Mud in my Veins
Join Date: Dec 2000
Location: Memphis, TN
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Re: alternator and headlights
The maximum output of the alternator isn't what makes the headlights bright or dim. It's the voltage regulator. Unless your alternator is running at its max right now, upgrading won't have any effect unless your regulator is bad and the replacement alt has a good one. But in that case, replacing the regulator is a lot cheaper - by about $140. The regulator is easy to install yourself if you have torx drivers and a paperclip (to hold the brushes back).
To see what's causing the dim lights, crank the engine and turn on all the stuff you normally drive with (don't forget the A/C). Then use a voltmeter to see what the voltage at the starter solenoid is. If it's less than 14, you have a problem with the alt or reg. If it's 14-15.5 at the solenoid, check it across the headlight bulbs. It should be 12.5-14.5V. If it's OK, your bulbs are just normally that color. (Check out the new "Cool Blue" line of GE bulbs.) If it's low there, check from the ground terminal of the headlight to the block. If it's more than .05V, clean and tighten the headlight ground screw (Haynes wiring diagrams tell exactly where it is). Otherwise, you'll have to trace the power wire back to the headlight switch to find the source of the low voltage.
83 Custom w/95 4.9L EFI, 78 NP435, 83 NP208, 83 D44IFS 3.07, 87 Ford 8.8ABS 3.08, Michelin LTX AT 31x10.50R15