Idle voltage
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Idle voltage
I have a 66 F100 with a 390 FE engine that the previous owner installed. The alternator is only a year old and I bought it along with the external voltage regulator by application for that engine and a '75 F-250 from CarQuest.
I noticed that my turn signal is slower at idle than it is when starting from a stop or revving the engine.
A friend suggested that the alternator just isn't making enough voltage at idle.
When revving the engine the voltage at the battery terminals is about 13.5
When idling it was a lot less. Maybe low 12's, or even less (high 11's, I forget exactly what . . . that important detail i know).
Since this is an older truck, is that normal? Should I do something about this like switch to a smaller pulley on the alternator? I can increase the idle, but it sounds good where it is (i have not used a tach to check).
The belt is nice and snug. I can not move the cooling fins on the pulley if I try to push it with my thumb.
I noticed that my turn signal is slower at idle than it is when starting from a stop or revving the engine.
A friend suggested that the alternator just isn't making enough voltage at idle.
When revving the engine the voltage at the battery terminals is about 13.5
When idling it was a lot less. Maybe low 12's, or even less (high 11's, I forget exactly what . . . that important detail i know).
Since this is an older truck, is that normal? Should I do something about this like switch to a smaller pulley on the alternator? I can increase the idle, but it sounds good where it is (i have not used a tach to check).
The belt is nice and snug. I can not move the cooling fins on the pulley if I try to push it with my thumb.
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Joined: Jan 2009
Posts: 49
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From: Melbourne
What you describe is a problem with original style alternators (new or not.)
I can't remember what the original output was, maybe 60amps or so, but that was max output with the rpm somewhere in the cruise range. So when its spinning at idle, it's putting out much less than its max rating. Add anything non factory to the slick like a big stereo, winches or lights and it just gets worse! It doesn't help going to an original style thats rated for higher output either, because the same problem exists at idle. What you're seeing is a lack of voltage at idle and the battery supplying 11.? volts instead of the 14ish when the alternator is up to correct speed. Nothing to be alarmed about, but it IS draining the battery until you're off idle again.
Changing to a smaller diameter pulley will help by speeding it up, but I found with all my vintage toys a 3G alternator conversion from a mustang is easier and gets a better result and you can ditch the external regulator. I don't know what the bracket setup is for the alternator on an FE, but if the mustang 3g fits, I would go that route as they put out full voltage at idle and probably half of their max amperage (Usually 100, 130 or 200!!)
Theres plenty of info online, but I have pics of the mod on my cleveland powered 66 if you decide to go that way that I'd be happy to share and it really is a dead easy mod to do.
I can't remember what the original output was, maybe 60amps or so, but that was max output with the rpm somewhere in the cruise range. So when its spinning at idle, it's putting out much less than its max rating. Add anything non factory to the slick like a big stereo, winches or lights and it just gets worse! It doesn't help going to an original style thats rated for higher output either, because the same problem exists at idle. What you're seeing is a lack of voltage at idle and the battery supplying 11.? volts instead of the 14ish when the alternator is up to correct speed. Nothing to be alarmed about, but it IS draining the battery until you're off idle again.
Changing to a smaller diameter pulley will help by speeding it up, but I found with all my vintage toys a 3G alternator conversion from a mustang is easier and gets a better result and you can ditch the external regulator. I don't know what the bracket setup is for the alternator on an FE, but if the mustang 3g fits, I would go that route as they put out full voltage at idle and probably half of their max amperage (Usually 100, 130 or 200!!)
Theres plenty of info online, but I have pics of the mod on my cleveland powered 66 if you decide to go that way that I'd be happy to share and it really is a dead easy mod to do.
Last edited by F100 wannabe; Oct 30, 2016 at 07:15 PM. Reason: Spelling
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