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How do you test an alternator?
#142595
10/28/08 02:42 PM
10/28/08 02:42 PM
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Joined: Jan 2004
Posts: 803
Zebulon
Stationwagonguy
OP
A Closet Buick Man
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OP
A Closet Buick Man
Joined: Jan 2004
Posts: 803
Zebulon
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I have a '78 Impala, just got it running again after it sat for almost 15 years. Anyhow, the "generator" light is on on the dash, and I'm trying to figure out what the issue is. The battery will obviously take some charge, but not an awful lot. I haven't actually had a chance to fully charge it either, but it took a jump pack to get it running and after running a half hour (mostly at idle), when it stalled out, I had to jump it to start it again. It would only crank about one revolution then give up. However, all the lights seemed to still work fine.
I don't know- I'm guessing since the generator light is on, it's either the alternator or the battery. How do I test either to be sure if either are good or bad? I'd rather troubleshoot than just throw money at it.
I do have a multimeter, but I'm not that good with it.
Thanks!
[image][/image]
Bob
SWG
'68 Chrysler Town & Country, 'Katy Jo'
'70 Chrysler 300 2-door
'78 Impala 2-door
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Re: How do you test an alternator?
[Re: Stationwagonguy]
#142596
10/28/08 02:50 PM
10/28/08 02:50 PM
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Joined: May 2003
Posts: 15,487
Florida
scratchnfotraction
I Live Here
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I Live Here
Joined: May 2003
Posts: 15,487
Florida
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take the hot wire off the batt while it is running....if it shuts off the alt is bad if it stays running its a low or bad batt use the multi meter to check for 14 volts while its running at the batt...may be a little more than that if its got a low batt and the alt is chargeing at the time of the reading have the alt tested at a parts store before replacing it to be sure good luck.... 
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Re: How do you test an alternator?
[Re: scratchnfotraction]
#142597
10/28/08 02:54 PM
10/28/08 02:54 PM
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Joined: Nov 2007
Posts: 21,843
Kirkland, Washington
Pacnorthcuda
Too Many Posts
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Too Many Posts
Joined: Nov 2007
Posts: 21,843
Kirkland, Washington
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Quote:
take the hot wire off the batt while it is running....if it shuts off the alt is bad
Don't do this, not only is it an unreliable test of an alternator, it can damage other components when the battery is removed from the system. The battery serves as a cushion to protect against spikes.
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Re: How do you test an alternator?
[Re: Pacnorthcuda]
#142598
10/28/08 03:38 PM
10/28/08 03:38 PM
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Joined: Dec 2006
Posts: 3,533
Indiana
Fury Fan
master
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master
Joined: Dec 2006
Posts: 3,533
Indiana
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When the alternator is spinning try touching a screwdriver blade to the bearing cap on the rear of the alternator (if it has one, Mopar alternators are configured like this). Be cautious of hitting any wire terminals to ground, though.
If you get magnetism, the alternator is doing something (although you don't know how much output it has) but could still be a culprit. If there is no magnetism, though, the alt is not doing anything.
This tip is more of a first-strike diagnosis and is NOT the full story.
Parts I seek:
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Send a PM.
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Re: How do you test an alternator?
[Re: Pacnorthcuda]
#142599
10/28/08 03:43 PM
10/28/08 03:43 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
Quote:
take the hot wire off the batt while it is running....if it shuts off the alt is bad
Don't do this, not only is it an unreliable test of an alternator, it can damage other components when the battery is removed from the system. The battery serves as a cushion to protect against spikes.
i disagree. its not the best test but it would work. and these cars are pretty stout when it comes to electronics susceptable to voltage spikes.
If youre really concerned, then dont reconnect the battery if the car stays running with the ignition on. have someone turn off the ignition before reconnecting it. then there will be no spike so no possible damage
but a voltmeter would be best. check battery voltage at the alt outpput with the car off and then with it running. voltage should be a few volts higher with the car running
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Re: How do you test an alternator?
#142600
10/28/08 03:51 PM
10/28/08 03:51 PM
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Joined: Nov 2007
Posts: 21,843
Kirkland, Washington
Pacnorthcuda
Too Many Posts
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Too Many Posts
Joined: Nov 2007
Posts: 21,843
Kirkland, Washington
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Quote:
Quote:
Quote:
take the hot wire off the batt while it is running....if it shuts off the alt is bad
Don't do this, not only is it an unreliable test of an alternator, it can damage other components when the battery is removed from the system. The battery serves as a cushion to protect against spikes.
i disagree. its not the best test but it would work. and these cars are pretty stout when it comes to electronics susceptable to voltage spikes.
