Headlights get Brighter when engine rev'd
Postmaster
Joined: Apr 2000
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From: SD
I've had my '73 for 10+ years and it's always done this. I've even replaced the alt and regulator (for other reasons) and there was no change in the dimming. Never have problems with charging or voltage, just annoying to watch the headlights dim and hear the heater fan slow down at idle.
Marty
Marty
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From: Virginia
Originally Posted by ford390gashog
very common problem. the alternator only works after 900 rpm so if your idle is below that it will just be battery voltage being used.
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From: Black Hills South Dakota
The alternator makes its juice by spinning an energized coil within another coil. No spinny, no juicy! If it's turning too slowly, it just won't create enough to be more than the battery is providing, so lights are dimmer than when the engine is revved. The regulator can only regulate a higher voltage downwards, not a low battery-only voltage upwards. If you made your alternator charge enough to keep your lights bright at idle, it might overcharge the battery when the lights are off. Mine does it too.
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From: Virginia
Originally Posted by GeneStoner
So is this a function of the voltage regulator?
My idle is about 1000rpm...
My idle is about 1000rpm...
It's hard to say since we are not there to compare it to what we normally see, but it will always do it a little bit.
Post Fiend
Joined: Jun 2002
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From: Atlanta GA
Alternators have AMPERAGE curves. Usually more AMPs with more RPMS. More AMPs means brighter lights because more AMPs are available to draw upon while the VOLTAGE only goes up a bit.
At idle you might see 13.2v while at high idle 14.4v, while at idle AMPs might be 40 and high idle 110. So at high RPMs, your voltage will only increase maybe 9-10% while the AMPs increase over 2x.
FWIW: When installing a larger battery, you go by CCA, not just size.
At idle you might see 13.2v while at high idle 14.4v, while at idle AMPs might be 40 and high idle 110. So at high RPMs, your voltage will only increase maybe 9-10% while the AMPs increase over 2x.
FWIW: When installing a larger battery, you go by CCA, not just size.
Junior User
Joined: Aug 2003
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From: Polson, Montana
I also have the same problem with my 72 390, but my lights are not only dimmer but they also flicker. Is this the alternator as well? It is an old alternator. The regulator is new, battery new. Checked resistance and it is good, so don't think ground is the issue. The brightness doesn't really bother me but the flickering makes me wonder. Any ideas?
Posting Guru
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From: Asheville, NC
Deputy, your brushes in your alt. are probably sparking. Probably need either to have your rebuilt, or, get another alternator.
Battery size: 24F is larger. and usually has more Cold Cranking Amps available.
Battery size: 24F is larger. and usually has more Cold Cranking Amps available.
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From: Virginia
Originally Posted by deputy1515
I also have the same problem with my 72 390, but my lights are not only dimmer but they also flicker. Is this the alternator as well? It is an old alternator. The regulator is new, battery new. Checked resistance and it is good, so don't think ground is the issue. The brightness doesn't really bother me but the flickering makes me wonder. Any ideas?