A few weeks back my charging system started going bad.  It's gotten to the
point where the battery won't run the radios for more than three hours.  The
Comanche has an alternator, installed under an STC; both the alternator and
regulator passed a load test.  The battery is new.  Three mechanics at three
FBOs have tinkered with the wiring and pronounced the machine airworthy.
But today I did my second manual gear drop in a month.

Now I think the problem may be in the overvoltage relay -- it may not be
resetting after a voltage spike.  The alternator is wired to the main,
rather than to a separate field switch.  When I shut down the radios and
cycle the main, the system does *not* reset and begin charging again.

Another symptom:  The jump-start connector has no continuity -- and I notice
that it's wired through the relay.

Can anyone in this group offer an insight?

Seth
N8100R

In article <bjet5.502$7K4.230...@news.uswest.net>,
  "Seth Masia" <sma...@hotmail.com> wrote:
hours.

If your OVP trips, and you cant reset it in flight, I would be surprised
if your battery lasts three hours...

  The

If your OVP trips, and it takes the alternator off line, your ammeter
will show a net discharge, or a load meter will show zero. Dont know
which a Commance has?

That's not normal. Even without a switch in the Field Circuit, if your
OVP trips, it is supposed to break the field circuit between the Main
bus and the VoltageReg, thereby shutting off the alternator. That may be
working correctly, because once tripped, the alternator is off line.

However, if you turn off the Master switch, it is supposed to disconnect
the battery from the Main bus, removing the source of power which keeps
the OVP relay pulled in, thereby allowing it to reset. Sounds like
whoever added the alternator to your bird didn't wire it
correctly,preventing you from resetting a tripped OVP. It could be that
the OVP relay is mechanically hanging-up, or that due to some miswiring,
there is some path from the alternator output back to the OVP which
allows the alternator to self-excite itself, even when isolated from the
battery by turning off the Master.

This is normal in Cessnas. The external power plug is isolated from
the Main Bus, and the Avionics bus with a complicated relay/diode logic
network to prevent reverse polarity or spikes from getting into the
radios from external power.

Look at the Form 337 with was supposed to have been done at the time
the STCed alternator was installed, or find the installation
instructions which came with the STC and check if it was installed
correctly.

--
MikeM, Skylane '1MM

Sent via Deja.com http://www.deja.com/
Before you buy.

Seth, read this:

www.burgoyne.com/pages/datkins/pulsing.doc

and look at the drawing referenced. Even though this was written to
addres a different problem, it shows how the OVP is supposed to be
wired.
--
MikeM, Skylane '1MM

Sent via Deja.com http://www.deja.com/
Before you buy.

What is your ammeter/load meter doing during all this?
 John

I assume that, even though your battery is new, you have tested it. Fully
charge the battery, see if it holds a charge. Discharge it with a known
load of so many amps, see it the amp*hours are somewhere near the spec
on the battery.

Gotta be one of the following
1. Battery
2. Alternator
3. Voltage Regulator
4. Wiring

In article <39B67F74.332F...@worldnet.att.net>,
John Kunkel  <JohnLKun...@worldnet.att.net> wrote:

Thanks, Mike, for your very helpful contribution.  Actually, after I did my
post I saw the thread on Cherokee ammeters, so I already have the
pulsing.doc printed out.  I'll hand all your stuff to my local mechanic, and
will pull all the paperwork out of the logbook and call the STC holder for
the schematics.

And FYI, the Comanche has an ammeter

Seth

news:8p4mr2$h6i$1@nnrp1.deja.com...

Charging sporadically.

Seth

news:39B67F74.332FFAC@worldnet.att.net...

Mike is correct.  You've probably got something wired wrong in your system.
The typical master switch has four connections.  The two on the left are for
the Battery Relay and the two on the right are for the Alternerator system.
The Left side grounds the battery relay.  The Right side provides power to
the OVP.

This is what you should get when measuring the voltage at the master switch
with the engine off:

SW Open:     12V     Battery               OVP        SW Open:     0V
SW Closed:   0V       Relay                                 SW Closed:   12V

SW Open:     0V       Ground               FLD        SW Open:     12V
SW Closed:   0V                                 Circuit     SW Closed:
12V
                                                           Breaker

If you really want to know what is going on with your aircraft.  Beg,
borrow, or steal a voltmeter, connect it to the buss (cigarette lighter
etc.) and go flying.  Have a passenger keep his/her eyes on the meter while
you fly and see what the bus voltage does.  Remember anything over 12.5
volts is coming from your alternator.  If the bus voltage is 12.5 volts or
lower, you are using your battery.  If the voltage ever drops below 12.5
volts, turn off your master switch, count to 10, turn on your master switch
and see if your charging system resets.

Philip Bolton

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