Need a second opioion. Is it the battery or the alternator?
#16
Guest
Posts: n/a
Re: Need a second opioion. Is it the battery or the alternator?
"Matt Ion" <soundy106@gmail.com> wrote in message
news:cOtph.615412$5R2.156297@pd7urf3no...
>
> Definitely sounds like an alternator issue. Your first test of
> disconnecting the battery was the right one; you don't want to run that
> way for very long, but it's still a valid way to isolate battery from alt
> problems.
>
I agree with Curly; the risks of disconnecting the battery with the engine
running are terrible. With a good alternator it is a little risky, with one
that has bad diodes (you do suspect the alternator is bad, right - why else
do the test?) you seriously risk huge spikes to all the electronics in the
car. It's not a big deal with a car made before about 1970, but don't *ever*
do it with a modern car. It never was a very good test, either; alternators
with bad diodes would still run the car but would fail to keep up with loads
like driving at night with the heater fan on.
The way to determine whether the alternator is working is with tests as Elle
describes. Personally, I also check for AC voltage with a digital voltmeter
under the same test conditions. Good diodes will produce less than 0.1 volt
AC, while bad diodes will produce 1/2 volt AC or more. The last bad
alternator I've seen (in a friend's Taurus) had two bad phases and the AC
voltage was 1.7 volts! An oscilloscope is even more definitive, as the
waveform of a bad phase is unmistakable, but in practice I've never seen
high AC voltage from an alternator that didn't have bad diodes. Alternators
with bad diodes will also fail Elle's test #3.
Mike
news:cOtph.615412$5R2.156297@pd7urf3no...
>
> Definitely sounds like an alternator issue. Your first test of
> disconnecting the battery was the right one; you don't want to run that
> way for very long, but it's still a valid way to isolate battery from alt
> problems.
>
I agree with Curly; the risks of disconnecting the battery with the engine
running are terrible. With a good alternator it is a little risky, with one
that has bad diodes (you do suspect the alternator is bad, right - why else
do the test?) you seriously risk huge spikes to all the electronics in the
car. It's not a big deal with a car made before about 1970, but don't *ever*
do it with a modern car. It never was a very good test, either; alternators
with bad diodes would still run the car but would fail to keep up with loads
like driving at night with the heater fan on.
The way to determine whether the alternator is working is with tests as Elle
describes. Personally, I also check for AC voltage with a digital voltmeter
under the same test conditions. Good diodes will produce less than 0.1 volt
AC, while bad diodes will produce 1/2 volt AC or more. The last bad
alternator I've seen (in a friend's Taurus) had two bad phases and the AC
voltage was 1.7 volts! An oscilloscope is even more definitive, as the
waveform of a bad phase is unmistakable, but in practice I've never seen
high AC voltage from an alternator that didn't have bad diodes. Alternators
with bad diodes will also fail Elle's test #3.
Mike
#17
Guest
Posts: n/a
Re: Need a second opioion. Is it the battery or the alternator?
"Matt Ion" <soundy106@gmail.com> wrote in message
news:cOtph.615412$5R2.156297@pd7urf3no...
>
> Definitely sounds like an alternator issue. Your first test of
> disconnecting the battery was the right one; you don't want to run that
> way for very long, but it's still a valid way to isolate battery from alt
> problems.
>
I agree with Curly; the risks of disconnecting the battery with the engine
running are terrible. With a good alternator it is a little risky, with one
that has bad diodes (you do suspect the alternator is bad, right - why else
do the test?) you seriously risk huge spikes to all the electronics in the
car. It's not a big deal with a car made before about 1970, but don't *ever*
do it with a modern car. It never was a very good test, either; alternators
with bad diodes would still run the car but would fail to keep up with loads
like driving at night with the heater fan on.
The way to determine whether the alternator is working is with tests as Elle
describes. Personally, I also check for AC voltage with a digital voltmeter
under the same test conditions. Good diodes will produce less than 0.1 volt
AC, while bad diodes will produce 1/2 volt AC or more. The last bad
alternator I've seen (in a friend's Taurus) had two bad phases and the AC
voltage was 1.7 volts! An oscilloscope is even more definitive, as the
waveform of a bad phase is unmistakable, but in practice I've never seen
high AC voltage from an alternator that didn't have bad diodes. Alternators
with bad diodes will also fail Elle's test #3.
Mike
news:cOtph.615412$5R2.156297@pd7urf3no...
>
> Definitely sounds like an alternator issue. Your first test of
> disconnecting the battery was the right one; you don't want to run that
> way for very long, but it's still a valid way to isolate battery from alt
> problems.
>
I agree with Curly; the risks of disconnecting the battery with the engine
running are terrible. With a good alternator it is a little risky, with one
that has bad diodes (you do suspect the alternator is bad, right - why else
do the test?) you seriously risk huge spikes to all the electronics in the
car. It's not a big deal with a car made before about 1970, but don't *ever*
do it with a modern car. It never was a very good test, either; alternators
with bad diodes would still run the car but would fail to keep up with loads
like driving at night with the heater fan on.
The way to determine whether the alternator is working is with tests as Elle
describes. Personally, I also check for AC voltage with a digital voltmeter
under the same test conditions. Good diodes will produce less than 0.1 volt
AC, while bad diodes will produce 1/2 volt AC or more. The last bad
alternator I've seen (in a friend's Taurus) had two bad phases and the AC
voltage was 1.7 volts! An oscilloscope is even more definitive, as the
waveform of a bad phase is unmistakable, but in practice I've never seen
high AC voltage from an alternator that didn't have bad diodes. Alternators
with bad diodes will also fail Elle's test #3.
Mike
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