alternator problems
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alternator problems
Is there anything in the charging circuit that would fry an alternator if it went bad. I have a 02 F250SD 7.3L that the batteries would not hold a charge (I thought being 5 years old) so I replaced with Optima's. To make a long story short the alt was only putting out 3v so the idiot light never went on. why? so I replaced new alt had 14v lasted 3 days then the idiot light goes on. so I test the voltage at the cable on the alt with the charging cable to the bat disconected or I would have 12v there no mater what. .25v is what I show with the newest one it was .5v but the idiot light is off. what should I be reading? and is there something else in the charging circuit that could be bad?
Thanks in advance for your response.
Thanks in advance for your response.
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Joined: Jul 2004
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From: Park Ridge, Illinois
It is very possible that the replacement alternator you got was bad. That is what happened with the last one I replaced in my old Aerostar. Took me a couple days to believe and verify that the new altenantor went bad. The next replacement lasted until I traded the van in.....
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From: Burnsville, MN
You asked if something could fry an alternator, and the easy answer is a short or other low resistance path to ground. However, it sounds like you ran for three days with your new batteries just fine, though of course they were never being recharged by the alternator, which suggests the problem is in the alternator (or its field coil).
You can't generally put a meter on an alternator by itself and declare it good or bad. If the field coil isn't energized, by design it won't put much out. However, when connected to new batteries, and with the engine running, if you don't get a 14v output, you have an alternator problem. Take it back and ask them to test it.
You can't generally put a meter on an alternator by itself and declare it good or bad. If the field coil isn't energized, by design it won't put much out. However, when connected to new batteries, and with the engine running, if you don't get a 14v output, you have an alternator problem. Take it back and ask them to test it.
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Here is my ? What should you read in voltage at the alternator cable post. Should you test it with the cable connected? If the cable is connected I test 12v there with the engine not running. And being there are 2 batteries it looks like the cable leads to some solenoids on top of the engine what are they?
Thanks
Thanks
Last edited by rustyoilpan; Nov 17, 2006 at 09:06 AM. Reason: spelling
Fleet Mechanic
Joined: Aug 2005
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From: NJ
The alternator main post should read battery voltage with the engine off and 13-14v when running. All test's should be made with the cable connected. As far as I have seen the 2 battery's should be wired together, as for the solenoid's they must be a diesel thing, not related to the charging system.
What kind of alternator are you using, new, rebuilt, oem, aftermarket?
What kind of alternator are you using, new, rebuilt, oem, aftermarket?
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Thank you for your reply.
Its a Napa rebuilt.
The first Rebuilt (that failed) unit was showing 14v output before I instaled the cable to the batteries. Thats what confused me when the replacement only showed about .5v. So a good alternator (or good voltage regulator on the alternator) will not show voltage when the cable is disconnected. It must work like a floating charger.
Thanks again for everyone's reply.
Its a Napa rebuilt.
The first Rebuilt (that failed) unit was showing 14v output before I instaled the cable to the batteries. Thats what confused me when the replacement only showed about .5v. So a good alternator (or good voltage regulator on the alternator) will not show voltage when the cable is disconnected. It must work like a floating charger.
Thanks again for everyone's reply.
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Never disconnect the alternator from the battery. I don't know about the Ford regulator used in this particular alternator, but you can fry 'em on other vehicles.
The regulator sees the disconnected cable as a dead battery and tries to shove as much current (voltage) at it as possible. Some regulators don't like that very much
The regulator sees the disconnected cable as a dead battery and tries to shove as much current (voltage) at it as possible. Some regulators don't like that very much
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