98 Civic - Possible electrical problem?

> #1. learn how to use a meter properly. you do /NOT/ measure a dc
> voltage with the meter set to ac.

Before I go any further, I want you know that I'm grateful for your
help. That said... I'm pretty sure I know how to use a multimeter. My
intention was NOT to learn the DC voltage, but rather the amount of AC
ripple, which exists in any DC generated by a rectifying circuit.
Obviously, since I measured pretty much the same thing on another
"good" vehicle, either my meter was wrong, or I'm not using it right,
but I'm going to blame the sub-$10 digital multimeter, unless you tell
me there's a different way to measure AC ripple on a DC voltage.

> #2. suspect a crock from autozone. there's no way an 80A alternator is
> going to output 175A, even intermittently.

I'm leery of what they told me, but I'm confused as to how two
different stores, 6 months apart, managed to measure the same thing,
and make the same diagnosis. I'd believe you over them any day, but
I'm still curious as to what they're measuring...

> #3. check the brushes on the alternator. they're cheap to fix. the
> rest of the honda alternator is good for 300k miles very often.
> #4. re mileage, check for gotchas like brakes stuck on, dying oxygen
> sensors, etc.

This is great advice. I never even considered the brakes/O2 sensors,
either of which could be faulty by 150k miles, and would definitely
impact fuel economy. Hmmm, O2 sensors... Maybe that cheap-***
multimeter will come in handy, after all! At least I know how to get
DC voltage readings from it, for sure!

Thank you again for your time and help.

-Randy

> wrote in message
oups.com...
>> You must be using an analog voltmeter, which reads like that on AC
>> ranges.
> <snip>
>> All digital voltmeters work
>> right.
>
> Nope, it's a digital multimeter, but admittedly a dirt-cheap (sub-$10)
> one. Still, I guess I should have realized that it was probably just
> doubling whatever voltage it saw, hence that reading.
>
> More in a reply to "jim beam" later in this thread.
>
> Thanks again for your time and response!
>
> -Randy
>
>

That's good to know. I have a couple of really cheap DVMs to carry in the
cars and they may do the same. I ought to check. Thanks!

Mike

> wrote in message
ups.com...
>> #1. learn how to use a meter properly. you do /NOT/ measure a dc
>> voltage with the meter set to ac.
>
> Before I go any further, I want you know that I'm grateful for your
> help. That said... I'm pretty sure I know how to use a multimeter. My
> intention was NOT to learn the DC voltage, but rather the amount of AC
> ripple, which exists in any DC generated by a rectifying circuit.
> Obviously, since I measured pretty much the same thing on another
> "good" vehicle, either my meter was wrong, or I'm not using it right,
> but I'm going to blame the sub-$10 digital multimeter, unless you tell
> me there's a different way to measure AC ripple on a DC voltage.
>
>> #2. suspect a crock from autozone. there's no way an 80A alternator is
>> going to output 175A, even intermittently.
>
> I'm leery of what they told me, but I'm confused as to how two
> different stores, 6 months apart, managed to measure the same thing,
> and make the same diagnosis. I'd believe you over them any day, but
> I'm still curious as to what they're measuring...
>

Me, too. Overcurrent should be nearly impossible, because of the physics of
the windings. http://tegger.com/hondafaq/altcharge.html Maybe the regulator
is running wide open? I'm grasping at straws here.

I'm sure your situation is frustrating, but you're providing some
interesting glimpses into the "shouldn't work that way" areas :-} As Mr.
Spock would observe in the old Star Trek series, "Fascinating."

Mike

wrote:
>> #1. learn how to use a meter properly. you do /NOT/ measure a dc
>> voltage with the meter set to ac.
>
> Before I go any further, I want you know that I'm grateful for your
> help. That said... I'm pretty sure I know how to use a multimeter. My
> intention was NOT to learn the DC voltage, but rather the amount of AC
> ripple, which exists in any DC generated by a rectifying circuit.
> Obviously, since I measured pretty much the same thing on another
> "good" vehicle, either my meter was wrong, or I'm not using it right,
> but I'm going to blame the sub-$10 digital multimeter, unless you tell
> me there's a different way to measure AC ripple on a DC voltage.

i doubt a meter that cheap will give you ripple since it'll need to
"offset" the dc. also, the ripple should be something of a funky
waveform since the alternator is 3-phase - it'll be 3 full-wave
rectified bumps, superimposed at 120 degrees each - so a cheapo r.m.s.
approximation will be off too. if you have one rectifier out, waveform
will be real ugly - ugly enough to cause noise on the audio system.
borrow an oscilloscope if you want to get serious about that stuff.

in the mean time, measure the dc voltage at idle, no load and at idle,
full load. if the readings are within spec, it's probably just brushes
giving you that startup flicker. also, check the gotcha of belt tightness.

>
>> #2. suspect a crock from autozone. there's no way an 80A alternator is
>> going to output 175A, even intermittently.
>
> I'm leery of what they told me, but I'm confused as to how two
> different stores, 6 months apart, managed to measure the same thing,
> and make the same diagnosis. I'd believe you over them any day, but
> I'm still curious as to what they're measuring...
>
>> #3. check the brushes on the alternator. they're cheap to fix. the
>> rest of the honda alternator is good for 300k miles very often.
>> #4. re mileage, check for gotchas like brakes stuck on, dying oxygen
>> sensors, etc.
>
> This is great advice. I never even considered the brakes/O2 sensors,
> either of which could be faulty by 150k miles, and would definitely
> impact fuel economy. Hmmm, O2 sensors... Maybe that cheap-***
> multimeter will come in handy, after all! At least I know how to get
> DC voltage readings from it, for sure!
>
> Thank you again for your time and help.
>
> -Randy
>
>

"Earle Horton" > wrote in
:

> wrote in message
> ps.com...
> ...
>>
>> By the way, I measured 30 volts AC across the battery with the
>> engine running, and then went and measured another vehicle, and
>> got nearly the same reading, so I assume that most likely, that's a
>> normal reading.
>>
> I admit I never thought to measure AC across a vehicle battery. The
> alternator is supposed to have diodes in it, which filter out the AC,
> but I suppose you are going to get some fluctuation with the
> alternator making output. What is the DC voltage?
>
> Earle
>
>
>

his meter could measure pulsating DC on the AC mode,especially if it's a
True RMS meter. 30VAC seems odd.

DCV is more important,though.

--
Jim Yanik
jyanik
at
kua.net

jim beam wrote:

<snip>

> i doubt a meter that cheap will give you ripple since it'll need to
> "offset" the dc.

Plenty of cheap meters (including my $7.99 no-name digital DMM) will
read the ripple on top of a DC voltage. On the AC setting, the input
signal is AC coupled (through a capacitor).

Regardless, both the poster's car and a known good car measure 30V
ripple. This simply isn't so. There is something wrong with the meter or
the method.

<snip>


> in the mean time, measure the dc voltage at idle, no load and at idle,
> full load. if the readings are within spec, it's probably just brushes
> giving you that startup flicker. also, check the gotcha of belt tightness.

With the caveat that on some cars (like my '94 Civic) the set point for
the voltage regulator is lowered while at idle (if a few other
conditions are met). Bumping the RPMs up a tad from Idle makes the set
point return to 14.5V (nominal).