OT? - Bad alternator?

[pheasant]

Just happened to be in the Subaru, but this is a pretty generic question
about charging system.

Yesterday went out to start the car, stone dead. Figured battery is about 6
years old, go get a new one.

Car starts fine, tach needle is bouncing up and down, (rpm is not varying by
sound), am showing about 14v on gauge, flip defroster on, and down in
discharge, maybe 10-11v. Revving engine doesn't change discharge.

Swap out alternator, system is happy.

Can a bad alternator drain a battery, or was this coincidence that the
battery was dead, and the alternator failed?
(had suspected it was going, as voltage was climbing a bit, I'm figuring the
regulator was getting tired).

Thanks
Mark

 

[Edward Hayes]

First: Check ALL ground connections by removing sanding and reattaching.
Check Battery connections, ground connections to engine,body and starter
connections as this is a common tie point. Yes a shorted diode in the
alternator will drain a battery but, you clamed 14 volts charging at some
point so I think that eliminates that part of the alternator. Post back on
this post on what you find. eddie

 

[Rick Courtright]

Can a bad alternator drain a battery, or was this coincidence that the
battery was dead, and the alternator failed?

Hi,

My understanding, backed up by the literature that comes in the box with
a new alternator, is that it can be a chicken or egg situation: battery
may not have been properly charged by a failing alternator and died,
alternator could have died due to bad battery causing it to overheat a
diode, etc.

Warning I've read several times is when replacing alternator to have the
battery checked. A six year old battery is definitely getting toward the
end of its life anyway, so the fact you swapped out both should keep you
in good shape. Follow Eddie's advice on cleaning everything, and let us
know how it goes!

Another tip I've read on new batteries is to make sure they're first
well charged up from an external source to avoid excess strain on the
alternator. If you've got a trickle charger, you could stick it on
overnight to do this. I really don't know if that helps, but I've tried
it a time or to and it did no harm.

Rick

 

[David Coggins]

Sounds like you had a short circuit diode in the alternator - allows the
battery to discharge back through the alternator windings.

Dave

 

[Edward Hayes]

As Rick suggested: I ALWAYS charge a new battery to a full charge ASAP
using a 10 amp taper charger. Stores give you a battery about 1/2 charged
usually and then you run the heck out of your alternator to charge it.

 

[pheasant]

First: Check ALL ground connections by removing sanding and reattaching.
Check Battery connections, ground connections to engine,body and starter
connections as this is a common tie point. Yes a shorted diode in the
alternator will drain a battery but, you clamed 14 volts charging at some
point so I think that eliminates that part of the alternator. Post back on
this post on what you find. eddie

Hi Guys;

Gonna be lazy and have it chauffeured to the happy dumping ground via the
city sanitation department. When they run 20 bucks rebuilt on
www.car-parts.com ain't gonna lose any sleep over what happened. Just
trying to figure out why the tach and voltmeter went nuts after putting the
new battery in. Thought I detected a whiff of ozone, but they were pumping
a lift station not too far away also.
I had cleaned the terminal connectors, and all cable attachments are clean
and tight. The alternator had a "moment" about 3 weeks ago, where the
needle jumped a bit indicating a charging state a bit higher than it had
ever been before accompanied by a grinding and funny smell, that's why I
wasn't too surprised when I had to replace both. I've never seen a
concurrent failure, so like you say, which came first the chicken or the
egg, most likely never know, but figured someone that reads the NG that has
a better understanding than I might be able to guess what may have happened.

Thanks again.
Mark

 

[Jim Stewart]

Just happened to be in the Subaru, but this is a pretty generic question
about charging system.

Yesterday went out to start the car, stone dead. Figured battery is about 6
years old, go get a new one.

Car starts fine, tach needle is bouncing up and down, (rpm is not varying by
sound), am showing about 14v on gauge, flip defroster on, and down in
discharge, maybe 10-11v. Revving engine doesn't change discharge.

Swap out alternator, system is happy.

Can a bad alternator drain a battery, or was this coincidence that the
battery was dead, and the alternator failed?

If the alternator failed from a shorted diode,
it most certainly could run down the battery.
There are other ways the alternator could fail
though.