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Is this alternator test no longer valid + other
#799963 Tue Nov 22 2011 03:03 PM
Joined: May 2005
Posts: 1,635
beltfed Offline OP
Shop Shark
I was helping my buddy work on his daughters car. Battery was discharged, so we will pulled it and charged overnight. Performed two tests on it after 12+ hour charge. Specific gravity test indicated all cells being marginal (yellow) as did the load test. We reconnected the battery, started car, lifted one battery lead, and engine died. Bad alternator, right? He took car to his 'wrench' as we don't work on 'sideway motors'. His 'wrench' said two strange things...never lift the battery lead on newer cars as this puts high voltage AC on the electrical system. Also, he said you have to let the battery rest after charging before taking a specific gravity reading. It will read low otherwise. Is this correct? Thanks

Six volt guy living in a twelve volt world
Re: Is this alternator test no longer valid + other
beltfed #799975 Tue Nov 22 2011 03:32 PM
Joined: Jul 2001
Posts: 3,904
Master Gabster
yes on not disconnecting the battery when running a car with an alternator [or computer], for an alternator test, read voltage across battery terminals while running, should be about 14V .... "resting" before testing applies to voltage tests after charging not SG reading


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Re: Is this alternator test no longer valid + other
squeeze #799989 Tue Nov 22 2011 04:17 PM
Joined: Apr 2005
Posts: 2,003
Shop Shark
Another alternator test loads the system to read the amps the alternator puts out.

Re: Is this alternator test no longer valid + other
beltfed #800049 Tue Nov 22 2011 07:44 PM
Joined: Feb 2004
Posts: 20,608
I've watched voltage spikes of over 100V on an oscilloscope when somebody pulled a battery cable with the engine running. If you're doing it on a stone age vehicle without a lot of electronics you might get away with it, but even a built-in regulator on a 10SI alternator can be fried that way. Ditto for that sound system you just spent a couple of paychecks on. Computers really dislike voltage spikes.

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Re: Is this alternator test no longer valid + other
beltfed #802587 Thu Dec 01 2011 04:30 PM
Joined: Jan 2000
Posts: 124
Shop Shark
Removing a battery cable used to work on the generator type charging systems but is a big NO NO for modern alternator type charging systems especially the new new computerized creatures. The better way to check alternator output is at the 10/32 stud on the back of the alternator using a volt meter. If you have none you can use a pocket knife or a screwdriver to touch the smooth bearing surface on the back of the alternator when the engine is running. If the alternator is working that smooth surface will be magnetized. The more the electrical load ie...headlights on etc etc the stronger the magnetism will be if the alternator is working. No magnetism...alternator not working.....thanks Randy

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