My mustang battery gauge go down when I stop at a light

if it is a "NEW" battery must be the 1/0 cable as it attaches to the frame break out the coarse wirebrush and clean and re-assemble this connection and the 1/0 battery cable going to the motor block (near the alternator)....these should be comfortable with pullin' 300 cold cranking amps~

if you believe there to be a parasitic drain, you can test for this~ http://www.youtube.com/watch?v=KF1gijj03_0

your alternator belt could be loose~

Idle speed is too low... that sound interesting how would make turn the idle speed up

seems these are fully computer integrated...would have to research how to do this...mostly chooses the idle speed based on prevailing conditions (i.e. A/C switched on) it is really not hurting anything if is relying on the power of mostly the battery as when you get goin' again will replenish immediately...the dim headlights are a concern, as it represents a faulty ground yet~

Maybe a ground could be the problem. It only happen if I turn the lights on or radio I replace the battey and alt and the problem still there. A few people told me it could be the underdrive pulleys. But I dont see how a pully would cause that

no buddy, it's the 1/0 battery cable as it attaches to the frame...what most people DO NOT know is that this connection "welds" itself with each use...pulling so much amperage is just like an electric welder and punches this connection red hot and hopefully fuses it together (with no contaminants)...but often is THE source for "Brown power" as the poor ECU perhaps 75o mA device gets it's ground by this partially correct offering varying amperages and varying voltages creating a "Brownout" condition....this is why I like the old ways~ buggy and whip~

...as I was saying this 1/0 connection will WELD itself...and in that light probably DO want the larges area in contact...use a coarse, unfriendly wire brush on the contact surfaces...be lucky if we get a quarter sized area in contact~

Sorry im having problem understand ing was a 1/0 connection? Are we talking about the positive cable or negative? Thanks for the help your giving me sir

the 1/0 cable is the HIgh amperage cable what is used to connect to the battery...both negative and positive...both must be free of corrosion....but the black one that connects to the chassis/frame is the one of concern~

http://custombatterycables.com/what_size_cable_do_you_need.htm