Low Voltage Warning Light

[Captain Noodle]

When running my C172 at slow idle the red low volatge warning light glows. The ammeter was positive and the red warning went off once I increased revs to normal idle of 1000rpm. People I have spoken to say this is OK. Can someone explain the intricacies of this part of the C172 electrical system?

Thanks
Noodle

[DownIn3Green]

Read the AFM. It should be detailed in there.

[Ham Phisted]

<font face="Verdana, Arial, Helvetica" size="2">intricacies of this part of the C172 electrical system</font>

I've heard everything now.

[hassel]

I used to have an old mini (car) and a red light came on when it was idling.

[DownIn3Green]

Cannon Fodder...

[Squawk 8888]

Unless it's an ancient generator-equipped 172 I think you're referring to the High Voltage light. It indicates that the alternator was shut down due to an overvoltage, or regulator failure.

Because the light came on and the ammeter still shows charging it sounds like you've got potentially serious problems with the regulator. The light should only be on when the alternator is off, so if the master is set with alternator on and the ammeter is showing a charge, the protective circuit is definitely wonky.

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Per dementia ad astra

[Lucifer]

Alternator only comes online above a certain RPM!

[Code Blue]

Both Squawk & Lucifer are correct.

I would check carefully the PoH to make absolutely sure the light is not an Overvolt light, esp. as you have an ammeter which reads positive.

The older 152s have a Low voltage light which came on with rpm less than 1700.


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[email protected]

[Captain Noodle]

DownIn3Green: What I was making fun of with my remark, "Cannon Fodder" (See GenAv thread), was that people seem to love coming on to this forum and creating arguments, or as some people put it, "winding people up".

I am low hour PPL. I don't fly every day, I hardly fly every week, sometimes not for a month at a time. And for someone like me, a warning light on the dash of a C172 that I don't know about, I shouldn't feel ashamed nor embarassed to find out about it. Bugger the "so called" professionals out there who love nothing more then to make us feel small, I shall keep asking questions. I thought this was a good place to find out information to make me a better pilot, not somewhere that I have to put up with belittlement from the likes of you!

Thanks to Squawk888, and Lucifer, and Code Blue who actually tried to answer my question for me. It was a low voltage light, and did only did go off after a certain RPM.

Ham, "Intricacies" was used tongue in cheek, here. I know to you "big boys", nothing is as complicated as the "trucks" you drive!

Noodle

PS I edited this. I have calmed down. I admit I don't know everything. Don't stop me asking questions.




[This message has been edited by Captain Noodle (edited 12 June 2001).]

[Ham Phisted]

Chill, I understand irony. Nobody knows everything. :)

But don't ask for opinions, get the facts, use the POH.

<font face="Verdana, Arial, Helvetica" size="2">C172R has a Low Voltage Annunciation light incorporated in the annunciator panel which activates anytime the voltage falls below 24.5 volts.

In the event an overvoltage condition occurs, the alt control unit automatically pops the ALT FLD circuit breaker, removing alt field current and shutting down the alternator. The battery will then supply system current as shown by a discharge on the ammeter. Under these conditions, depending on electrical system load etc etc

Illumination of the low voltage light and ammeter discharge conditions may occur during low RPM conditions with an elctrical load on the system, such as during a low RPM taxi. Under these conditions, the light will go out at higher RPM. </font>

Pause for breath. So, according to the Cessna POH, it sounds like perfectly normal characteristic of the 172.

HP

[Squawk 8888]

Sound advice, Ham, but I might add that he should make sure which model that applies to. Even the 172s have considerable variation so always be sure the POH is for the model you're actually flying; my guess would be 172R- the M & N have lights for high voltage, not low, and run on 12 volts.

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Per dementia ad astra

[Captain Noodle]

Thanks Ham and (others)for good advice.

Merely asked a straight forward question. Used the right forum. Then copped smart alec response from one writer. (Not Hassel)

"Don't know what Capt N is laughing about, though. He apparently doesn't know what a low voltage light on a C-172 means.

I'll bet every 747 Capt knows what electrical fault warnings on their a/c mean."

Do you have to be clever to write this kind of tripe?

Sure I replied to GenAv in a previous thread, but that was purely in the context of that thread, and in the context of the previous reply. GenAv would have expected that kind of reply. Even the moderator felt he had to respond.

On the whole the reponses to my question have been good. Very helpful and some even amusing. As I would expect from professionals assisting the amateurs.

