If your regulator case is about 3" wide by 5" tall and 1" thick and
has a round plastic plug then it has a built in over voltage relay
function. This regulator also has remote voltage sensing for both the
positive and the ground. You need to check the resistance for both of
these sense wires.


I almost died once when a Cessna with this type of regulator developed
a high resistance ground fault. If I had pulled up the electric gear
before I sucked up the electric flaps I most likely would not be here
today. The NTSB report would have said "Pilot Error". There was not
enough juice left in the battery to suck up the gear and there was
not enough power to fly level with 30 degrees of flaps on the go
around. The regulator thought the battery was charged but the battery
was slowly discharging and the under voltage light was not on nor was
the amp meter showing a discharge either.

I would look to see where the excessive voltage drop is on the
regulator power. Measure across each connection with a DVM.
You also need to check the fat ground braid between the engine case
and the motor mount frame for high resistance.
0.05 ohms will give you a 2.5 volt drop at 50 amps.
You need to get the service manual for the aircraft to check the
wiring correctly. Cessna sells the manuals for about the cost of one
gas tank fill up.

John

On 8 Jul 2004 11:54:28 -0700, (John Clonts) wrote:

(John) wrote in message ...
Most of the 24 volt Cessna's have the over voltage relay built into
the regulator. The over voltage protection has two stages. The first
stage turns off the field pass transistor. If the voltage still goes
higher the second stage turns on a crowbar SCR that blows an internal
5A fuse in the regulator.


I think mine is separate, per my comparison with the docs at
www.zeftronics.com. ("Type B" system using R25101 or R25102, not sure
which yet).

BUT I have not been able to find the OVR yet I assume its up under
the panel somewhere, but in that rats nest I can't find it. Can
anyone tell me which side? or high/low? Mounted or just hanging?
There are a few barrel-like connectors hanging around in there that
look about the same size is what I imagine the OVR to be.

From what you describe the alternator has field voltage (21.6) so it
should be generating current. With power off the field should measure
15-25 ohms. Your brushes may be worn out.

The field resistance was 18 ohms. I didn't try moving the prop to see
if it varied though-- I'll try that.


I had a problem a few years back where new brushes would not last 50
hours. I finally found that all of the replacement brush springs
were too stiff. There is two ways to fix this. Get the springs out
of a "OLD" junkyard alternator or cut a 1/4 inch off of the new
stiffer springs. I have no idea why the spring stiffness changed but
it is noticeable and measurable.


Wow. We DID replace our alternator just last August at annual time,
and it has about 50-75 hours on it since then...

Thanks for your input!

Cheers,
John Clonts
Temple, Texas
N7NZ