Kinda OT: Electrical Problem on New Ford

The car is a 2000 Escort ZX2.
The problem is that at RPMs above an idle, the battery light comes on. I checked things out with a voltmeter, and indeed, when the engine is
running fast the voltage at the battery drops from 14.1V at idle to 13.low, with an excursion to 12.high if you drop the throttle fast.
Presumably the engine just normally needs more amps to run fast, to run the injectors and ignition and whatnot. But I don't know if what I'm seeing is a battery going dead, a bad charging system, or some fault developing in the ignition or injectors that's pulling an unusual amount of current.
So -- is this diagnostic of anything specific? Does anyone have any suggestion for how I might measure the alternator output current to get a yes/no determination on whether that's going on the fritz?
(I may throw in the towel and take it to the dealer; that'd be admitting defeat, but on the other hand it's a good dealer so it'll probably get the problem fixed).
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Tim Wescott wrote:

Most car parts places will test the battery for free. They also test alternators, but they have to be removed from the vehicle first.
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Just a shot in the dark... likely a centrifugally induced, open alternator field winding... or brushes floating on the slip rings for some reason.
Were it mine, I'd slap on a rebuilt alternator.
If your not going to do it yourself, by all means avoid dealers, they're mega bucks. Pretty much any competent automotive shop should have no issue with this... (and/or check the phone book for an auto electric shop.)
Good luck!
Erik
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    Actually, what I think you are seeing is badly worn brushes, which start bouncing off the slip rings at speed, and thus are making contact only part of the time.
    That is 12 years on that original set of brushes?

    At 12 years -- the cheapest thing is to first pull the alternator apart and replace the brushes.

    Do you have an oscilloscope? If the problem is what I suspect, you should see a steady output when running at an idle, and intermittent voltage at the higher speeds.

    Good luck,         DoN.
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Tim Wescott wrote:

Take it to an Advance/Autozone/OReilly/PePBoys type auto place. They can toss a charging system tester on it in the parking lot and tell you if it's battery (not likely) or alternator. I'm betting the alternator is dying. Sounds like either a bad rotor or a bad slip ring. Either way it isn't a tough job to swap out.
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wrote:

Ditto
Gunner
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I

Just played that game a month back with the van. Happened on a 1200 mile trip, finished it with the last bit of charge in the battery, no auto parts places open at 2 AM in the middle of nowhere. Like you, I wasn't sure whether it was battery, alternator or something in the wiring. Started with the obvious, pulled the alternator and had the parts place test it, didn't even make it to the diode test, was a deader. Mid-trip, the light would go off if I gunned the engine. Then, not. I was lucky, the alternator is on top of the engine, pull the belt, 4 bolts and it's free. Took longer to find the socket set than to get it off. So I'd check that first. HF sells a copy of a decent battery load tester, have one of those. Said the battery was OK, but I replaced it anyway. Pulled it for recharging and it had a "recycled" sticker at the rear, explains the lack of any stickers on it. Bubba was cheap-ass on maintenance.
So don't overthink this and spend hours on it. Get another buggy, throw BOTH alternator and battery in and head to the local parts place. You MAY need both, have had that happen, too. Alternator goes, takes out the battery, generally a regulator failure when that happens.
Stan
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wrote:

Firstly, if you are reading 14 volts than the alternator is working, at least at the moment you read the 14 volts.
As for reading less at higher RPMs it is very unlikely that it is other then an alternator problem as a healthy alternator will put out a significant amount of amperage and for the engine to use so much electricity that it reduces circuit voltage to 12 volts while the alternator is producing 60 amps would likely mean a direct short.... which would shortly become very apparent.
My suggestion would be to take the alternator off and see if you can find a shop that repairs them to have a look at it.
I once had an Isuzu diesel pickup that exhibited similar symptoms. Took it to the shop and never got to the end of my explanation before the mechanic said "bad brushes" :-)
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[...]

That's my take on it too.
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Check the brushes.
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wrote:

Bad brushes in the alternator - and possibly worn or out-of-round slip rings. The brushes are pretty common cause for the described problem. ALMOST guaranteed a new alternator will solve the problem if you don't want to fiddle around fixing the old one.
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Den 24-12-2012 23:54, Tim Wescott skrev:

Most likely the fault is on the alternator. There are severeal faults that will give this indication.
1: Worn brushes 2: Worn sliprings 3: Defective rectifier
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Flea bay sells cheapo cigarette lighter voltmeters:
<http://www.ebay.com/sch/i.html?clk_rvr_idC0876222790&_nkw=cigarette+lighter+voltmeter&_trksid=m194&ssPageName=STRK:MEFSRCHX:SRCH I have one in each car. You can easily tell if it's charging or not. I don't trust their accuracy, but you can see the trend.
Further, you can test an alternator by ear. At idle, load it with headlights, and rear-window defrogger. You can hear the engine slow slightly.
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On Wed, 26 Dec 2012 01:21:03 +0000 (UTC), David Lesher

Won't test for intermittent operation like the OP described.
You CAN however, test for bad diodes with a standard AC voltmeter. Bad diodes cause ripple, which is read as low voltage AC And of course a DC voltmeter will tell you the output.
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snipped-for-privacy@snyder.on.ca wrote:

On the subject of quick/dirty alternator tests:
The rotor 'field' windings, when energized turn the alternators shaft into a fairly decent electro magnet.
With the engine running, and the alternator charging, the end of the shaft should attract a screwdriver or other ferrous object. One that isn't charging might still retain a very small amount of residual magnetism. You can even turn on something like turn signals, and 'feel' the magnetic strength fluctuate as the voltage regulator modulates the field current up and down. Try it!
Sometimes, you can also kinda hear the belts working a little harder on a working unit... you can have someone load & unload the alternator with lights, blower, R window defogger or whatever while you listen. This was never really conclusive, and worked better back in the 'V' belt days of yore.
As usual, a clear head, common sense, good judgment and situational awareness are paramount... especially working with and/or around running engines/machinery.
Erik
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snipped-for-privacy@snyder.on.ca writes:

That is quite true.

That Depends. Some AC voltmeters will read ~RMS ie AC+DC; so test the voltmeter first.
An easier test is a AM radio. Listen between stations, if you have lost a diode (or diodes) the whine under load will be FAR louder. A baseline help, give a listen before it fails.
When I am in doubt, I use a scope.,
And:

I find bad diodes to be the most common failure mode.
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On Tue, 25 Dec 2012 23:04:45 +0100, Uffe Brentsen

And the third one is a real long-shot.
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wrote in message wrote: >Den 24-12-2012 23:54, Tim Wescott skrev:

And the third one is a real long-shot.
---------------------------------------
No, the third one is not the problem. One open diode in the rectifier will give low voltage at any RPM. I've had this one twice. Once, I replaced the rectifier assy., as a local shop did alt. work and had the parts. 2nd time, just replaced the alt. Which is what I'd do with this one.
Garrett Fulton
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