'll know that the alternator is not working and thus not charging the battery.
NOTE: You'll need to make sure that the battery has enough of a charge to keep your S10 (Sonoma, Hombre) pickup running for at least 5 minutes to do this test. If the battery is completely dead, charge it up enough so that it can crank and start the engine.
These are the test steps:
Crank and start your Chevy pickup and select Volts DC mode on your multimeter.
Probe the positive battery terminal with the red multimeter test lead.
With the black multimeter test lead, probe the negative battery terminal on your Chevy pickup's battery.
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.
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.
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.
2.) It will register 12.5 V DC and this voltage will decrease more and more as you turn on stuff inside your Chevy vehicle.
OK, let's interpret your multimeter test results:
CASE 1: Battery voltage with the engine running was 13.5 to 14.5 Volts. This voltage test result tells you that the alternator is working (charging the battery).
No further testing is required, since this multimeter test result eliminates the alternator on your Chevy pickup as bad.
Now, if you're having to jump-start the pickup to get it going, 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 pickup, for example: a dome-light) and draining the battery while the engine is off.
CASE 2: Battery voltage with the engine running steadily dropped down to 9-10 Volts. This is a clear indication that your Chevy pickup's alternator IS NOT charging the battery.
Replacing the alternator at this point usually solves around 90% of the No Charge conditions on any 2.2L Chevy pickup around the world. That's right, you could stop testing here and say: ‘The alternator is fried' and be done but..
I suggest two more easy tests to be absolutely sure it is bad. For the first test of the two, go to: TEST 2: Checking The Continuity Of The Bat (+) Cable.