How to test alternator

[fredsmedina]

At low RPM's(idling in drive), my stereo shuts off because it doens't have enough power. It doesn't do it all the time..but does it enough to be annoying. Also when the car is in park, and i open the door, which turns on the dome light, the stereo wil shut off.. Also when the car is off, and i turn a blinker on, the stereo will shut off..

So i'm thinking that I either have a bad battery, or an alternator thats going out.. How do i test the alternator? i'm thinking I am going to go to the local auto parts store tomorrow and have them test the battery

 

[bigdemo1]

Where is your stereo getting power from?On the back of your alt.there is a small silver coloured cap it's a bearing cover,while the car is idlong take anything magnetic and see if it sticks to this cap,DO NOT ANYTHING IN THE ALT.just to the center cap this will only tell you if the alt is good nothing else.

 

[fredsmedina]

Where is your stereo getting power from?On the back of your alt.there is a small silver coloured cap it's a bearing cover,while the car is idlong take anything magnetic and see if it sticks to this cap,DO NOT ANYTHING IN THE ALT.just to the center cap this will only tell you if the alt is good nothing else.

I haven't looked yet where its getting power from.. going to do that in the morning. I didn't have the problem anytime before though. It just started about the last week or so.. i've been driving it for a couple months just how it is though. Previous owner installed stereo, so this is why I am not sure where its getting power from..

 

[bigdemo1]

Is this in your dart?any progress on it?

 

[fredsmedina]

Is this in your dart?any progress on it?

its actually on my satellite. satellite is my daily driver right now.

Dont know if you've been following any of the progress on my dart, but it is turning into a bit more of a long term project. been hard with college and work. But right now, i'm in the process of building a chasis dolly so that I can roll by car outside and start some bodywork. everything is stripped right now..I have pretty much everything I need to get it finished, just need to find the time! Once I paint her, it should get finisheed pretty quickly

 

[169dart4y]

Take it to kregan, they will test the alternator, load test the battery and tell you how many cold cranking amps the battery is putting out. That way you will know what it is doing before you change anything. Good luck

 

[fredsmedina]

Take it to kregan, they will test the alternator, load test the battery and tell you how many cold cranking amps the battery is putting out. That way you will know what it is doing before you change anything. Good luck

thanks..i fugured kragens would know how to test it all..

just didn't know if there was anything i could do on my own with my voltage meter..

 

[mpgFanatic]

At low RPM's(idling in drive), my stereo shuts off because it doens't have enough power.

Wait, careful about making an assumption as to the "because", until you've tested and traced a couple more more items. You may be right, but the process of thinking through the diagnosis can easily be tainted if you already think you know what the problem is.

So i'm thinking that I either have a bad battery, or an alternator thats going out.. How do i test the alternator?

A voltmeter will tell you mountains. Attach to cigarette lighter, or any fusebox terminal-- the basic system voltage should be the same no matter where you test. 12.6V indicates a fully charged battery, engine off. With engine running, a functioning alternator should provide at least 13.5, probably 13.8, maybe even low 14s. If the battery has been really run down recently such that the alternator is doing a great job of bringing it back up in a hurry, mid 14s isn't unreasonable.

It's a little hard to imagine that the voltage drop caused by a dome light energizing is enough to trigger audio shut-off circuitry, unless you've been running the battery down below 11V or so... and if so, I suspect you'll find out pretty darn soon when there's not enough juice left to start the car.

- Erik

64 Valiant, 170 3-spd
82 Volvo wagon, 5.0 5-spd

 

[fredsmedina]

Wait, careful about making an assumption as to the "because", until you've tested and traced a couple more more items. You may be right, but the process of thinking through the diagnosis can easily be tainted if you already think you know what the problem is.


A voltmeter will tell you mountains. Attach to cigarette lighter, or any fusebox terminal-- the basic system voltage should be the same no matter where you test. 12.6V indicates a fully charged battery, engine off. With engine running, a functioning alternator should provide at least 13.5, probably 13.8, maybe even low 14s. If the battery has been really run down recently such that the alternator is doing a great job of bringing it back up in a hurry, mid 14s isn't unreasonable.

It's a little hard to imagine that the voltage drop caused by a dome light energizing is enough to trigger audio shut-off circuitry, unless you've been running the battery down below 11V or so... and if so, I suspect you'll find out pretty darn soon when there's not enough juice left to start the car.

- Erik

64 Valiant, 170 3-spd
82 Volvo wagon, 5.0 5-spd

I guess the quickest thing I can do also is throw the battery on the charger for a bit and see if it fixes my problem. Thanks for the tip on not jumping to any assumptions..I wont do that! so I put the (+) probe into the cigarette lighter and then ground the (-)?

I did leave my head lights on about a week or so ago for about 5 hours while I was at work. I've driven my car every day since though, shouldn't the alternator have re-charged my battery completely by now? or does what i'm saying not make sense?

 

[mpgFanatic]

I did leave my head lights on about a week or so ago for about 5 hours while I was at work. I've driven my car every day since though, shouldn't the alternator have re-charged my battery completely by now? or does what i'm saying not make sense?

Ah, more clues. Telling the doctors everything makes it easier, ya know... ;)

Yes, it should have recharged by now, IF it were working properly. On the other hand, an alternator is NOT a battery charger, it's designed to merely top off whatever the starter depleted. Trying to use it to top off the effects of leaving headlights on for 5 hours shouldn't be that hard, but may have been the last straw if it was on its last legs. The alternator may now be working only partially-- not enough to keep up with a week of daily driving. If so, you're not going to see 13.8V when running, and yes, putting a battery charger on it for awhile should change the symptoms.

All of *these* results will be useful, too...

- Erik

64 Valiant, 170 3-spd
82 Volvo wagon, 5.0 5-spd

 

[fredsmedina]

Ah, more clues. Telling the doctors everything makes it easier, ya know... ;)

Yes, it should have recharged by now, IF it were working properly. On the other hand, an alternator is NOT a battery charger, it's designed to merely top off whatever the starter depleted. Trying to use it to top off the effects of leaving headlights on for 5 hours shouldn't be that hard, but may have been the last straw if it was on its last legs. The alternator may now be working only partially-- not enough to keep up with a week of daily driving. If so, you're not going to see 13.8V when running, and yes, putting a battery charger on it for awhile should change the symptoms.

All of *these* results will be useful, too...

- Erik

64 Valiant, 170 3-spd
82 Volvo wagon, 5.0 5-spd

THanks. I actually didn't even remember that I had left the headlights on until now. totally slipped my mind! so hopefully that will make a difference tomorrow! we shall see

so how to i test the voltage through the cigarette lighter?

 

[cantcatch06]

positive in the well and negative on the side of the well. or you could just open the hood and test at the battery on pos and negative terminals. Then crank the car and check again. 12.5+ shut off and 13+ while idling should be a good. any less and you should be looking at either the alternator or the battery accordingly. At least that's the check i do every day up at the good ole autozone i work at.

 

[fredsmedina]

positive in the well and negative on the side of the well. or you could just open the hood and test at the battery on pos and negative terminals. Then crank the car and check again. 12.5+ shut off and 13+ while idling should be a good. any less and you should be looking at either the alternator or the battery accordingly. At least that's the check i do every day up at the good ole autozone i work at.

thanks..