can a faulty voltage regulator kill a battery? 91 Fox alternator (90A

my big question here is CAN A FAULTY VOLTAGE REGULATOR KILL A (relatively) NEW BATTERY??..
i posted a few days ago about my 91 fox alternator (90 Amps) problems...
q quick recap--
my battery didn't have enough juice to actuate the starter. was jumped and drove down the street to autozone. took battery out of car, had it tested at autozone. it tested bad. they gave me a new battery (as it was just one year old and under warranty). i installed the battery in the vehicle and had the autozone people bring their alternator testing equipment out to the car. we were getting a reading of about 11 V and 5 A output...i think these numbers should be about 13 V and 90 A, respectively. i was told that the alternator is bad and should be replaced.
i got to wondering why the alternator light didn't come on. tonight, i put the key in the ignition and turned to the engine run position. the alternator warning light did not come on. there was no activity from it when i switched to start and back to run.
my bently manual tells of testing the light (removing the instrument cluster)..which i really don't want to do.
it also mentions that this condition (battery not being properly charged & light not coming on) could be caused by a faulty regulator. if anyone has any knowledge in this area, please advise.
i am planning on removing the alternator and having it tested in-store. as i am sure it will be much easier to remove the alternator and have it tested rather than tearing the dash and steering wheel apart to test the light itself.
thanks andy
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Yes.
It should be somewhere around 14.2V.
If the charging circuit is not producing enough power, but almost enough, that can miss turning on the "battery" light but still cause problems.
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Joseph E. Meehan

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The alt light not coming on can be caused by 2 things. 1- faulty alt & 2 wiring problem to alt warning lamp. Also alt will not charge if the alt light doesn't work.

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A faulty regulator can fail in several ways. In my experience the most common fault is that you get too high a voltage at high RPM's. This will permanently ruin a battery fairly quickly.
Voltage regulators are quite precise. The VW that I have worked on ('87 GTi) uses a 14V voltage reguator (like most cars) and is accurate to within 1%. Try measuring the voltage between the positive terminal of the alternator and ground as you rev the engine to 3 - 4000 RPM. If you see more than 14.5V it means the regulator is bad.
The other failure mode is not undervoltage. This does not directly harm the battery, but will cause no or insuficient charging, which reduces the life of the battery. If the battery is left completely drained it can warp the plates and thereby destroy the battery.
If you measure the output from the alternator as described above you should see at least 13.5V as long as the enigne is running at 1500 RPM or more. More often than not you will see the correct 14V even at idle.
On the '87 GTi the regulator (mounted on the alternator) can be replaced separately. I don't remember if the regulator comes with a set of brushes or not. If the alternator is original to the car, you may want to replace the whole unit rather than messing around with brushes and regulators.
Also, a bad battery puts a heavy load on the alternator and can wear it out prematurely.
Agingric37 wrote:

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got the alternator out successfully....
used my test light to test for current in alternator's red wire. OK
used jumper wire between alternator's blue wire, and found to my horror, that the alt. warning light did not come on. i found that the blue wire was broken near the battery. repaired that, and the light comes on.
so i now know that my problem was the broken blue wire OR a faulty alternator. in that case, the faulty alternator caused the battery to fail, upon removing or replacing the battery, i broke the blue wire.
i am going to take the alternator to autozone now to be tested.
thanks andy
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alternator and battery tested OK.
i'm going to assume that
1. the battery was defective, and i broke the blue wire while removing battery.
2. the blue wire broke on its own and that's why my light didn't go on.
thanks to all who wrote.
off to philly to see my beloved dodgers take on the phils.
later andy
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