Ammeter failure?

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Mtns2Skies

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As some of you know, I had a total electrical failure in the air in a Cessna 152 but this thread isn't about that specifically. We think the problem was caused by a failed alternator however during flight the ammeter showed NO discharge and NO low voltage light came on. On run-up I checked the ammeter, I turned on the landing light it showed negative charge, Turned it off it went back to the center. So if not the alternator what did fail? or what would have caused the ammeter/low voltage light to fail? (This aircraft N757LU has a loooong history of electrical problems, but none this severe)
 
Some ammeters are just not very sensitive, and might not show the routine discharge with enough deflection to notice. Example: when you turn on the landing light, you're turning on the device which, by far, consumes the most current of anything in the plane (except for the starter, the current for which does not flow through the ammeter).

Hence, there might be a slight deflection of the needle for the radios and beacon, and it might be enough that you can't really tell. Thus, you can merrily motor along, using up the battery, without realizing it until the radios go "Vrrp!" and nothing works.

I prefer a good, accurate voltmeter to an ammeter anyway, as it gives you a better idea of the state of the electrical system, especially under load.

As for the alt-out light, I believe that, on the 152, it is handled by the voltage regulator, and that circuit can fail while the alternator still functions; have you noticed whether the alternator light illuminated when the master was on and the engine not running? That should be observed, as part of your pre-start.
 
When you turned the light on I would expect a momentary discharge indication followed by a return to 0 indicating that the alternator is working.

The needle staying on discharge was your indication of a problem.
 
SCCutler said:
As for the alt-out light, I believe that, on the 152, it is handled by the voltage regulator, and that circuit can fail while the alternator still functions; have you noticed whether the alternator light illuminated when the master was on and the engine not running? That should be observed, as part of your pre-start.
Click to expand...

Aus, based on what you wrote, I agree w/ SC. While Alt. can fail, there seems to be a much higher rate of failure w/ voltage regulators. Electrical issues can be challenging w/ these older aircraft. Noting that there have been issues in the past w/ this particular aircraft just proves it even more.
Hopefully, due dilegence while win and problem will be solved.:rollercoaster:
 
HawkEye007 said:
As some of you know, I had a total electrical failure in the air in a Cessna 152 but this thread isn't about that specifically. We think the problem was caused by a failed alternator however during flight the ammeter showed NO discharge and NO low voltage light came on. On run-up I checked the ammeter, I turned on the landing light it showed negative charge, Turned it off it went back to the center. So if not the alternator what did fail? or what would have caused the ammeter/low voltage light to fail? (This aircraft N757LU has a loooong history of electrical problems, but none this severe)
Click to expand...

An ammeter is a lousy instrument for determining alternator and battery health, a voltmeter is considerably better. Chances are the full scale range on your ammeter is between +/-50 and +/-100 amps. With solid state radios and no lights or pitot heat on you're probably only drawing an amp or two in a 152 and that's not enough to move the ammeter needle enough to notice. A voltmeter would normally show around 14v with the engine running and would drop quickly to 12v if the alternator quit. As long as the voltage is above 12.6 you aren't discharging the battery either. To check your alternator in flight, you must turn on something that draws enough current to register on the ammeter such as a landing light or pitot heat. But you should also know that on some airplanes the alternator capacity isn't enough to power all the lights at the same time (it should always be able to handle pitot heat though). When you apply the load you may or may not see a brief dip on the ammeter towards the discharge (-) side but it should return to and stay at or above the zero mark. I'd test this with at least two different loads since a lack of needle movement could also mean that the load you used was inop. Finally, the '0' point on many ammeters isn't exactly where the '0' is on the scale. You can check this before turning on the master, whatever shows on the ammeter then is the true '0'.

You mentioned no low voltage light. Does that come on before you start the engine (this should be checked before each start)? If not either the bulb is burned out, the undervoltage detect isn't working, or your alternator warning light is only supposed to show an overvoltage trip of the regulator.
 
RMCN172RG said:
When you turned the light on I would expect a momentary discharge indication followed by a return to 0 indicating that the alternator is working.

The needle staying on discharge was your indication of a problem.
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It returned to 0 after the light went off
 
Alright thanks guys, but what about the low voltage light failing to turn on?
 
HawkEye007 said:
It returned to 0 after the light went off
Click to expand...

