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Apr 26th, 2026 | | AD Chevy Trucks  Over 6,000 pictures Brad Allen has an awesome collection of Chevrolet factory pictures that he has set up from film strips. Check them out!
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| | | | Joined: May 2005 Posts: 1,624 Shop Shark | | Shop Shark Joined: May 2005 Posts: 1,624 | 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
| | | | | Joined: Jul 2001 Posts: 3,887 Cruising in the Passing Lane | | Cruising in the Passing Lane Joined: Jul 2001 Posts: 3,887 | 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
Bill | | | | | Joined: Apr 2005 Posts: 1,971 'Bolter | | 'Bolter Joined: Apr 2005 Posts: 1,971 | Another alternator test loads the system to read the amps the alternator puts out. | | | | | Joined: Feb 2004 Posts: 29,850 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,850 | 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.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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| | | | | Joined: Jan 2000 Posts: 124 Shop Shark | | Shop Shark Joined: Jan 2000 Posts: 124 | 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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