Cessna Alternator Warning Light

[OntheNumbers]

Here's the problem - alternator warning light comes on intermittently, always can be reset with the split rocker but sooner or later it comes back on again and we repeat the cycle. The battery and alternator have been "bench tested" and the regulator has been replaced. The problem has been happening for a while and seems to be nothing more than an inconvenience. The wiring has been inspected closely but nothing seems to be wrong with any of the major components.

I want to ask - since this is an intermittent problem is it possible that bench testing the alternator might not detect anything wrong? If it's not that and the regulator is new, what else could it be?

Any ideas would be appreciated.

'best

OTN

[Goodwrench]

Could you provide more information? Were you able to determine if this is an indication problem (light only) or is the alternator going off line? What is the ammeter or voltmeter doing? Is this a twin? Model? Some have a alternator out sense module which I have seen fail in the past.
GW

[Colonel Sanders]

The Cessna "high voltage light" is a bit of a misnomer.

When it comes on, all it's saying is that the alternator
has been taken off-line, and isn't producing current
any more. That's all. Now, one cause of the alternator
being taken off line is over-voltage, but ..

Older aircraft have separate voltage regulators - often
mechanical, with points that stick - and separate over-
voltage relays. The more separate boxes you have - a
and the more wires and connectors you have between
them - the more likely you have a problem.

One source of problems with this antique arrangement
is that if there is a poor connection to the voltage
regulator, it will see a low voltage due to voltage drop
across the contact resistance. So, it ups the field to
the alternator, driving the volts over 15V which causes
the over-voltage relay to kick in and take the regulator
offline.

What I prefer is to throw out both the old mechanical
voltage regulator and separate overvoltage relay and
replace it with a single solid-state box. Far less to go
wrong with that simple arrangement. Only problem I
have found is that it doesn't like heat - it prefers to be
aft of the firewall. Obviously you need paper to do this
mod - a drawing or SB from the airframe manufacturer,
or some kind of STC or PMA approval from the parts
manufacturer.

http://www.aircraftspruce.com/catalog/e ... icsreg.php

These new boxes are marvellously simple. Three
wires - ground, input and output (to alt field). A lot
less crap to go wrong.

[culver10]

I have dealt with this issue on many of the Cessna's that I maintain. It has always been the Overvoltage Sensor P/N C593001-0101. It seems to fail open when it gets 30 plus years old. When it opens, it turns off the power between the circuit breaker and the 2 position switch, which shuts down the Alt. If you want to test it, jumper the connection between the circuit breaker and the switch with a chunk of solid wire and your problem will go away. I do agree also with CS about mechanical vs solid state regulators. I have also used a $12 solid state regulator from NAPA in a pinch and they worked great!

[Big Pistons Forever]

The Cessna "high voltage light" is a bit of a misnomer.

When it comes on, all it's saying is that the alternator
has been taken off-line, and isn't producing current
any more. That's all. Now, one cause of the alternator
being taken off line is over-voltage.

This is a very good point. Most pilots who fly the simple Cessna's, including most instructors , think if the light isn't on the alternator must be working. This is not true as there are many other failures which could cause the alternator to stop providing power. The only way sure way to know if the alternator is working is to monitor the ammeter. BTW if you are flying along and all of a sudden ATC says that they are no longer receiving your transponder the first thing you should do is check the ammeter. Transponders are very sensitive to voltage in the event of an alternator failure the first thing to drop off line as the battery voltage drops will be the transponder. Still far better to keep a good scan of the ammeter.

It should also be noted that later models (post 1978) have a "low voltage" light instead of the "high voltage" light. This light comes on any time the bus voltage is below 24.5 volts. If the alternator is working then the bus voltage will be 28 volts, so this light is positive indication that the charging system is not working. That is it will always come on regardless of what or how an element of the charging system has failed.

