If the battery is at 12.75 volts, it should be fully charged. Even with zero charging from the alternator, there should not be drivability or accessory problems like you described until the battery discharges. I'd suspect some kind of bad connection..
Sure, either battery or alternator. But, if you have a voltmeter, why not test the alternator yourself? Fully charge the battery, put it in the car, and get a voltage reading across the terminals. Should be
12.5-13 volts or so. Now start the car and rev it to a fast idle. The voltage should increase to about 14.5 volts or so. If the voltage does not increase when the engine is reved, the alternator is not charging. Otherwise, probably the battery.Harry
You say the battery measured 12.75 volts and that is good. However, the test is what the voltage drops to under load. You can have a battery that immediately drops to 3-4 volts or whatever the instant a load is applied. Check voltage with the engine off and all the lights on for a minute. If the voltage holds up at about 12.4 or 12.5 then look elsewhere for the problem. Be aware that a battery problem can be intermittent. If that happens to be the original battery then forget checking. Just go buy a battery and continue testing.
He was being a "smart ass?"
Re check the open circuit voltage, if it hasn't dropped that much you can skip that step.
The sound of a failed diode. Connect your DVOM between the alternator ouput stud and battery negative, set DVOM to AC volts, jump-start Mustang (running) if AC voltage measures over 100mv, there's your problem. AC leaking into the electrical system certainly explains the driveability symptoms. The fact that it took a jump to get it cranking enough to start suggests that there is a loose or rotten connection somewhere between battery and the starter, the fact that it took a jump to get the window back up suggests that there is a loose or rotten connection somewhere where the low current circuits tap off of the battery supply. Both problems combined suggest a loose or rotten ground (common denominator). What ever it is, I wouldn't risk the second alternator until I determined where the bad connection is.
Actually..you're sounding like an ass right now......
Then take the damn battery out, bring it to your local parts store, and have them charge it!!!!!!
Bad battery no drivability problems? A car cannot run off an alternator alone. Maybe a carubrated engine can...maybe. But todays fuel injected engines w/ I don't know how many electrical components? No damn way. And I speak from personal experience there..been there, tried that.
So yea, a bad battery sure as hell could cause driveability problems.
Stephan
But intermittent cables/connections could. Including those inside the battery..
It's got to be one of those. Even with the symptoms the alternator and voltage regulator actually passes every test I give it. The only thing I couldn't check in one way or another was the field because the test port is not easily reachable. So I'm going to get a new battery and try not to go far with it for a few days.
In pulling the battery I found one of the pockets of the battery tray filled with water and the bolt rusted away. The clamp bolt sheared on me... what fun....
You can check for AC over DC by checking right at the battery with the engine running. Use a capacitor in series with the VOM and read on the AC scale. There should be no AC present.
Whoa! Hey, sorry if what I said was a bit obvious. I have no way of knowing if you are an expert or a notice, nor anything besides what you post. Excuse me if I have somehow offended.
You have a voltmeter and no battery charger? Fine, my apologies for not knowing what you have in your garage. Like the other guy said, take it to Autozone and wait an hour while they charge it for you. Testing the electrical system when the battery is not fully charged is something of a waste of time, if the battery is low it can screw all sorts of things up.
Yeah, the high-pitched whine is an odd one. However, the fact that the voltage increased with RPM's (if that's what you're saying, I don't know for sure) tends to imply that the alternator is charging. If the battery is the original one, I would definitley replace it before I replaced the alternator, especially given what I think you're saying. Also, odds are if the battery has died by this point if it wasn't bad already. Being completly drained is hard on even a new battery, much less one that's six or seven years old. You'll probably have to replace it anyway.
But hey, good luck, it sounds like you've done a whole battery of tests.
Harry
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