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Old 03-18-2011, 02:11 AM
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car wont start without a jump

can somone please help i have a 75 chevy caprice with new battery and new 200amp alternator but car wont start unless i jump it as soon as i jump it starts so i read the volts it show 12.5 while car is on i checked the connections to starter everything is good heres the killer part its happend twice where i take the car out without jumping it and battery reading 14.5 or so i shut off the car and then it reads under 13v i do have a sound system and video equip in the car if i higher volume my headlights start to blink and radio blinks as well can someone please help summer is almost here

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Old 03-18-2011, 03:52 AM
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I know a couple things about the electrical system but don't know beans about the sound system. A normal system will show around 12.5 not running and 14-15 with the engine running. A good test for a charging system is to note the volts at rest, 12.5 or so is right. Then start the car, turn everything you can on and at idle you should see one volt higher than you did without the engine running.
First off I would suspect a ground, that's always a good place to start. Any connection that's bad should be hot after a failed start, like the battery terminals ect. I have seen many old dying starters that one battery wouldn't spin but 2 would. A starter draw test would be great right there. And sometimes it takes more amps when they are hot.
Now the dicey part, fixing what someone else might have messed up. The sound system is a red flag. I have seen more butchered up crap come through my shop, even systems installed by the places that sell them with serious issues, like undersized wire, inadequate grounds and no circuit protection. Probably half of them over the years looked like they were installed by a caveman with a big rock and a couple rolls of tape.
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Old 03-18-2011, 04:18 AM
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Unhook the main pwr feed to all your sound system..... see if it does the same thing. If it doesn't.....
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Old 03-19-2011, 11:18 AM
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If you have a 200 amp alternator in a '75 Caprice, there have been some significant changes to your car, other than the stereo. The biggest alternator in '75 was the 63 amp 10si (or maybe the 94 amp 12si).

If this is standard GM alternator hookup, they probably have the ignition-on terminal connected wrong, or have left out a diode. This is causing the alternator to drain the battery after you shut it off. You need to find out the model of alternator they used, and then check the wiring.

Normal running voltage will show about 13.8-14 volts when charging, and once its charged it will drop down to about 12.5. 14.5 volts is a little high, but probably not the source of your problem.

If your sound system makes the lights start flashing when you turn it up, you are drawing too many amps. Either the alternator is not sized right, or they haven't used big enough wiring and relays. They also might have exaggerated the alternator output - it might not be a 200 amp alternator.

Bruce
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Old 03-20-2011, 06:02 PM
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if the headlight are blinking off an on under load you have a bad connection check grounds and main term on starter. do a voltage drop test while cranking. hope this helps
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Old 03-20-2011, 09:46 PM
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genes10, you might want to explain a voltage drop test, I don't think that many hobbyists have a clue how to perform the test. Great idea for any electrical problem and a test that should be used more often by all of us. Also I might add that attempting any electrical troubleshooting with out a basic trouble light and volt/ohm meter is just futile.
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