The DVM reading (-) is correct. The battery being the source would read
drain current as positive if the red lead was connected to the battery +
post.

As far as the magnitude goes, did you read with all the accesories off?
It strikes me as a tad high, was the dome light on? something else on?

Good luck.

VanguardLH wrote:
> Mike Copeland wrote:
>
>
>>>Hmm, come to think of it, I should put the charged battery back into the
>>>car and connect it up. If there is a leak in the wiring or other
>>>component (or even through the alternator) then I might see this by next
>>>weekend (although the battery might still be good enough to start the
>>>car so I'm not sure this test will reveal anything).
>>
>>Before you reconnect the battery, put an ammeter in series with the
>>positive cable. You will see any current draw. If you get a current draw
>>pull the plug on the alternator and see if goes away.
>
>
> Did as you mentioned. The old ammeter didn't budge when I put it across
> the "+" on the battery to the positive cable (with the negative cable
> already connected to the "-" battery post), even on the 6A range. That
> seemed weird since I know the radio is still hooked up and has a clock
> in it. So I got out my DVM which has a 2A scale and it showed 0.438A,
> or a little under half an amp. Odd part was that it showed -0.438A
> (with red probe to cable and black probe on "+" battery post), as though
> current was running to the battery instead of out. In any case, there
> is a little bit less than an ampere showing on the DVM from the battery.
>
> I left the battery disconnected until it's time to take in the car to
> have the alternator tested. My buddy isn't showing up today so maybe
> tomorrow.

"VanguardLH" <V@nguard.LH> wrote in message
newsoqdnUZxHvMqV-zVnZ2dnUVZ_gadnZ2d@comcast.com...
> AS wrote:
>
>
> Yeah, I figured that after thinking about it. The current would go out
> from the "+" battery post so the red probe on the DVM on there would've
> been backwards to the electron flow.
>
>> As far as the magnitude goes, did you read with all the accesories off?
>> It strikes me as a tad high, was the dome light on? something else on?
>
> The only device that I know of that would've been on was the clock in
> the radio. No lights, including dome lights, were on. The key was not
> in the ignition switch so it wasn't even partially turned to enable some
> of those circuits.
>
> I figured if a diode in the rectifier were shorted then that might be
> the cause of the drain through it. I found a basic schematic at:
>
> http://www.alternatorparts.com/under...lternators.htm
>
> If a diode were shorted, wouldn't that mean that there would be current
> flowing through it (back to ground)? I always thought a diode would
> blow open due to the heat of the leakage current but then these are
> high-current diodes and the leakage current is only half an ampere.

The reason the amp draw may be a little high is because of the keep alive
memory in the radio and computer. Here's how to get around that. Hook up
your amp probe as before between the +post and +cable. Now take a jumper
wire and place it between the +post and +cable. Leave it there for at least
15 seconds (some take up to a minute). At this point your amp probe will
read zero. Once you remove the jumper wire the probe should read much lower
than in your initial test.

All that is moot though as I agree with JohninKY's original post, you have a
bad diode since it's only charging 12.5 volts. BTW... that shop that says
they can't jump the car, then test the alt is suspect. It sounds like they
have an idiot proof tester of some sort and only have idiots to test with.
;-)

NAPA sells a very good reman alt with a lifetime warranty. Be sure to ask
for their premium line as they also have a cheaper line with a 3 year
warranty. They also sell a new one for many applications for not too much
more.

"Dave" <mecdave@hotmail.com> wrote in message
news:g4vlg3$29q$1@registered.motzarella.org...

>
> The reason the amp draw may be a little high is because of the keep alive
> memory in the radio and computer. Here's how to get around that. Hook up
> your amp probe as before between the +post and +cable. Now take a jumper
> wire and place it between the +post and +cable. Leave it there for at
> least 15 seconds (some take up to a minute). At this point your amp probe
> will read zero. Once you remove the jumper wire the probe should read much
> lower than in your initial test.
>
> All that is moot though as I agree with JohninKY's original post, you have
> a bad diode since it's only charging 12.5 volts. BTW... that shop that
> says they can't jump the car, then test the alt is suspect. It sounds like
> they have an idiot proof tester of some sort and only have idiots to test
> with. ;-)
>
> NAPA sells a very good reman alt with a lifetime warranty. Be sure to ask
> for their premium line as they also have a cheaper line with a 3 year
> warranty. They also sell a new one for many applications for not too much
> more.