What I do not expect though is crap from smart alecs, especially based on another topic. Especially when that topic in question was (and in my opinion, meant to be)inflametry.

Enough said.

My email is available should you want to have more digs off this forum.

Noodle

[This message has been edited by Captain Noodle (edited 12 June 2001).]

[Luftwaffle]

My two bits in the high voltage/low voltage question.

Ham Phisted's advice to use the POH is right on, but some early model Cessnas have very skimpy manuals, and don't explain things in the detail you might like. From my ASA C172 guide:

"Up to 1978 aircraft, a HIGH VOLTAGE warning light is fitted. In the event of a high volatage condition occurring, the overvoltage sensor will shut down the alternator. The red high voltage warning light will then illuminate to show that the battery is supplying all electrical power. On 1979 models and after, this light is replaced with a red LOW VOLTAGE warning light.

"This light will illuminate at any time when the alternator has failed or been shut down, or when voltage has dropped below approximately 24 volts, and the battery is supplying all electrical power. On earlier aircraft this light may be fitted in addition to the high voltage warning light. During operations at low RPMs (i.e. during taxiing) it is not unusual for the low voltage light to illumiate intermittantly."

Squawk 8888 is right on when he says know what model you fly. The V-speeds and systems are NOT all the same.

[Captain Noodle]

Thanks people,
As usual, its been interesting(?) and informative. I appreciate the efforts.

Stay tuned for my next question

Noodle

[Bomber Harris]

There are quite a few 'intricacies' about every aircraft and I am certainly no expert on C172's but I do have a couple of hundred in them.

The POH is the place to go but let me give you a stand back type answer to this. In general it is a good rule of thumb that if a red electrical light comes on in a light aircraft then the battery isn't being charged from the altenator or generator(I do accept the comments about high volts light). If your on the taxiway a bit of power or cycling the gen switch normally fixes it. This normally means the aircraft has a specific anomoly that frequent flyers know about or the gene/altenator needs a shop visit. Sometimes one of the diodes gone inside the altenator will reduce the output causing this.

There's no great difference between this and a big aircraft. Low volts and your on batteries. So it's a good time to learn to deal with it. Nothing really to worry about in a 172 but it's an opportunity to learn. Look at issues like load shedding when it does happen and know what your time on batteries is. Know what to switch off and what the biggest drain is. Know the fixes (switching it on and off works on the big ones too!!).

In short, good question but don't focus on the technically correct answer.....look at the other issues like 'what to do'.

[Captain Noodle]

Bomber: Thanks heaps for that post. I really appreciate the practical aspect of your advice. I have learnt a lot from posting this question.

Thanks again to all who gave great answers.

Noodle

[Ham Phisted]

In addition to Bomber Harris' advice regarding load shedding, take a look at the fuses. They should tell you which utilities cause the largest drain on the battery. Thus if it looks like your alternator has failed you can make an informed decision about which utilities make a difference to the battery load.

[OnTheStep]

I drive a 172S which is loaded quite nicely, almost to the point of lunacy- gps(kln-94), mfd(pity there's no wx rad or tcas though), elec trim, dual axis autopilot(altitude/hdg preselect and all), leather interior, decent sun visors and trick cup holders. i think there's even a way to get it to do your laundry. anyhow, here's my 2p on the electrics...

the 'volts' anunn on the panel is a low voltage light, the only indication of an overvoltage condition (effectively an excessive rate of charge) would be the ammeter showing more than two needle widths from centre. if you're in an overcharge condition it's a good bet that most of the electrolyte in the battery will be evaporated by the time you get down and if the voltage builds to 31.5v then the acu will automatically shut the alternator down and trip the 'alt fld' cb.

your case of the 'volts' annun being illuminated is quite normal in low rpm conditions (comes on when the voltage drops under 24.5v. it should flash a couple of times (10 i think) and then stay on. if you're flying confirm the ammeter discharging (indicating the alt has been shut down by the acu due to a preceeding overvoltage condition) and recycle the avionics master & alt. the details are all in the poh.

as far as the 'intricacies' of the 172 electrical system, there's not much to 'em, there should be a couple of schematics in the poh as well. off the top of my head- your basic 28v(dc) system, 60 amp alternator, 24v batt. the ammeter shows current through the starter and alternator relays indicating the charging rate. power runs through a split primary bus bar with each side connected to an avionics bus through the avionics master.

...but we learned all that on our 172 ppc course right?

(i need to get out of these things, alas at least i'm rid of that cursed carburettor)

[This message has been edited by OnTheStep (edited 15 June 2001).]