I know you can check an alt. w/ a meter. How??

i believe u put the black wire from the meter on the negative post and then the postive wire from the meter on the postive wire on the back of the alternator... and it should read around 14.1 volts i believe... what kinda vehicle is this for anyways..

i believe u put the black wire from the meter on the negative post and then the postive wire from the meter on the postive wire on the back of the alternator... and it should read around 14.1 volts i believe... what kinda vehicle is this for anyways..

1999 XJ, 2.5L, manual

shin is right.

You don't need a meter just to "pass/fail" it though. Start the engine and turn the headlights on. Take a screwdriver or pocketknife or something metal and touch the back of the alternator, right where the rear bearing is. Don't acidentally short any wires. The harder your alternator is working, the more magnetic that spot will be. If it is not magnetic enough for you to feel, your alternator isn't doing anything. Just a quick check FYI...

EDIT: Apparently what I said is wrong. Oops, sorry,

Ryan

You can also start the car, and then unhook the poistive terminal of the battery (be careful to not touch anything with it). If the car stays running, the alternator is working, if it dies, the alternator is not. You can take this a step further and check the charging voltage while the battery is unhooked as well. In order to check charging amp, the only way I have done it is with a clamp type (inductive) amp meter. You need to be able to apply a load to the system in order for this to be effective though.

Ryan

Please don't do this. It used to work DECADES ago when cars had generators. If you do this with today's cars, the least you'll do is probably damage the alternator; the worst is far worse.

Don't do it.

Take it to one of the Auto-Zone/Advance Auto type places if you have to, they'll check it for free on the car.

Good luck.

Al

Please don't do this. It used to work DECADES ago when cars had generators. If you do this with today's cars, the least you'll do is probably damage the alternator; the worst is far worse.

Don't do it.

Take it to one of the Auto-Zone/Advance Auto type places if you have to, they'll check it for free on the car.

Good luck.

Al


AND,you can also spike the computor by doing this. NOT RECOMMENDED.

or the easy way woule be to check for more than 12 volts. your battery should show no more than 12.1-12.2 volts MAX when the car/truck is not running. if it' sup around 13.5-14 wjile running then your alternator is charging.

-Scott

You can also start the car, and then unhook the poistive terminal of the battery (be careful to not touch anything with it). If the car stays running, the alternator is working, if it dies, the alternator is not. You can take this a step further and check the charging voltage while the battery is unhooked as well. In order to check charging amp, the only way I have done it is with a clamp type (inductive) amp meter. You need to be able to apply a load to the system in order for this to be effective though.

Ryan


I work at a parts house......

This is an urban myth on newer vehicles. I don't know who started it or where, but you don't know how many people do this and say that they tested their alternator. PLEASE DON"T DO IT PEOPLE.

Glad someone has mentioned taking the battery cables off is a very, very bad thing. Did you know you also do the same amount of damage boosting. Not as likely but still very possible.
Batteries when the vehicle is off should have 12.72 to 13volts in them. 12.5 volts is considered to be 50% discharged in a start battery. If a battery is really old it might drop to 12.6 volts at rest. When you first shut a vehicle off it has a surface charge in the battery, you need to turn your headlights on for about 1 minute to remove this charge before testing.
When the vehicle is running an alternator needs to have a minimun 13.2volts to be doing anytihng. It is between 13.2 and 13.8 it is either working way harder than it was designed to or it is going bad. 13.8 to 14.1 is an alternator that is working hard. 14.1 to 14.4 in an import vehicle it is an alternator that is working well and all should be good. In a domestic vehicle it is an alternator that is near its max output at that rpm. 14.4 to 15.0 volts is a domestic alternator that is working well within its range. 15.1 to 15.3 is an alternaotr that is over charging and can boil a battery depending on the batterys cemistry.

or the easy way woule be to check for more than 12 volts. your battery should show no more than 12.1-12.2 volts MAX when the car/truck is not running. if it' sup around 13.5-14 wjile running then your alternator is charging.

-Scott

Best answer so far. Around 12v with engine off.

Should be higher when running. Alternator is putting out enough current to run everything. If the alt is not working it will be lower.

Straight to the point for Newb tech :flipoff2:

deleted link, I posted one that wasn't any use

Dave

Best answer so far. Around 12v with engine off.

Should be higher when running. Alternator is putting out enough current to run everything. If the alt is not working it will be lower.

Straight to the point for Newb tech :flipoff2:
If your battery is around 12volts, i.e. 12.2volts or lower, it is considered dead if it is a starting battery. DC electricity is very easy to work with most of the time. The reason why it is so hard for mechanics and nonmechanics to grasp is the lack of information.

