Alt. light on while still charging
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New User
Joined: Mar 2011
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Alt. light on while still charging
'91 F250 Custom, 460. While about 200 miles from home (because problems always happen far from home), I notice the alt. light is on, at about half the brightness it has KOEO. The volt gauge on the dash is still reading normal, does the same sweep it always has, drops at idle, etc, all those 2G traits. Made it home fine, with the lights, fog lights, heater, stereo, wipers, and powering a trailer, with no issues. Why is the alt. light on?
Fleet Owner
Joined: Aug 2005
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From: Easton,Ks
That would be a 3G, he said he has a 2G.
Senior User
Joined: Dec 2010
Posts: 203
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From: Middle of nowhere, Mo
When I used to have my 89 6 cyl 4x4 it started doing the same thing. Then one day the battery light came on and the gauge started to go over normal. Popped the hood and my battery was leaking from the top around where you would fill it. Changed the alt out and it never did it again. Idk if the voltage reg is internal or external from the alternator or not but basically i guess it started over charging or something.
pedant
Joined: Jan 2008
Posts: 23,576
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From: EXTREME southwest CT
2G's are internally regulated.
It would be a good idea to put a VOM across the battery terminals while the engine is running and see what the alternator is putting out.
Bring the rpm's up to 2k or so and check again.
"Why is the alternator light on?"
Because there is a voltage difference across the two terminals of the bulb's socket.
It would be a good idea to put a VOM across the battery terminals while the engine is running and see what the alternator is putting out.
Bring the rpm's up to 2k or so and check again.
"Why is the alternator light on?"
Because there is a voltage difference across the two terminals of the bulb's socket.
pedant
Joined: Jan 2008
Posts: 23,576
Likes: 39
From: EXTREME southwest CT
... and your alternator seems to be working fine. 
I would check for dirty copper foil contacts in the bulb socket or some other reason for the instrument cluster not seeing full voltage.
I would check for dirty copper foil contacts in the bulb socket or some other reason for the instrument cluster not seeing full voltage.
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I'll have to get some ambition up to do that, ha. (Yes, I know the cluster is simple to pull out)
It just seems odd that it would start out of the blue.
It just seems odd that it would start out of the blue.
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but I do agree with what was said by Ard...I did have the same issue (in a different foreign made car that is now driven by my younger brother) and when the same type of behavior started it wound up being the alt its self showing signs of break down and then just one day died...
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So I'm thinking the old girl was trying to give me a warning. Noticed the dash volt gauge was looking lower at idle, and acting odd at revs. Got home and retested it...
Idle load: 11.95
Rev load: 13.01
Idle no load: 12.41
Rev no load: 13.32
Fortunately, I just happened to get my hands on a 3G, from a '97 3.0 Taurus. Unfortunately, I need to tow the race car tomorrow at 10:30 AM. Tine to get swapping...
Idle load: 11.95
Rev load: 13.01
Idle no load: 12.41
Rev no load: 13.32
Fortunately, I just happened to get my hands on a 3G, from a '97 3.0 Taurus. Unfortunately, I need to tow the race car tomorrow at 10:30 AM. Tine to get swapping...
Postmaster
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From: E.Lincoln County, NC
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