[OntheNumbers]

Could you provide more information? Were you able to determine if this is an indication problem (light only) or is the alternator going off line? What is the ammeter or voltmeter doing? Is this a twin? Model? Some have a alternator out sense module which I have seen fail in the past.
GW

Not sure about that - I had it happen 3 times during a flight on Saturday but didn't look at the ammeter - fixated on the lamp instead. Fixation. Bad for flying

I'll keep an eye on the systems next time and gather some more facts.

BTW - this is a C-177 we're talking about and it has a Zeftronics regulator installed - so solid state.

The replies are educational as usual, thanks

OTN

[torquey401]

One of the easiest things to check these days is the voltage your electrical system is receiving from the alternator. Most GPS's can be set up to display voltage (Garmin handhelds I know for sure). Alternatively you can pick up a simple lighter plug in voltage analyzer that simply plugs in and will show what the voltage level is doing. Battery voltage (12VDC or less) = alternator is offline. 13-14 VDC = alternator online. This can also be described as "nervous ammeter needle syndrome". I would perform a good run-up, load up the electrical system and watch the voltage and the ammeter. Most likely the over-volt sensor as was mentioned.

[OntheNumbers]

It's finally happened again, twice during a 1-hour flight and within about 10 minutes apart. I checked the ammeter and indeed it was showing discharge. Both times, cycling the Alternator half of the Master switch brought the alternator back online. Since this problem has been observed with two separate Zeftronics regulators (I now have a perfectly good spare), then if the regulator is the problem it would have to be a design flaw. I don't think that is likely.

I've flown twice since I posted this, both flights were less than 1.2 hours. We'd already been flying for 2 hours with stops in between before the first occurrence happened - so maybe it is heat related as the Colonel suggested. I don't mind paying to get this fixed but without a definite cause anything anyone tries will be guesswork and that can get expensive. Looking for some popular wisdom on how to proceed!

Cheers all,

OTN

[Colonel Sanders]

Could you clarify - does this aircraft have a separate
overvoltage relay, or is it integrated into the newer,
solid-state regulator?

If it has a separate over-voltage relay, that is where
I would look next - and at it's wiring.

Especially on older aircraft, it's not just the components
of the electrical system that you want to be checking -
also the wiring and connectors!

One common failure mode is poor wiring (eg contact
resistance) to the regulator, so that it "sees" a lower
than actual system voltage, and in response, drives
the field too high, and thus drives the actual system
voltage too high, and the overvoltage relay correctly
takes the regulator offline.

Or, it could be that the overvoltage relay has an internal
fault, and is incorrectly taking the regulator offline.

As I said before, I really prefer the combined single-box
voltage regulator and over-voltage relay. They are
solid state these days and don't like the heat, but are
dead nuts simple to wire and understand.

If you have really addressed all the relevant components
of the electrical system and are convinced that they are
serviceable in the presence of heat and vibration:

- alternator
- regulator
- overvoltage relay

then the next thing I would look at is replacing any sketchy
connectors, and wiring. Insulation deteriorates over time.
It's quite possible you have an intermittent connection, or
short.

[Colonel Sanders]

I might point out that R&R'ing an alternator or
voltage regulator is not rocket science, diagnosing
the root cause of a problem can be pretty tricky.

Even doctors have trouble doing it, and they're supposed
to be the brightest of the brightest, and have many
many years of education.

Some years back I stopped in at a Big School and talked
to the DOM. They had a rough-running engine on a twin
and thus far had changed the prop governor, carb and
mags and still had the problem. Think about what that
mis-diagnosis cost.

[culver10]

I told you how I have solved this problem on 3 other Cessna's in a previous post.

[torquey401]

Some years back I stopped in at a Big School and talked
to the DOM. They had a rough-running engine on a twin
and thus far had changed the prop governor, carb and
mags and still had the problem. Think about what that
mis-diagnosis cost.

Whatever, dude.

[Colonel Sanders]

So, you've never had difficulty diagnosing a problem,
and spent a bucket of time (money) and/or parts (money)
trying to fix the problem? Sorry, gotta wave the BS flag.

