High Voltage

[Jim Rosinski]

Probably way past due for me to post something not off-topic...

Today while descending from a leaf-watching flight at 13K down to 5K,
the "high voltage" indicator flickered for about a minute and then
went out. It has never done that before. I forgot that a useful
procedure is to cycle the alternator half of the master switch so I
cannot report a result of having done that. Throughout, alternator
readings were fine, and battery water levels checked fine after the
flight. A brand new voltage regulator was installed 3 years ago.

Thanks in advance for any advice on what might be wrong, or
appropriate further courses of action (obviously talking to my
mechanic is high on the list). The airplane is a 1972 Skyhawk. BTW,
googling for "high voltage" gave some useful results, but inadequate
so I thought posting would be appropriate.


Jim Rosinski

Off-topic aside: I recently completed a leaf-watching drive around
Colorado and posted the pics at
www.burningserver.net/rosinski/FallColors2004

[Roger Long]

Here's what I have posted for our club members about the "Idiot" (well
named) light in our 172 N.

The "High Voltage" light is actually a low voltage light. Cessna calls it a
high voltage light because the most probable cause of low voltage is high
voltage causing the alternator to go off line. In that case, the light
labeled "High Voltage" would be telling you that you now have low voltage
because you used to have high voltage. Perhaps it should have been labeled
the "Formerly High Voltage" light.

If your high voltage protection is disabled due to equipment malfunction or
someone putting a wire on the wrong terminal, and you get a high voltage
condition, the high voltage light will not come on. It will remain dark
until frying radios, smoke, or some other subtle indication alerts you to
the fact that something is wrong. In fact, the light will never come on
under a high voltage condition, only a low voltage one. It will come on if
there is low voltage due to something compromising the alternator field
circuit but, it's a "High Voltage" light so you might not know that you
actually have low voltage.

They could have called it a "Low Voltage" light. That would explain why it
always comes on when there is low voltage such as when the alternator is
off. The pilot would then have to figure out that the most probable cause
of low voltage was not having an alternator any more. Cessna must have
figured that was too much brain work for someone who meets the minimal
standards for a PPL.

I always wondered where Microsoft got the engineers who designed a computer
that you turn off by clicking "Start". Now I know. They hired them from
Cessna!

No, this isn't a joke and I'm not making it up.

Not all alternator failures will cause the light to come on, only those that
cause an over voltage trip or interrupt the field current. You need to keep
an
eye on the ammeter as well.

-- Roger Long


"Jim Rosinski" > wrote in message
om...
> Probably way past due for me to post something not off-topic...
>
> Today while descending from a leaf-watching flight at 13K down to 5K,
> the "high voltage" indicator flickered for about a minute and then
> went out. It has never done that before. I forgot that a useful
> procedure is to cycle the alternator half of the master switch so I
> cannot report a result of having done that. Throughout, alternator
> readings were fine, and battery water levels checked fine after the
> flight. A brand new voltage regulator was installed 3 years ago.
>
> Thanks in advance for any advice on what might be wrong, or
> appropriate further courses of action (obviously talking to my
> mechanic is high on the list). The airplane is a 1972 Skyhawk. BTW,
> googling for "high voltage" gave some useful results, but inadequate
> so I thought posting would be appropriate.
>
>
> Jim Rosinski
>
> Off-topic aside: I recently completed a leaf-watching drive around
> Colorado and posted the pics at
> www.burningserver.net/rosinski/FallColors2004

[David Brooks]

"Roger Long" > wrote in message
...

> I always wondered where Microsoft got the engineers who designed a
computer
> that you turn off by clicking "Start". Now I know. They hired them from
> Cessna!

Probably after they had designed the feature in various UNIXes where you
shut the system down with the "init" command.

-- David Brooks

[Dan Thomas]

(Jim Rosinski) wrote in message >...
> Probably way past due for me to post something not off-topic...
>
> Today while descending from a leaf-watching flight at 13K down to 5K,
> the "high voltage" indicator flickered for about a minute and then
> went out. It has never done that before. I forgot that a useful
> procedure is to cycle the alternator half of the master switch so I
> cannot report a result of having done that. Throughout, alternator
> readings were fine, and battery water levels checked fine after the
> flight. A brand new voltage regulator was installed 3 years ago.
>
> Thanks in advance for any advice on what might be wrong, or
> appropriate further courses of action (obviously talking to my
> mechanic is high on the list). The airplane is a 1972 Skyhawk. BTW,
> googling for "high voltage" gave some useful results, but inadequate
> so I thought posting would be appropriate.
>
>
> Jim Rosinski

Worn alternator brushes. How many hours on the alternator? How many
with lots of load, such as night flying?