PS I just reread your first post. That $118 alt from Pep boys is a great
price for a lifetime warranty! I once bought a $22 alt for a Chevy from
AutoZone. I asked if they had anything of better quality. He said it had a
Lifetime Warranty so I bought it. Darn thing lasted 8 years!

"VanguardLH" <V@nguard.LH> wrote in message
news:u9adncsjtdLy9O7VnZ2dnUVZ_r3inZ2d@comcast.com. ..

> While the alternator isn't
> that tough to remove (and put back in), I'd rather not do it if not
> necessary, plus an in-car test might find a different problem, like a
> short in the harness, a short in a dash guage, or whatever.

You're making this more complicated than it is. Shorts cause fuses and
fusable links to burn out, not drag the voltage output down. You've already
determined the alt can't put out more than 12.5 volts even though your
battery voltage was well below that. It should have been putting out 13.8v,
if not more on a dead battery.

If you had a problem with your harness or dash gauge not energizing the
alt, or the alt voltage not getting to the battery, the battery voltage
would have read well below 12 volts, not 12.5v. 12.5v is sort of a magic
number. It shows the alt is trying but one of the diodes is burned out.

Testing it for amperage output like the shops want to do, whether on car or
on the bench tester, or testing for starter draw while cranking, isn't going
to show a different result. You really need another alt.

"Dave" <mecdave@hotmail.com> wrote in message
news:g4vtr4$qot$1@registered.motzarella.org...

> Shorts cause fuses and fusable links to burn out, not drag the voltage
> output down.

Let me rephrase that: Shorts cause fuses and fusable links to burn out, not
drag the voltage output down - for more than a few seconds anyway.

"Dave" <mecdave@hotmail.com> wrote in message
news:g4vtr4$qot$1@registered.motzarella.org...
> "VanguardLH" <V@nguard.LH> wrote in message
> news:u9adncsjtdLy9O7VnZ2dnUVZ_r3inZ2d@comcast.com. ..
>
>> While the alternator isn't
>> that tough to remove (and put back in), I'd rather not do it if not
>> necessary, plus an in-car test might find a different problem, like a
>> short in the harness, a short in a dash guage, or whatever.
>
> You're making this more complicated than it is. Shorts cause fuses and
> fusable links to burn out, not drag the voltage output down. You've
> already determined the alt can't put out more than 12.5 volts even though
> your battery voltage was well below that. It should have been putting out
> 13.8v, if not more on a dead battery.
>
> If you had a problem with your harness or dash gauge not energizing the
> alt, or the alt voltage not getting to the battery, the battery voltage
> would have read well below 12 volts, not 12.5v. 12.5v is sort of a magic
> number. It shows the alt is trying but one of the diodes is burned out.
>
> Testing it for amperage output like the shops want to do, whether on car
> or on the bench tester, or testing for starter draw while cranking, isn't
> going to show a different result. You really need another alt.

Dontcha hate it when they never come back to tell you you were right, and
thanks for the help. I know I do. </talking to myself>

"Dave" <mecdave@hotmail.com> wrote in message
news:g5dh7l$2m6$1@registered.motzarella.org...
> "Dave" <mecdave@hotmail.com> wrote in message
> news:g4vtr4$qot$1@registered.motzarella.org...
>> "VanguardLH" <V@nguard.LH> wrote in message
>> news:u9adncsjtdLy9O7VnZ2dnUVZ_r3inZ2d@comcast.com. ..
>>
>
> Dontcha hate it when they never come back to tell you you were right, and
> thanks for the help. I know I do. </talking to myself>

just wantja to know..I feel your pain.