Reminds me, a few years back. Aircraft with Lycoming on
the field, hadn't flown much. Owner started it up, it ran
like crap. Obviously a sticking valve. But the genius AME
on the field thought that clearly the problem was that both
mags had failed (WTF - nice diagnosis) so he pulled them
and sent them off for service.

First mistake. Owner spent a crapload of money "servicing"
the mags, and got back his old ones with the worn-out
bearings, old coil, etc.

Then, the AME runs and hides away. Won't put the mags
on. I am not making this up. I have to put the goddamned
mags on and time them, because the AME has run away.

Found another AME to sign the maintenance release on the
mags, then drained the oil, added new oil, tossed in 2 cans
of Avblend and ran it up on the ground. Let it sit overnight,
and the stuck valve freed up.

Same AME talks big to a homebuilder. What a blowhard.
Pulls a jug, then runs and hides again. I have to help the
homebuilder put jug back on, show him how to torque the
throughbolts and holddown studs, put the valvetrain back
together. Sigh.

Yeah, all AME's are rocket scientists that carry through.
I just wish I didn't have to clean up after them all the
time.

[Colonel Sanders]

Especially when it comes to electrical/avionics problems,
it can be really hard to diagnose.

A while back, one of the C421 developed horrible avionics
noise (and other weird problems). At great expense, it was
flown to a distant Very Big Name Avionics shop.

Their diagnosis - change both mags and harnesses with new.

Which I knew was complete BS. It was the tach generator,
as I suspected.

But what would I know about electrical/electronic problems?

[SeptRepair]

Reminds me, a few years back. Aircraft with Lycoming on
the field, hadn't flown much. Owner started it up, it ran
like crap. Obviously a sticking valve. But the genius AME
on the field thought that clearly the problem was that both
mags had failed (WTF - nice diagnosis) so he pulled them
and sent them off for service.
First mistake. Owner spent a crapload of money "servicing"
the mags, and got back his old ones with the worn-out
bearings, old coil, etc.

So did the mags come back with a servicability tag? If they did, then why did he not send them back and have them repaired properly?

Then, the AME runs and hides away. Won't put the mags
on. I am not making this up. I have to put the goddamned
mags on and time them, because the AME has run away.

So let me get this straight, by your own admission the mags are worn out and need repair and the AME chooses not to install these mags. So YOU decide you know better, install them and then find another guy to sign out the mag install, since your NOT an AME. Got it. Im sure you let the new AME know the truth behind these "worn out" mags.

...... then drained the oil, added new oil, tossed in 2 cans
of Avblend and ran it up on the ground. Let it sit overnight,
and the stuck valve freed up.

WHAT? no rope trick...oh the horror. The self proclaimed rope expert doesnt apply the "secret" technique?

Same AME talks big to a homebuilder. What a blowhard.
Pulls a jug, then runs and hides again. I have to help the
homebuilder put jug back on, show him how to torque the
throughbolts and holddown studs, put the valvetrain back
together. Sigh.

You forgot the part where the cylinder had a crack between the fins and the AME never wanted the liability, even though the cylinder had great compressions. sigh.

Yeah, all AME's are rocket scientists that carry through.
I just wish I didn't have to clean up after them all the
time.

Yup, thanks for cleaning up after us, Your doing the aviation world a great service.

[Colonel Sanders]

What crack in the jug?! The homebuilder bought a NEW jug.

As expected your impaired reading comprehension caused you to miss the point about the mags. They were not the cause of the problem, but again the AME's poor decision meant the owner spent a crapload of money on the old mags instead of buying new. Too complicated for you to understand, I guess.

You talk of something you clearly know nothing about, which I find is quite common these days in AME's.

Reminds me of a recent pre-pruchase I reviewed. AME said because of prop strike back in 80's - NOT a Lyc, no AD - engine had to be torn down. What an idiot. He didn't understand that the CARs - specifically 625 App G - are NOT retroactive. If they were, TC would charge the estate of Alex G Bell with no C of A or C of R for the flight of the Silver Dart in 1906.

