High Voltage Light On (Temporarily)

I think you've diagnosed the problem. Bad master or connections around
it. Although, there has been some history of failing master contactors,
but you should have heard it clunk when the master switch in energized.

So, the potential of a bad master switch leads me to a couple of other
questions I have had....

Is there an advantage of having the split master (alt/bat) vs. a single
master?

Along those lines, I have flown 172's with an avionics master, and have
been told that it would be pretty easy to add one. I thought that
would be pretty nice, as you could shut down the whole avionics stack
with one switch rather than each individual component.

Any thoughts on what the ideal configuration might be for a master and
avionics solution? I don't have a ton of money to throw at the plane
for major panel upgrades, which would be ideal, but I would like to
take the approach of improving things over-all as I touch/fix things.

Finally... Can anyone point me to any documentation or literature on
doing these kinds of things myself? (Of course I would have an A&P
looking over my shoulder or checking my work)

Thanks for the input!

Todd

The advantage is that you can turn on the master power to the A/C
without exciting the alternator rotor (which is an additional 1-2 Amp
load). It is useful if you want to check/use a radio on the ground or
extend the electric flaps when you are not planning to immediately
start the engine. There may be other advantages, but I can't think of
them. I do use the feature.

I have an avionics master contactor/relay for the electronics stack
that I installed almost 30 years ago for which I got a one time GADO
approval. It automatically drops out each time the master is turned
off. On startup, I must manually re-engage the self latching relay
with a panel pushbutton after being done with the engine starter. The
radio etc knobs and switches are rarely touched.

Cessna in those years did have an auto-disconnect relay that dropped
out when the starter was engaged, but my engineering concern (real or
imaginary) was still starter-induced transient spikes getting thru the
relays' relative timing, vs the wear and tear of operating the
electronic stack controls each time I started it.

In hindsight it is probably much easier nowdays to install a manual
switch, but it requires pilot memory to use it. However, FWIW, I've
never had electronic problems in the airplane, except for some Narco
manufactured-in ones. I also have been fussy about maintaining the
stack cooling system.

Maybe I'm just keeping elephants away?

"nrp" wrote in message
ups.com...
The advantage is that you can turn on the master power to the A/C
without exciting the alternator rotor (which is an additional 1-2 Amp
load). It is useful if you want to check/use a radio on the ground or
extend the electric flaps when you are not planning to immediately
start the engine. There may be other advantages, but I can't think of
them. I do use the feature.


How about shutting down the alternator in flight in the case of some types
of alternator-failure??

Yes that would be another use - good for a runaway alternator.

I've turned the Master off inflight to reset the overvoltage lite, but
never liked doing it with the system electronics.

Could be the master switch or it could be the master contactor, it's
usually located right next to your battery box. Replace the contactor
first as it's very easy to do and about a $20 part.

three-eight-hotel wrote:

I assume you mean the red LED Overvoltage light on the passenger side.
In my 172M, when it came on, it would latch on until the master was
turned off and back on again. Is that what you are doing in flight?
Or maybe you have an intermittent in the indicator too?


Correct... it's the red LED, however, mine is on the pilots side. I
did not have to turn off the master though. It came on the first time
when I was in a constant 30 degree bank, and went off by itself.

Something else just occured to me... A couple of months back, I
switched on my master switch, and nothing??? I switched it back off,
and switched it back on, and it worked fine. A friend of mine said
that my master switch was probably on its way out, and that I'd
probably be replacing it soon. It happened about two more times after
that, but has been working fine ever since. Is there a potential
relationship here, or are these two separate issues? Was there
something else going on besides the master switch starting to fail, or
could the failing of a master switch be tied in, some way, to cause the
voltage LED to illuminate?

Best Regards,
Todd