Dan
>
> Off-topic aside: I recently completed a leaf-watching drive around
> Colorado and posted the pics at
> www.burningserver.net/rosinski/FallColors2004

[Dan Thomas]

(Jim Rosinski) wrote in message >...
> Probably way past due for me to post something not off-topic...
>
> Today while descending from a leaf-watching flight at 13K down to 5K,
> the "high voltage" indicator flickered for about a minute and then
> went out. It has never done that before. I forgot that a useful
> procedure is to cycle the alternator half of the master switch so I
> cannot report a result of having done that. Throughout, alternator
> readings were fine, and battery water levels checked fine after the
> flight. A brand new voltage regulator was installed 3 years ago.
>
> Thanks in advance for any advice on what might be wrong, or
> appropriate further courses of action (obviously talking to my
> mechanic is high on the list). The airplane is a 1972 Skyhawk. BTW,
> googling for "high voltage" gave some useful results, but inadequate
> so I thought posting would be appropriate.
>
>
> Jim Rosinski

Worn alternator brushes. How many hours on the alternator? How many
with lots of load, such as night flying?

Dan
>
> Off-topic aside: I recently completed a leaf-watching drive around
> Colorado and posted the pics at
> www.burningserver.net/rosinski/FallColors2004

[Jim Rosinski]

(Dan Thomas) wrote

> (Jim Rosinski) wrote
> > Today while descending from a leaf-watching flight at 13K down to 5K,
> > the "high voltage" indicator flickered for about a minute and then
> > went out. It has never done that before. I forgot that a useful
> > procedure is to cycle the alternator half of the master switch so I
> > cannot report a result of having done that. Throughout, alternator
> > readings were fine, and battery water levels checked fine after the
> > flight. A brand new voltage regulator was installed 3 years ago.
> >
> > Thanks in advance for any advice on what might be wrong, or
> > appropriate further courses of action (obviously talking to my
> > mechanic is high on the list). The airplane is a 1972 Skyhawk. BTW,
> > googling for "high voltage" gave some useful results, but inadequate
> > so I thought posting would be appropriate.
>
> Worn alternator brushes. How many hours on the alternator? How many
> with lots of load, such as night flying?

Alternator has about 1800 hours and 22 years on it, so your suggestion
sounds reasonable. My hangar neighbor also suggested a loose
alternator belt, which I have yet to check. Both suggestions seem
consistent with the low rpm experienced during a long descent, which
was when I noticed the flickering.

Jim Rosinski

[Jim Rosinski]

(Dan Thomas) wrote

> (Jim Rosinski) wrote
> > Today while descending from a leaf-watching flight at 13K down to 5K,
> > the "high voltage" indicator flickered for about a minute and then
> > went out. It has never done that before. I forgot that a useful
> > procedure is to cycle the alternator half of the master switch so I
> > cannot report a result of having done that. Throughout, alternator
> > readings were fine, and battery water levels checked fine after the
> > flight. A brand new voltage regulator was installed 3 years ago.
> >
> > Thanks in advance for any advice on what might be wrong, or
> > appropriate further courses of action (obviously talking to my
> > mechanic is high on the list). The airplane is a 1972 Skyhawk. BTW,
> > googling for "high voltage" gave some useful results, but inadequate
> > so I thought posting would be appropriate.
>
> Worn alternator brushes. How many hours on the alternator? How many
> with lots of load, such as night flying?

Alternator has about 1800 hours and 22 years on it, so your suggestion
sounds reasonable. My hangar neighbor also suggested a loose
alternator belt, which I have yet to check. Both suggestions seem
consistent with the low rpm experienced during a long descent, which
was when I noticed the flickering.

Jim Rosinski

[Dan Thomas]

(Jim Rosinski) wrote in message >...
> (Dan Thomas) wrote
>
> > (Jim Rosinski) wrote
> > > Today while descending from a leaf-watching flight at 13K down to 5K,
> > > the "high voltage" indicator flickered for about a minute and then
> > > went out. It has never done that before. I forgot that a useful
> > > procedure is to cycle the alternator half of the master switch so I
> > > cannot report a result of having done that. Throughout, alternator
> > > readings were fine, and battery water levels checked fine after the
> > > flight. A brand new voltage regulator was installed 3 years ago.
> > >
> > > Thanks in advance for any advice on what might be wrong, or
> > > appropriate further courses of action (obviously talking to my
> > > mechanic is high on the list). The airplane is a 1972 Skyhawk. BTW,
> > > googling for "high voltage" gave some useful results, but inadequate
> > > so I thought posting would be appropriate.
> >
> > Worn alternator brushes. How many hours on the alternator? How many
> > with lots of load, such as night flying?
>
> Alternator has about 1800 hours and 22 years on it, so your suggestion
> sounds reasonable. My hangar neighbor also suggested a loose
> alternator belt, which I have yet to check. Both suggestions seem
> consistent with the low rpm experienced during a long descent, which
> was when I noticed the flickering.
>
> Jim Rosinski

1800 hours on an alternator that's probably never been opened is
a fairly positive affirmation that the brushes are shot. We check ours
every 500 hours, and find that after two checks (1000 hrs) the brushes
are far enough gone that we won't leave them for another 500.
Better get them changed before one of them comes out of its guide
and the spring chews up the slip rings. Things get more expensive at
that point.

Dan