If youre really concerned, then dont reconnect the battery if the car stays running with the ignition on. have someone turn off the ignition before reconnecting it. then there will be no spike so no possible damage
but a voltmeter would be best. check battery voltage at the alt outpput with the car off and then with it running. voltage should be a few volts higher with the car running
So if you disco the battery from a running car and the car dies you figure you just reliably determined that the alternator is bad? Really?
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Re: How do you test an alternator?
[Re: scratchnfotraction]
#142602
10/28/08 04:22 PM
10/28/08 04:22 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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i guess I should have said it was a good test for the charging system.
mind you, this is for people who are just looking for an easy way to check battery or something else. I have a meter and know how to do it. Im just tired or rewriting it again and again
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Re: How do you test an alternator?
#142605
10/28/08 08:43 PM
10/28/08 08:43 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
Quote:
Quote:
take the hot wire off the batt while it is running....if it shuts off the alt is bad
Don't do this, not only is it an unreliable test of an alternator, it can damage other components when the battery is removed from the system. The battery serves as a cushion to protect against spikes.
i disagree. its not the best test but it would work. and these cars are pretty stout when it comes to electronics susceptable to voltage spikes.
If youre really concerned, then dont reconnect the battery if the car stays running with the ignition on. have someone turn off the ignition before reconnecting it. then there will be no spike so no possible damage
but a voltmeter would be best. check battery voltage at the alt outpput with the car off and then with it running. voltage should be a few volts higher with the car running
!!! DO NOT EVER DO THIS !!!
I've actually checked this years ago with a "test pig" that had no electronic anything. Disconnecting a charging unit from the battery generates OVER A HUNDRED volts of spike. I don't have it anymore, but I once had a 'scope photo of this spike, and used to haul it out for guys like you
One modern cars, with electronics of all kinds, solid state controls, radios, regulators and ignition, this is a GUARANTEED way of frying something. Yep---I know you "got away with it"---you might get by over a hundred times, and the 103rd time you do it, "the guy" might say, "Ya know, it's funny, when YOU worked on my alternator 'thu udder day' the RADIO quit working."
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Re: How do you test an alternator?
#142607
10/28/08 10:49 PM
10/28/08 10:49 PM
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Joined: Aug 2004
Posts: 5,033
Ontario
5537SG
super gas
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super gas
Joined: Aug 2004
Posts: 5,033
Ontario
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Quote:
Quote:
Quote:
Quote:
take the hot wire off the batt while it is running....if it shuts off the alt is bad
Don't do this, not only is it an unreliable test of an alternator, it can damage other components when the battery is removed from the system. The battery serves as a cushion to protect against spikes.
i disagree. its not the best test but it would work. and these cars are pretty stout when it comes to electronics susceptable to voltage spikes.
If youre really concerned, then dont reconnect the battery if the car stays running with the ignition on. have someone turn off the ignition before reconnecting it. then there will be no spike so no possible damage
but a voltmeter would be best. check battery voltage at the alt outpput with the car off and then with it running. voltage should be a few volts higher with the car running
!!! DO NOT EVER DO THIS !!!
I've actually checked this years ago with a "test pig" that had no electronic anything. Disconnecting a charging unit from the battery generates OVER A HUNDRED volts of spike. I don't have it anymore, but I once had a 'scope photo of this spike, and used to haul it out for guys like you
One modern cars, with electronics of all kinds, solid state controls, radios, regulators and ignition, this is a GUARANTEED way of frying something. Yep---I know you "got away with it"---you might get by over a hundred times, and the 103rd time you do it, "the guy" might say, "Ya know, it's funny, when YOU worked on my alternator 'thu udder day' the RADIO quit working."
Disconnecting the battery on any vehicle while the vehicle is running is nothing but stupid. Its just as you say, it generates a huge spike, thats capable of serious damage. Oh, it might not be immediate, but it damaged something!
Ya know, I argued back and forth with a few know-it-all hacks here a few years ago over this very topic. Some people just dont want to learn. 
The good news is most people get away with it because their alternator is actually bad (disconnecting it also can damage it too) and there is no spike. However, if its working....its a different story.
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Re: How do you test an alternator?
