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charging problem

2366 Views 9 Replies 3 Participants Last post by  Evan
I've worked on the duster all day, and I can't figure out where the problem is. The ammeter shows -15 during the day and -30 with the lights on. It's a good battery and a new alternator. I tought the alternator would solve the problem, so I switched it out - but it did the same thing with a new alternator. Any ideas? I really appreciate it. -Evan
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Mearsure the voltage at the battery when the car is running with a digital voltmeter. You should have 14 to 14.5VDC. Turn the car off, the voltage should drop to 12.0 to 12.5VDC. The battery needs 14 volts to charge. If you never get above the battery voltage (12V), then it will never charge.
i will check that today. If I'm not getting enough voltage, what would my problem be? I've tried with two different good batteries. thanks -Evan
I was wondering if it could be the voltage regulator, but I don't know how to check it.
The only way you can test the regulator is to measure the voltage at the battery when the car is running. Turn off all lights etc. so the only load on the system is the ignition. The voltage should be approx. 14. As you add load (turning on headlights) you should see the voltage drop a little, the alternator only has so much capacity at lower RPMs. Increasing the engine rpm will bring the volatge back up. If something is realy wrong with the alternator, like blown diodes, it will have even a more reduced capacity. When all is working correctly the alternator should be capable of suppling all the vehicles electrical needs and have some extra to charge the battery.
I have a multimeter - I test between the posts for voltage, right?
Yes, between the posts on the battery. It could also be a field problem. What year car? What year alternator? Has anything been modified on the car?

its a 70 duster with electronic ignition. The wires are old and hard, but it has not been hacked together - it looks original. The old alternator was from a 72 fury, but i got a new alternator at the parts store and told them i wanted one for a 70 duster - thats all i know about the alternator. -Evan
Evan, get some voltage readings on the battery, first with the car not running, next running will all accessory stuff off, and then with full lights + high-beams + heater. Then, bring up the rpms and measure the voltage at 2 grand or so.
Well, don't laugh at me too hard. I chased all the wiring from the alternator, and somebody's sad looking splice had come apart. i hooked it back up and it runs fine ....go easy on me lol . thanks for your help! -Evan
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