Home testing an alternator?

[AJMBLAZER]

My buddy's cars are giving me fits again.

'98 Kia Sephia. We put in a junkyard alternator that worked and tested fine a month or so ago. Had the original battery in it that didn't test well but worked fine. I told them to replace it but various issues prevented this.

Today his wife texts me in the morning saying it died on her completely on the way to work.
He went out and got a new battery, dropped it in, and the car started right up.

I'm going to check it out tomorrow but have had bad luck with the local places testing for alternators and batteries. Someone told me the "home test" for a dead alternator was to pull the battery cables while the car was running and if it ran the alt was good, if it died the alt was bad. Is this correct?

Don't neglect your main vehicles folks. Your car guy friends will thank you.

 

[bradshawtech]

That is a quick test that works. A voltmeter will let you test many other things.

As you do this test try not to have too many electrical accessories on and running at the time of the test. The only load on the alt being the fuel pump and the ignition and computer.

 

[AJMBLAZER]

I've got a basic multimeter and know how to get the 12v reading out of it. That's about it.

 

[bradshawtech]

Well you can check the battery voltage before starting the car and then verify that the alt is charging after starting it with at least a voltage of 13.5 or more.

The static battery voltage will be somewhere around 12.6 or more

 

[dremu]

I've got a basic multimeter and know how to get the 12v reading out of it. That's about it.

That's all you need. I don't like disconnecting the battery while the motor is running as it can lead to spikes on the electrical system and you'll end up frying something, either an expensive computer or the alternator.

Check the battery voltage with the key off, should be about 12V. Then start the car; the voltage should go up a bit at idle, maybe 12.5, 13V. At WFO the alternator should be putting out its max and you should be close to 14V.

The numbers aren't critical so much as just that the voltage goes up with the alternator spinning, which means it's charging. If the voltage stays low, then the alternator *isn't* charging -- check belts and wiring.

Car that new, the battery cables shouldn't be corroded through like we get on our 70's and 80's trucks, but undo the battery connections, make sure they're clean (shiny), and tighten 'em.

I spent two days chasing a loose negative connection once... LOOKED fine, and would pass enough current to turn on the dome light, but when you tried to crank the motor, the heat would bend the terminal or something and it would go loose for a few hours. Drove me nuts until I wiggled the cable, and then had that sinking "oh sh!t you dork" feeling. Three-quarters of a turn with the 5/16" ratcheting wrench and voila, the thing starts.

-- A

 

[AJMBLAZER]

Okay, I'll give that a try.

I learned a bit about batts fixing that Miata last month so I'm not entirely clueless on this stuff now. I am however realizing I can replace parts but not diagnose much.

 

[dremu]

Okay, I'll give that a try.

I learned a bit about batts fixing that Miata last month so I'm not entirely clueless on this stuff now. I am however realizing I can replace parts but not diagnose much.

Ah, charging systems are simple -- voltmeter and the old "visual inspection" on the cables covers 90% or more of failures.

Now if I just remember that "visual inspection" includes jiggling the cable with my hands and not JUST looking at it...

-- A

 

[AJMBLAZER]

I'm thinking with this thing it's the ancient battery killed the worked good but used alternator. Alternator tested fine and seemed to work well but we knew the battery was the weak link.

I think that link broke finally.

 

[Fordum]

Please don't pull the battery cable with it running. That can and will go wrong so many ways you would not belive.
That way was fine with simple points and carbs, but not anymore.

Fastest way to check an alt. is like they said. If the battery is decently charged, and not shorted, then you should see about 14.5V across the battery terminals with it above fast idle.

If the battery is almost dead, then the voltage might be as low as 13. Depending on how stout the alternator is.

A bad battery can easily cook a good alternator.

Don't forget to carefully touch the alt belt pulley with the engine off after it has run for a while.
If you hear a sizzle and your finger tends to stick, then the belt is slipping.

You can check on bad connections using your voltmeter to do a voltage drop test. If you don't find anything else wrong, let me know and I will talk you through it if you don't know how.

J.

 

[Blue85]

I like to put a current clamp on the alternator wire. You can also measure current with a shunt, like by buying an old school ammeter.

 

[AJMBLAZER]

Cool, thanks guys. Will let you know.

 

[AJMBLAZER]

Hahaha, well, that FAILed but still, I'll keep visiting these places.

 

[AJMBLAZER]

There are men in Yoga studios...









...gay men.

 

[AJMBLAZER]

I get target fixated with my hobbies. I'd be playing trucks and women be damned.

Unless a woman was there and actually interested, then I'd pay attention to her.

Too many "dragged along" experiences with my ex's.

 

[Greg72]

Don't forget to carefully touch the alt belt pulley with the engine off after it has run for a while.
If you hear a sizzle and your finger tends to stick, then the belt is slipping.

J.

Good one!

