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Old 08-02-2008, 07:12 PM
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Need some volts!

Evening all, I'm running a Cobra with a bored & stroked 351. The car only has the basics, no a/c, radio (the side pipes are all the music I need), or anything else. During the day I have no problem with the volts. Gauge reads around 12 to 13 volts. Now when I drive at night, at a steady cruise the volts drop to around 12. I stopped at a traffic light and while listening to the pipes, I realize that my turn signals have stopped and my lights have dimmed. My volt gauge is at 10. I throttle up and the gauge moves up to 12. Even at speed she's only putting out around 12 with the headlights on. The alternator is a newer unit but not sure what amp. The regulator is the external type that's mounted on the fire wall. What's my best bet, replace the alt, or can I adjust the regulator? Is this a adjustment I can make or better to take to a shop? Jack of all trades, master of none, but better at some.
Thanx and a good weekend to all,
Everett.

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Old 08-02-2008, 08:05 PM
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sounds like a bad regulator. you shouldnt be dropping that much at idle. if its a autoparts alternator its probably crapped out on ya.
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Old 08-02-2008, 10:06 PM
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It sounds like the alternator may not be spinning fast enough... or, like 69satellite said, your voltage regulator's bad.
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Old 08-03-2008, 05:09 AM
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forgot bout that. are you running underdrive pullies?
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Old 08-03-2008, 07:37 AM
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Thanks for the info. I'm running the stock pulley's and the alternator is, I believe a 61 amp version. I'll check into the regulator, would it be best to replace it or can the regulator be adjusted?
Good-day,
Everett.
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Old 08-03-2008, 08:49 AM
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step one is tighten the belt.....that will often gain you a full volt plus (and verify you have the correct V angle pulley for the belt V angle for no slippage)

step two is verify the guage is reading atleast close to correct (those numbers are way low)....with a multimeter at the alt....

with no accessories on you want/need a minumum 13.4V to 14.4V max at 2,000rpms at the alt....

when a alt doesn't put out enough amps at only idle rpms to support all the normal accessories the guage should only drop to approx 12.4V which is the charge reading for a good condition batt

based on the numbers you posted, my gut feeling is your guage is screwed up...

???? if this is a glas Cobra "fair chance" you don't have nearly enough batt neg grounds sources causing voltage drop (voltage drop indicates resistance on the neg batt supply side so add more and better grounds wires)

???? you do also need to measure and calculate the damper pully diameter versus alt pulley diameter (normal ratio is roughly/typically close to 3 to 1, (at 6000 motor rpms the alt is turning 18,000 rpms)....alt don't turn on with less than about 750 motor rpms typically (750 x 3 = around 2k alt rpms determined by the damper versus pulley diameters)

one fix for more idle amps is to use a just slightly smaller pulley diameter (so it spins faster at idle)....
DO verify that the smaller pulley ratio won't exceed the max alt rpms allowed when the motor is at the hp peak

Last edited by red65mustang; 08-03-2008 at 08:56 AM.
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Old 08-03-2008, 01:22 PM
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If everything is normal under in a lightly loaded situation such as you said, daytime with no lights but low voltage at night when under a load with the lights on, then it will not be a regulator problem. The problem here is not enough AMPs to maintain battery voltage so this could be a problem with the alternator but more like what red65 said it would be a grounding problem with the glass body. A loose belt could cause this and sometimes they can slip without squealing but I seriously doubt it would be a pulley size issue because the voltage would increase to normal when the engine revs up much above idle if that were the problem, lights on or not.
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Old 08-03-2008, 05:34 PM
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If it is only an issue at idle then it is not spinning fast enough or it is under rated. From what you have explained it does not sound like you have a huge electrical load so a 60amp should be more than enough. It looks like it is never charging enough to keep the battery up. At a traffic light for the minute or so you are idling a properly sized fully charged battery should be able to maintain 12 volts or so.
You may be able to play with the regulator to improve the situation a little but if it were me I would get an electronic regualtor and then have the alternator tested on the car to see what the amperage output is. Taking an externally regualted alternator off to test it is sort of useless, it is better to test it in the environment it lives in. You can bypass the regulator by unplugging it and shorting the a and f terminals together if I remember correctly (look it up please, dont take my word for it). This will full field the alternator and it will put out at full capacity.

Chet
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