Light dimming?
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Junior User
Joined: Oct 2005
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Light dimming?
I have a 1994 F150 302. My friend's 96 302 5spd and my other friends 96 300 5spd and my father's 1992 E-150 351 all have this same problem. When driving our trucks with the headlights on and the fan on high when we are idling the headlights and instrument panel lights all get dim and the battery guage reads down around O on normal. When we hit the gas to take off again the guage goes back up, fan picks up speed, and lights get bright again. Is this a design flaw with Ford that have crappy alternators or what can I do to correct this problem. Maybe a different alternator pulley would adjust the rpms to correct this? Thanks for the help.
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Junior User
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I figured that a new alternator would correct the problem but didn't know if the pulley just spun it too slow at idle. Ford must have had a design flaw or something because my 1984 Chevy has a stock alternator and everything else and it never has done this.
Post Fiend
Joined: Oct 2003
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From: New Jersey
Ford had several alternators they put into the F-series, and the most common one was the 63 amp alternator, which my 93 F350 crewcab has. At idle, that particular alternator can only put out a hair over half it's full RPM rating, which is about 37 amps. Headlights draw 20ish, blower motors also draw about 20ish. Plus the EFI system, the fuel pump, and so on.
Like ford390 said, you're running off your battery when the voltage gauge dips and the headlights dim.
A larger alternator may provide more current than stock, however most of them do not provide much more at idle, for the same reasons, so before you upgrade your alternator to a larger size, research and see what it will put out at idle.
It's also possible your idle is too low. My crewcab's idle was typically 750ish and I was able to get rid of the headlight dimming issue by adjusting that to 850, which is closer to where it should be anyway.
Like ford390 said, you're running off your battery when the voltage gauge dips and the headlights dim.
A larger alternator may provide more current than stock, however most of them do not provide much more at idle, for the same reasons, so before you upgrade your alternator to a larger size, research and see what it will put out at idle.
It's also possible your idle is too low. My crewcab's idle was typically 750ish and I was able to get rid of the headlight dimming issue by adjusting that to 850, which is closer to where it should be anyway.
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Joined: Aug 2005
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Also check and make sure the belt is put on correctly. When we put a head gasket on my 1987 f-150 4.9 we got in a hurry and put the belt on incorrectly. When i stopped the volts guage went really low and when I took off the volts guage would go back up to normal.
-Matt
-Matt
