Battery Voltage drop at higer revs - 99 Taurus

I have a 99 Taurus with the 3.0 engine.
Today on a long trip I noticed that the battery 'idiot light' came on under hard acceleration. Upon further investigation with a OBDII tool
while driving the vehicle I see that the battery voltage drops from a steady 13.5V to around 12v only when the engine revs are increased to around 4,000 or more. The idiot light comes on at these times.
This is indicative of failure of something in the charging system. Does a knowledgeable mechanic here know if it is likely to be the voltage regulator or the alternator? I'm not even sure if the Taurus has a separate voltage regulator or if it's part of the alternator on this vehicle.
Any advice welcome on diagnosing the faulty component.
JP
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Brushes... really. They are probably available. Can usually happen between 125 and 190 kmiles
It ALSO COULD be the VR, but that's classic of worn brushes. But IIRC the charging voltage SHOULD be about 14.3 to 14.6
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Backyard Mechanic wrote:

Hey thanks for your quick advice!! The car has 72K on it, low for the year, my wife does about 8k a year.
Something always seems to 'go wrong' on these long trips. I suspect it's because I'm the driver on long trips and she simply doesn't notice the early warning signs.

Oh OK. I'll get the charging voltage checked independently, the ODBII system may not be well calibrated after 7 years of operation. Will changing the brushes help the average charging voltage also? Changing brushes sounds cheaper than buying a new alternator, just a little more work. I'll check online as well as ask at an auto parts store.
Thanks again.
JP
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Sounds early for brushes... go to google and search for my alt refurb article. Also check autozone's site to see if your Alt is the externally/PCM regulated 'new improved' type.
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On Fri, 24 Mar 2006 02:52:51 GMT, Backyard Mechanic

If the car just gets knocked around town as a "wife's car" 72000 miles can have the equivalent run time of a highway driven car with almost 200,000 on it - so I'd still say, most likely - like 90+%, brushes. *** Free account sponsored by SecureIX.com *** *** Encrypt your Internet usage with a free VPN account from http://www.SecureIX.com ***
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JP White wrote:

battery. check every high current power cable for corrosion and connection torque. Check the large high current wire at the alternator for a good connection.
How often does the car run at 4000 RPM +?.
Bob
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Bob Urz wrote:

All checked good with Battery and cables. Exchange alternator fixed it up. Charges at a higher voltage now.

Rarely. Just when passing and needing the kick down. Typically cruises at 2500-2700 on interstate.
JP
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JP White wrote:

I had the same problem with a 95' Escort. The charging light would come on when the engine speed went over 2000-rpm. Changing the alternator fixed it.
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Backyard Mechanic wrote:

Your memory is right. Now charging at 14.3 with exchange alternator.
Glad I caught it early before it let my wife down one day.
JP
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wrote:

Your brushes are worn out and/or sticking.

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JP White wrote:

load maybe the belt is slipping ... otherwise its the brushes .. the slip rings on the stator are not round or the brushes are so worn there is not enough spring pressure and at the higher rpm the contact is not good enough
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