How To Bench Test Your Car Alternator (2023)


How To Bench Test Your Alternator,

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Do you have an old alternator that you like to test just kick around the shop here? I'll show you how to do it? Okay.

So what we got here is an 82 for, for alternator came on 99 to 2006 GMC trucks, this great alternator, rated 140 amps.

The key thing about this alternator is that it gives you most of all your current at idle speed, so you don't have to rev the alternator or your engine up to 3000 to make a lot of current.

So all you need to test the alternator, so I got a plug here, factory plug it's a four pin plug.

Only three of them are used.

The only one you need to worry about is the brown wire, so this plug essentially there's letters associated to the plugs plug ports.

So you got PL is so pob for it's mostly used on diesels.

It'll, be essentially a rev counter.

So this is how your diesel vehicle that tachometers hooked up it's hooked up through this wire.

This came off a gas, a gas motor.

So this wire wasn't used this here's the L it's for the lamp, so your charging indicator dummy lamp.

This was this wires for PL I.

This is this will be usually an ignition wire, so hooked up to 12 volts, mostly in the pulse-width modulation, newer trucks, where the ECU controls the field of current.

This wire will be used.

This wire here actually GM that didn't use it that CS wire sensing wire.

So if you want to account for a voltage drop, you would run a wire here to wherever your voltage really matters and then that'll count for the voltage drop.

So the only wire you're concerned about is the brown wire here.

That'll be the lamp wire.

This is the only hookup you need to test the alternator.

So I got my voltage meter here, hooked up to the battery, we'll turn it on okay, so we're at twelve point: five: three volts just push it to tears.

You guys could see.

Okay, twelve point: five, three volts now I've got a light bulb here.

So get yourself.

A low five watt light bulb run a jumper wire from the brown wire to one side of the light bulb.

The other jumper wire I got going to the switch.

This is just for so that I could control when it's going to charge not, but you don't need to hook it up to this switch and then go to the positive post on the alternator.

Do not hook this wire up to the positive post directly without a load, because you will fry the voltage regulator, it needs to have a load on it or in series with it.

Okay, so give yourself a drill with a I got a thirty one.

Thirty seconds socket on here.

So right now, I got my switch off so nothing! Oh! It's not going to charge when I spin, the pulley here so we're at twelve point five: three volts so I'm gonna start spinning the pulley nothing's changing okay.

So now I'm gonna flick, my switch okay.

So this is your dummy light.

So this is telling you the alternators, not charging now you'll notice, as they start turning this pulley.

That light will go dim until it's all the way out and that's when it'll start charging.

So let's try it here: okay, I'm, not spinning it fast enough.

Let me kick it in the second gear here: okay, I'm not able to spin it fast enough.

Let me get another battery.

You need to be spinning us pulley at at least 1500 rpm.

So we may have a hard time but we're at twelve point three volts I'm.

Turning a switch on go little batteries at twelve point, one, zero or two so we'll start spinning it not charging, not charging you'll notice.

If I release the trigger the light will come back on saying it's not charging.

Now, if I turn that switch off listen for the the speed of the motor on the drill here, it'll really slow down once I flicked a switch at high speed showing that the alternator is putting a load on the drill, because it's actually creating current generating current thoughts.

Full power on the drill, I'll flick, the switch, it's really giving her.

No click, the switch, no longer charging drills going at full speed.

Okay so that's essentially how you, test the alternator like I say.

All you need is one wire, the brown wire while GM uses Brown, but it's essentially the El wire, which is light indicator light.

So take a jumper from that wire.

Go to your bulb again make sure you got if you don't have a bulb, get at least a 50 ohm resistor put it in series with these two jumpers, otherwise, like I, say you'll fry the voltage regulator, so the other end of the bulb when my case goes to switch, but that's not necessary.

The other end of the bulb will go to your positive ignition source.

Now these newer GM's, they don't have a incandescent bulb.

They actually have.

This goes to your computer, so the computer will ground this bulb on your dash, telling you that you're not charging, but they use pulse width, modulation, modulation through this wire here, so that's essentially sine waves and that will regulate the field of current that this alternator will output, but on an older truck like say a 98 or whatever.

Then all you need to run this alternator again.