 

[AJMBLAZER]

That's all you need. I don't like disconnecting the battery while the motor is running as it can lead to spikes on the electrical system and you'll end up frying something, either an expensive computer or the alternator.

Check the battery voltage with the key off, should be about 12V. Then start the car; the voltage should go up a bit at idle, maybe 12.5, 13V. At WFO the alternator should be putting out its max and you should be close to 14V.

The numbers aren't critical so much as just that the voltage goes up with the alternator spinning, which means it's charging. If the voltage stays low, then the alternator *isn't* charging -- check belts and wiring.

Car that new, the battery cables shouldn't be corroded through like we get on our 70's and 80's trucks, but undo the battery connections, make sure they're clean (shiny), and tighten 'em.

I spent two days chasing a loose negative connection once... LOOKED fine, and would pass enough current to turn on the dome light, but when you tried to crank the motor, the heat would bend the terminal or something and it would go loose for a few hours. Drove me nuts until I wiggled the cable, and then had that sinking "oh sh!t you dork" feeling. Three-quarters of a turn with the 5/16" ratcheting wrench and voila, the thing starts.

-- A

Please don't pull the battery cable with it running. That can and will go wrong so many ways you would not belive.
That way was fine with simple points and carbs, but not anymore.

Fastest way to check an alt. is like they said. If the battery is decently charged, and not shorted, then you should see about 14.5V across the battery terminals with it above fast idle.

If the battery is almost dead, then the voltage might be as low as 13. Depending on how stout the alternator is.

A bad battery can easily cook a good alternator.

Don't forget to carefully touch the alt belt pulley with the engine off after it has run for a while.
If you hear a sizzle and your finger tends to stick, then the belt is slipping.

You can check on bad connections using your voltmeter to do a voltage drop test. If you don't find anything else wrong, let me know and I will talk you through it if you don't know how.

J.

Okay. After sitting 17 hours the battery still showed 12.57v according to the multimeter. At idle it climbed up to around 14.4-14.5v. With the gas mashed it actually droops a bit down to around 14.3somethingv.

Seem okay?

We're heading down to AutoZone to get the alt tested and swap this batt for the correct one. Dumb ass counter kids.

 

[Fordum]

14.5 will keep any good battery charged.
The droop when you mashed the gas is either a regulator artifact that I would not worry about, because that it still plenty high enough to keep it charged, or belt slippage which you need to check.

If its serpentine, then check the belt for hardening, cracks, and the tensioner for strength. If they all look good, then I would not worry about the voltage droop.

 

[Fordum]

Good one!

That was a test procedure I wrote for Compuserve for a thread called Unorthodox Test Procedures.

If I remember it, it went something like this:

Hold car at 1/4 throttle for 5 minutes.
Gently touch pulley.

If pulley is cold, start car and hold 1/4 throttle for 5 minutes and retest.

If pulley is warm, belt is tight enough.

If pulley is hot, belt may be slipping slightly, or heavy load on alt.

If a red mark appears instantly, belt is slipping, but can probably be reused.

If a blister appears, belt is slipping badly, and should be inspected for damage before reuse.

If you hear a sizzle, and finger sticks to pulley, belt will be damaged, and should be replaced after prying finger loose.

If finger flys off hand, turn off engine, and retest using alternate finger.

I wish I had some of the others. They were quite good. There was usually an injury involved in each procedure.
J.

 

[AJMBLAZER]

14.5 will keep any good battery charged.
The droop when you mashed the gas is either a regulator artifact that I would not worry about, because that it still plenty high enough to keep it charged, or belt slippage which you need to check.

If its serpentine, then check the belt for hardening, cracks, and the tensioner for strength. If they all look good, then I would not worry about the voltage droop.

Sounds good. The new belt loosened up after the semi-epic water pump swap last spring and I think got a bit glazed. Starting to squeal a lot yet it's tight. Think that'll just be a new belt.

Went to AutoZone and got him the correct battery and had them test it. Got a guy who actually knew wtf he was doing and the alt passed the tests with flying colors.

 

[dremu]

That was a test procedure I wrote for Compuserve for a thread called Unorthodox Test Procedures.

If I remember it, it went something like this:

Hold car at 1/4 throttle for 5 minutes.
Gently touch pulley.

If pulley is cold, start car and hold 1/4 throttle for 5 minutes and retest.

If pulley is warm, belt is tight enough.

If pulley is hot, belt may be slipping slightly, or heavy load on alt.

If a red mark appears instantly, belt is slipping, but can probably be reused.

If a blister appears, belt is slipping badly, and should be inspected for damage before reuse.

If you hear a sizzle, and finger sticks to pulley, belt will be damaged, and should be replaced after prying finger loose.

If finger flys off hand, turn off engine, and retest using alternate finger.

I wish I had some of the others. They were quite good. There was usually an injury involved in each procedure.
J.

The human version of the redneck torque wrench -- turn until it breaks, then back off half a turn. That's the correct torque

-- A