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Classic Mustangs Tech Forum

Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 03-16-2009   #1 (permalink)
extreme69 is offline Apprentice

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Default lights dim at idle and are bright at high rpm?

I have a 1969 coupe with a 351w, When i am at idle my volts gauge says 12 volts, when my foot is on the gas it says like 17 volts. At idle the lights are dimmer then when my foot is on the gas. Are these normal values for the volt readings and is it normal for the lights to do this, its not a drastic change in light levels but noticeable.
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Unread 03-16-2009   #2 (permalink)
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sounds normal do u have aftermarket pullys that slow down ur alternator
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Unread 03-16-2009   #3 (permalink)
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there is a slight difference in brightness on my 65 as well from idle to pushing the gas. Not very much but it is noticable. I don't know what the volts should be though
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Unread 03-16-2009   #4 (permalink)
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Quote:
Originally Posted by extreme69 View Post
When i am at idle my volts gauge says 12 volts, when my foot is on the gas it says like 17 volts.
You need a voltage regulator.

You should be more concerned about the 17V than than the 12V. The battery by itself should be 12.6V. At idle you have essentially nothing coming from your alternator. The maximum voltage should never go over 14.5V or at the most 15V. More than that and you risk burning up things including the battery. If yours goes to 17V then the VR is not doing its job. It would appear to be stuck and causing too little charge at idle and way too much at higher RPM. It should adjust the charge rate as you change RPM to maintain something close to 14 - 14.5V A little bit outside that range isn't really bad, but NOT 17V.
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Unread 03-17-2009   #5 (permalink)
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I checked my volts at the battery (don't have a gauge in the car)and it was around 14.3
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Unread 03-17-2009   #6 (permalink)
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this might be a dumb question but where on the car would the voltage regulator be?
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Unread 03-17-2009   #7 (permalink)
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Not too sure on the voltage regulator location on a 69, but on my 72 it is right under the solinoide. On my girlfriends 68 it is next too the windshield washer resivoir. It is usually a silvery goldish color square box that has 6 or 7 wires going into it. I dont know what to say about the 17 Volts but find where the negative battery cable attaches to the block and unhook it from the engine. now take some emory cloth or steel wool and clean all the corosion, rust, paint, grease from the block and the battery cable. After you cleaned both re-attach them and spray a little paint on to keep it sealed from the environment. This should help with the dim lights.
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