Is this one brown wire one more thing: I wanted to touch on that I forgot to mention.

Is these newer trucks like the GMT, 900 platform, 2011 and up and stuff they? They really use a pulse width, modulation to actually vary the voltage output of the alternator, so at idle they'll develop, put maybe thirteen point two volts just so they don't put a huge load on the motor and on highway or it ever, though, output I think it's even twelve point eight volts or something it's very low.

Now this this is okay for the average user, but if you're running car II all car, audio and stuff you're not gonna like this.

So what I've heard is this pulse which pulse width, modulation wire if it's senses and open? So if you cut this wire, the alternator will run full field, so it'll default - its 14 and a half volts.

So that's very common in the car audio department.

If you're like I, say if you're running subs and stuff, you don't want that twelve point, eight volts going to your subs cut this wire.

You should have 14.5 volts all the time, so I got a different battery hooked up now.

The other one was dead, which is probably why it didn't want charged all that good.

So, let's just do a quick, we'll turn it again and see how many volts we could get bulb on okay, 12 point 7 volts and see your belt started slipping then your lights would come on there.

She goes yeah so that other battery was just straight dead.

So that's why it was taking up so much current that the I couldn't spin the alternator fast enough to get some current flowing there Makua maintaining the voltage.

So there you go.

We got 14 volts out of her again.

I'm, probably not spinning it quite fast enough, but if I had it on the car, it would spin much faster again hope you guys enjoy this video and.


What is the best method of testing a bad alternator? ›

Simply check the voltage of your battery by touching the multimeter prongs to the terminals with the car shut off. Take note of that number; it should be somewhere in the 12-to-13-volt range, according to Hines. Then, start the car and check the voltage at the battery again while the vehicle is running.

Can an alternator bench test good but still be bad? ›

Keep in mind, just because the Alternator tests fine, does not mean the system is fine. The charging system is composed of wiring that not only supplies the battery and system with DC power, it also “senses” the voltage that's out in the system in order to properly set the voltage leaving the alternator.

Can you load test an alternator? ›

The voltage should read a minimum of 13 volts. A good alternator should put out between **13.5-14.5 volts. Stress test the alternator - Place a load on the alternator by turning on the headlights, the radio and the air conditioning. The voltage should remain high with these circuits on.

How does a mechanic check an alternator? ›

How mechanics test the alternator. Mechanics uses a computerized charging system tester to check your alternator. This is called an AVR test, and it can show if there's a weakness in the charging system, or if you'll need to repair or replace your alternator.

Can you test an alternator without installing it? ›

One way is to use a voltmeter. If the alternator is not outputting enough voltage, then it is most likely bad. Another way to test the alternator is to start the car and then turn on all of the lights. If the lights dim, then that means the alternator is not charging the battery properly.

How many volts should a car alternator put out? ›

A good alternator should maintain battery voltage between 13.9 and 14.8 volts (14.2 is optimum). Even worst-case, with all accessories turned on, there should be at least 13 volts at the battery.

How to check for bad alternator by disconnecting the battery? ›

Disconnect the negative battery terminal

A good alternator will keep the engine running after you've disconnected the negative battery terminal, but if the engine performance decreases, stalls, or dies out your alternator may be faulty.

Does AutoZone bench test alternators? ›

Testing will ensure you identify a power issue correctly. How to get your alternator tested: Just visit a nearby AutoZone. We'll test the alternator while it's still in the vehicle.

What part most commonly fails in an alternator? ›

One of the most common problems you're likely to experience with an alternator is a failure in the bearings. There are needle bearings in the alternator that allow the rotor to freely spin inside the housing, and those bearings can break down over time as a result of exposure to heat and dirt.

Why is my alternator only putting out 13 volts? ›

For some alternators, it's normal to see as low as 13 volts. For some, a constant 13.6 volts is an indication of a problem. For others, it's normal to see as high as 16 volts for long stretches. Still, it's perfectly normal for others to see the alternator not charge at all intermittently.

Does O Reilly bench test alternators? ›

Our experienced parts professionals can test your starter or alternator while it's still on your vehicle. If your vehicle won't start and you believe the starter or alternator may be the culprit, simply bring the part to an O'Reilly Auto Parts store near you and we would be happy to bench test it for you.

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