Testing an alternator: Using a stiffening capacitor

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yonksteryouler@hotmail.com - 01 Jun 2006 03:38 GMT

Hi all,

I've heard that you can't disconnect the positive terminal of a battery
to test an alternator on newer cars:

http://www.repairfaq.org/ELE/F_Car_Battery.html#CARBATTERY_031

If I strapped one of those stiffening capacitors
(http://en.wikipedia.org/wiki/Stiffening_capacitor) for car audio (1-2
farads) across the positive and negative terminals, would I be able to
protect my electronic circuits enough to be able to disconnect the
positive terminal from the battery to test the alternator? My
suspicion is that it would, but I thought I'd throw this question out
there before I tried it...

Has anyone tried this?

Yonkster Youler

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Don - 01 Jun 2006 04:23 GMT

>Hi all,
>
>I've heard that you can't disconnect the positive terminal of a battery
>to test an alternator on newer cars:

You can -- at considerable risk to everything electronic in your
vehicle.

>http://www.repairfaq.org/ELE/F_Car_Battery.html#CARBATTERY_031
>
[quoted text clipped - 3 lines]
>protect my electronic circuits enough to be able to disconnect the
>positive terminal from the battery to test the alternator?

It would greatly reduce the risk to the electronics. It won't do much
for the testing aspect, however. Such a test is virtually
meaningless. You would do better to get a $9 multimeter and check
voltage across the battery with everything turned on -- high beams, AC
on full blower, rear defroster etc. You should have about 13 volts at
idle and 13.5 or more at about 2000 rpm. The high limit is
temperature compensated and goes down the hotter it gets under the
hood. you should never see much over 14.4 or so unless you are in
Antartica. A very discharged or internally shorted battery will lower
these readings. Be advised that at less than 12.6 volts across the
battery, unless the voltage is rising it is losing charge.

> My suspicion is that it would, but I thought I'd throw this question out
>there before I tried it...
>
>Has anyone tried this?

I haven't and see no reason to do so.

Don
www.donsautomotive.com

>Yonkster Youler

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Kevin - 01 Jun 2006 04:41 GMT

> Hi all,
>
[quoted text clipped - 14 lines]
>
> Yonkster Youler

Why would you even want to. The alternator can be tested without
disconecting the battery. Actually the battery should be tested first then
the altenator.

go here to learn how:

http://www.cdxetextbook.com/electrical/chargStartLight/charging/01.html

Signature

Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green

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Mike Romain - 01 Jun 2006 14:57 GMT

For less than five bucks you can get a multimeter which will allow you
to test the voltage and see what is going on.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590
(More Off Road album links at bottom of the view page)

> Hi all,
>
[quoted text clipped - 14 lines]
>
> Yonkster Youler

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AZ Nomad - 01 Jun 2006 16:07 GMT

>For less than five bucks you can get a multimeter which will allow you
>to test the voltage and see what is going on.

Don't forget to put on a load such as the headlights and rear window
defroster. A cheap meter won't tell you if the alternator has a blown
output diode or two. Without a load, you might still read 14.4V on
an alternator that'll let the battery run down while you're driving
with headlights.

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Lawrence Glickman - 01 Jun 2006 23:38 GMT

>>For less than five bucks you can get a multimeter which will allow you
>>to test the voltage and see what is going on.
[quoted text clipped - 4 lines]
>an alternator that'll let the battery run down while you're driving
>with headlights.

I just turn =all= the electrics on, everything including rear window
defogger, headlights on hi beam, etc etc and put a clamp meter on the
alternator B+ wire to the battery. "Normal" is 66 amperes. If I'm
reading less than that, then I will know there is a problem.

So, when you check your vehicle, IF you have a known good reference
value for this or that, a simple clamp type ammeter will tell you if
your alternator is working correctly I do believe.

Then, you can check for more or less than 100 ma AC at the
battery...by putting the voltmeter across the +/- while the meter is
switched to AC. If you see more than 100 ma AC then there is
something wrong with the alternator somewhere.

Lg

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Lawrence Glickman - 02 Jun 2006 00:20 GMT

>>>For less than five bucks you can get a multimeter which will allow you
>>>to test the voltage and see what is going on.
[quoted text clipped - 13 lines]
>value for this or that, a simple clamp type ammeter will tell you if
>your alternator is working correctly I do believe.

=================================================================
Then, you can check for more or less than 100 mv AC at the
battery...by putting the voltmeter across the +/- while the meter is
switched to AC. If you see more than 100 mv AC then there is
something wrong with the alternator somewhere.

>Lg

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AZ Nomad - 02 Jun 2006 02:24 GMT

>>>>For less than five bucks you can get a multimeter which will allow you
>>>>to test the voltage and see what is going on.
[quoted text clipped - 18 lines]
>switched to AC. If you see more than 100 mv AC then there is
>something wrong with the alternator somewhere.

More like 300-500mv.
100mv isn't much ripple and is normal.

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Lawrence Glickman - 02 Jun 2006 03:32 GMT

>>>>>For less than five bucks you can get a multimeter which will allow you
>>>>>to test the voltage and see what is going on.
[quoted text clipped - 18 lines]
>>switched to AC. If you see more than 100 mv AC then there is
>>something wrong with the alternator somewhere.

====================================================================

>More like 300-500mv.
>100mv isn't much ripple and is normal.

Okay, thanks. Just going by what aarcuda mentioned the other day in
another thread. I haven't even measured the "AC component" of the
alternator across the battery yet. I don't know why. The car is in
the garage about 20 feet away from me...one would think I was
_curious_. Evidently not at the moment ;-)

Lg

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Lawrence Glickman - 02 Jun 2006 04:23 GMT

>>>>>>For less than five bucks you can get a multimeter which will allow you
>>>>>>to test the voltage and see what is going on.
[quoted text clipped - 30 lines]
>
>Lg

========================================================================

UPDATE:

I just measured the AC component on 61 AMPERES DC coming out of my
alternator, and I get 20 ( Twenty ) milliVolts of AC ripple from the
alternator when its' B+ is connected to, and measured at, the battery.

So...it looks to me like aarcuda was/is correct. Anything over 100
millivolts AC means something is not right with your charging system.

Lg

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Mike Romain - 02 Jun 2006 14:18 GMT

> >For less than five bucks you can get a multimeter which will allow you
> >to test the voltage and see what is going on.
[quoted text clipped - 4 lines]
> an alternator that'll let the battery run down while you're driving
> with headlights.

I usually like to read the battery with the engine off, then at idle,
then with everything electrical turned on.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590
(More Off Road album links at bottom of the view page)

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