[Re: 5537SG]
#142609
10/28/08 11:42 PM
10/28/08 11:42 PM
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Joined: Oct 2005
Posts: 4,956
WI
Dcuda69
master
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master
Joined: Oct 2005
Posts: 4,956
WI
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Quote:
Quote:
Quote:
Quote:
Quote:
take the hot wire off the batt while it is running....if it shuts off the alt is bad
Don't do this, not only is it an unreliable test of an alternator, it can damage other components when the battery is removed from the system. The battery serves as a cushion to protect against spikes.
i disagree. its not the best test but it would work. and these cars are pretty stout when it comes to electronics susceptable to voltage spikes.
If youre really concerned, then dont reconnect the battery if the car stays running with the ignition on. have someone turn off the ignition before reconnecting it. then there will be no spike so no possible damage
but a voltmeter would be best. check battery voltage at the alt outpput with the car off and then with it running. voltage should be a few volts higher with the car running
!!! DO NOT EVER DO THIS !!!
I've actually checked this years ago with a "test pig" that had no electronic anything. Disconnecting a charging unit from the battery generates OVER A HUNDRED volts of spike. I don't have it anymore, but I once had a 'scope photo of this spike, and used to haul it out for guys like you
One modern cars, with electronics of all kinds, solid state controls, radios, regulators and ignition, this is a GUARANTEED way of frying something. Yep---I know you "got away with it"---you might get by over a hundred times, and the 103rd time you do it, "the guy" might say, "Ya know, it's funny, when YOU worked on my alternator 'thu udder day' the RADIO quit working."
Disconnecting the battery on any vehicle while the vehicle is running is nothing but stupid. Its just as you say, it generates a huge spike, thats capable of serious damage. Oh, it might not be immediate, but it damaged something!
Ya know, I argued back and forth with a few know-it-all hacks here a few years ago over this very topic. Some people just dont want to learn.
The good news is most people get away with it because their alternator is actually bad (disconnecting it also can damage it too) and there is no spike. However, if its working....its a different story.
Besides taking the chance of creating a spark near the batt.It's obvious some people have never seen a battery explode,not pretty!! If you have a Batteries Plus or even Auto Zone should test it for you,or follow the instructions above and use your meter!!!
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Re: How do you test an alternator?
[Re: RodStRace]
#142612
10/29/08 09:28 AM
10/29/08 09:28 AM
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Joined: Jun 2008
Posts: 1,822
Colorado
denfireguy
top fuel
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top fuel
Joined: Jun 2008
Posts: 1,822
Colorado
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Quote:
I wrote a long version a month ago, and don't want to again. Here you go. There are others on the net, too.
http://autorepair.about.com/od/electricaltroubleshooting/a/battery_check.htm
You are right, thought we had beaten this horse to death already.
No wonder there are so many bad regulators on the rallye dashes out there.
Craig
2014 Ram 1500 Laramie, 73 Cuda
Previous mopars: 62 Valiant, 65 Fury III, 68 Fury III, 72 Satellite, 74 Satellite, 89 Acclaim, 98 Caravan, 2003 Durango
Only previous Non-Mopar: Schwinn Tornado
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Re: How do you test an alternator?
[Re: stumpy]
#142613
10/29/08 09:45 AM
10/29/08 09:45 AM
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Joined: Aug 2004
Posts: 5,033
Ontario
5537SG
super gas
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super gas
Joined: Aug 2004
Posts: 5,033
Ontario
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Quote:
Guess I've been lucky for the last 50 years. I never had one spike enough to hurt the electrical system or a battery blow up either. But we must listen to the "experts". 
sometimes the damage isnt obvious or immediately apparent.
I dumped alot of Freon into the atmosphere too, but I never saw anything happen
As for the blown up battery, tell that to the guy at the garage down the road from me with one eye. It does happen.
But theres no need to be cranky about it, just accept the new found knowledge and move on. The day a mechanic stops learning is the day they are no longer a mechanic.
Keep an open mind, and aim to learn, cause it never stops.
the guy who started this thread sought knowledge, now he has some.
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Re: How do you test an alternator?
[Re: ahy]
#142614
10/29/08 09:50 AM
10/29/08 09:50 AM
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Joined: Feb 2008
Posts: 233
NH
Fastback67
enthusiast
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enthusiast
Joined: Feb 2008
Posts: 233
NH
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Quote:
if you take the alternator off, Autozone will test it for free.
Most shops will test it for FREE
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,call me lucky when it comes to me doin it 