Lets use these posts as an example. You have a problem, and test the battery and it reads 12.0 volts. So you think it is good. Then you start the truck up and test it again and it reads 13.2 volts. So now you think your alternator is good. So you get in drive down the road. 30 minutes later your truck starts to chug and run rough. Now you are not near home so you have to pull into the closest garage. He comes out and checks things properly. Comes back and tells you that your battery was bad and your alternator fried trying to recharge it. That you should have changed the $60 battery, but now you are getting a $300 bill for a battery and new alternator. OR he says it looks like your alternator was bad and it boiled your good battery. Same $300 bill.

I am not trying to pick on anyone, I have class A mechanics that would say the exact same thing. Like I said electrical is easy if done properly, but it is just that it has to be done percisely.

Take it to one of the Auto-Zone/Advance Auto type places if you have to, they'll check it for free on the car.

Good luck.

Al
The NAPA near me has a fancy computerized tester that checks the internals before it even spins the alt. I like it better than the generic on at AZ

If your battery is around 12volts, i.e. 12.2volts or lower, it is considered dead if it is a starting battery. DC electricity is very easy to work with most of the time. The reason why it is so hard for mechanics and nonmechanics to grasp is the lack of information.

Lets use these posts as an example. You have a problem, and test the battery and it reads 12.0 volts. So you think it is good. Then you start the truck up and test it again and it reads 13.2 volts. So now you think your alternator is good. So you get in drive down the road. 30 minutes later your truck starts to chug and run rough. Now you are not near home so you have to pull into the closest garage. He comes out and checks things properly. Comes back and tells you that your battery was bad and your alternator fried trying to recharge it. That you should have changed the $60 battery, but now you are getting a $300 bill for a battery and new alternator. OR he says it looks like your alternator was bad and it boiled your good battery. Same $300 bill.

I am not trying to pick on anyone, I have class A mechanics that would say the exact same thing. Like I said electrical is easy if done properly, but it is just that it has to be done percisely.


He asked for an easy way to check the alt with a voltmeter. Just wanted to try to keep it simple.

Generally that will get you by.

Not busting your balls, just throwing it out as food for thought. When you have a basic understanding of electronics, it is easy to forget not all people do. Like we keep talking about voltage when an alt is really a current source.

I raised three daughter that all thought they were a good place to put a knee when getting up on my lap. So no worries about busting them, LOL.

I understand what you are saying and what you are trying to do. Over the years i have found that someone will take info like that and think they are now a mechanic. (not pointing fingers or saying someone here, just in general). It is my opinion DC is an all or nothing thing, if you are not prepared to to go all the way, then you should leave it alone.

That is why I try to explian things more in depth. When I posted the ranges I consider that the minimum, or newbie type, knowledge. But you are right in that I sometimes can forget that there is also a lack of base knowledge. Heck it has only been a few months since I had a class A in my shop explaining to me he took a battery cable off while it was running.

About the alternators. I will have to say I dont know. I have tested them with only voltage and no amperage coming out of them. But I have never really thought about it before. What increases the voltage in the system if it isnt the alternator. You now have me curious.

I have never heard that taking the battery out of the system hurt anything, but now that I think about it, I can certainly see the potential for problems. My post above is edited, sorry for the confustion.

Ryan

For an in vehicle check make sure that you turn on a few accessories when checking to see if the alternator is working. With a single open diode it is possible for an alternator to show enough voltage, with no load on it, to make you think that it is within spec. It is also important to check the battery or batteries in order to properly evaluate a vehicle charging system.

Gus

When the car is running, you'll want to turn on your heater on full, turn on your brights, rear defrost if you got it, etc. to see if the alternator can keep up with the load that's demanded of it. It may be putting out 14.1 volts or whatever, but that doesn't mean that it is putting out enough amps.

It may be putting out 14.1 volts or whatever, but that doesn't mean that it is putting out enough amps.

Actually yes it does.

Actually yes it does.
suepahfly is correct, if the battery is FULLY charged, and no loads on it (the blower,headlights, etc), even with the alt only putting out 15 amps, it could still read 14+ volts easy.

A fully charged battery is close to an impossibility if the alt is bad though, so VERY unlikely to have 14 volts with low amps.

suepahfly is correct, if the battery is FULLY charged, and no loads on it (the blower,headlights, etc), even with the alt only putting out 15 amps, it could still read 14+ volts easy.

That is why you load it. The volts would drop to below engine off voltage if the alt is not keeping up.


A fully charged battery is close to an impossibility if the alt is bad though, so VERY unlikely to have 14 volts with low amps.

More real world.

Remember this was an easy test to quick check the alt with a voltmeter.