[torquey401]

Instead of being part of the solution Mr Sanders desires to add to the already existing problems in the aviation industry with the next generation of pilots he is "teaching". We don't need more egos - we need professionals.

Good luck.

[SeptRepair]

+1 We have an industry that for years has been plagued with the this and them attitude as it pertains to AMEs and Pilots. I thought it was pretty much extinct that that type of childish mentality was still in our industry. Way to go Sanders. You proved us wrong.

[Colonel Sanders]

Reminds me of an import I ran a few years back.

Beautiful, new 3-blade prop on the N-reg aircraft,
just coming up on 10 years old. Sigh. In the USA,
they'd just leave it alone, but in the Glorious People's
Republic of Canada it's gotta come off every 10 years.

So off it comes, drive it down to a Big Name Shop in
Toronto. A week or so later, drive down to Big Name
Shop in Toronto to pick it up again (2nd round trip).

Put it back on the aircraft. I run it up, it simply would
not come out of coarse pitch (counterweighted prop)
no matter what I did. I'm not very bright compared
to most AME's here, so off it comes again.

Back into the prop box, another round trip (3rd time)
to Toronto to the Big Name Shop who discovered that,
silly them, they used the wrong seals on the blades. Their
paper was really very nice, though. A really excellent
maintenance release.

4th round trip down to Toronto to the Big Name Shop
to pick up the prop - again. Put it back on the aircraft.
Again. Ran it up - and glory be, the blades moved.

Too bad we couldn't have just left the prop well enough
alone. Had to get it "maintained". What a clusterf__k.


Speaking of clusterf__ks, had a well-known Avionics
shop in Toronto, back in the mid 80's - pull an old
nav/comm and replace it with a nice new single digital
flip/flop comm.

Never worked worth sh1t. I thought it was just a crappy
radio. After a while, started poking around the aircraft,
and sure enough, the dinglenuts at the avionics shop
had plugged the comm antenna BNC into the VOR
antenna. No wonder it didn't work very well! Also,
the avionics shop, despite the exhorbitant bill, never
bothered removing the connector for the old radio - it
was just swinging loose, underneath the dash.


Another clusterf__k, this one quite recently. PPL buys
an aircraft - with fresh annual - and wants some type
famil training on it. Sure. I walk around the aircraft
an immediately snag the following:

- stall warning inop
- airspeed indicator reads 20-30 mph at rest
- tailwheel pops out of detent far too easily

All of the above are showstoppers, I tell him, for a
newbie. Get them fixed. So, he gets the ASI and
stall warning fixed, but the tailwheel repair drags
on for months and months. The AME changes one
part, then another, then another, then another.

Sigh. I ain't as bright as the average AME but I
know that when a tailwheel is a worn-out piece
of sh1t, toss it and replace it with new. It wasn't
a Scott, just a Maule tailwheel, so it wasn't even
very expensive.

And he'd lost all those months of flying, while his
AME fiddled and farted around, changing one piece
after another to no avail. I know it's not important
to AME's whether aircraft are flying or not for very
long periods of time, but sometimes it gets annoying.


I could go on, and on, and on, and on .... but what
I have learned, over the decades is that the more
most AME's work on an aircraft, the faster it is
reduced to a heap of rubble.

the next generation of pilots he is "teaching"

Excellent point. When I am teaching a pilot, he learns
that an aircraft is most dangerous when it just comes
out of maintenance. That's when it will kill you. Ask
any experienced pilot - his most exciting flights will
come from bad wx, and bad maintenance.

[Colonel Sanders]

When I am teaching a pilot, he learns that an aircraft is
most dangerous when it just comes out of maintenance

Impeccable timing! Look in the accident forum:

http://www.yourottawaregion.com/news/lo ... -cornfield

NEPEAN - The pilot of a single-engine plane was able to walk away from an emergency landing in a dirt field.

The 1997-model Katana two-seater lost power shortly after takeoff from Ottawa airport, said Ottawa fire department acting district Chief Donald Smith.

“He took off on a test flight and had engine trouble and lost power,” Smith said.