It sounds like you checked the electrical system, the check failed, you flew anyways and the battery went dead. This shouldn't be a surprise :)
 
Greg Bockelman said:
The CFI is culpable in this deal.
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Oh sure, I agree, Just pointing out that there is a lesson to be learned. If you do a check and the check fails you really need to think about that before you throttle into the sky.
 
jesse said:
Oh sure, I agree, Just pointing out that there is a lesson to be learned. If you do a check and the check fails you really need to think about that before you throttle into the sky.
Click to expand...

But you have to KNOW the check failed. The CFI screwed up on that score. He needs a checkout on a 152 electrical system.
 
Greg Bockelman said:
But you have to KNOW the check failed. The CFI screwed up on that score. He needs a checkout on a 152 electrical system.
Click to expand...

Yes..you need to understand what you're checking, why, and what it really means. Far too many people fall short on this.

Austin's desire to better understand this really speaks highly of him.
 
Is the ammeter not supposed to return to the center position after the check?
 
We were positive the light did not come one... however it is an old beaten up plane, its possible the light no longer worked.
 
HawkEye007 said:
Alright thanks guys, but what about the low voltage light failing to turn on?
Click to expand...

Don't know if this helps, but my 1973 172 does not have a low voltage light. It has an Over Voltage Light. (I have no idea if a 152 and a 172 would be the same in this respect) It is located above the co-pilot yoke. This is from my owners manual:

"In the event of an overvoltage situation, the over voltage sensor automatically removes alternator field current and shuts down the altenator. The red warning light will turn on, indicating to the pilot that the altenator is not operating and the aircraft battery is supplying all electrical power."

This may explain why the light did not come on as you probably did not have a Over Voltage situation.

Also on my plane, when the engine and master switch is on the ammeter is showing the charging rate applied to the battery. Unless you load it up excessively, you won't see much gauge movement. I say this to agree with those that encourage a voltmeter, much more telling. I replaced my ammeter with an amp/volt meter.

Tim Gibbs
 
HawkEye007 said:
Is the ammeter not supposed to return to the center position after the check?
Click to expand...

It should return very close to center after you turn the landing light off because at that point you aren't using much current. If everything were working OK, it would return to center with the landing light STILL ON!. If it shows discharge for longer than just a moment (which you suggest), it's because the alternator is not keeping up with the electrical load - i.e. it's broke.
 
Capt. Geoffrey Thorpe said:
It should return very close to center after you turn the landing light off because at that point you aren't using much current. If everything were working OK, it would return to center with the landing light STILL ON!. If it shows discharge for longer than just a moment (which you suggest), it's because the alternator is not keeping up with the electrical load - i.e. it's broke.
Click to expand...

Oh I see, thanks
 
Is the airplane that you had the trouble in yours of is it a rental? If it is yours then you might want to put in a volt meter.

When you get to larger and more expensive aircraft, you will start to see both volt meters and ammeters. That makes it much easier to see the battery condition and the load you are generating. But, with just a single ammeter in a 152 it makes it harder to see what is going on.

My advise would be to go and talk with an A&P that is really good with electrical systems. Talk to a different one than the one that works on the airplane. He may have an allegiance to the airplane owner or operator. That way you will get an impartial explanation.
 
HawkEye007 said:
Is the ammeter not supposed to return to the center position after the check?
Click to expand...

Ok, I think 'Charging systems 101" is in order here.

Note: This is a general description that pretty much applies to most aircraft and cars - actual numbers and some details may vary.

Lets define some terms:

Voltage - think of voltage like pressure - high voltage is high pressure - measured in volts.
Current - think of current like a flow rate - measured in amps (this is what your ammeter reads).
Power - this is the voltage times the current - measured in watts. Simce the voltage in your 150 is nearly constant, we can use the words "power" and "current" somewhat interchangably to indicate which direction or how much stuff is flowing (in or out of a device).

A pressure washer like you use to clean your parents deck is a high pressure, low flow device - high voltage, low current.
If you dump out a 5 gallon bucket, you get a lot of flow (current) but there isn't much pressure.

Ok, on to the electrical system.

You have a few different kinds of things in the electrical system

The battery stores electricity in the form of potential chemical energy and can either supply current/power (discharge) or absorb current/power (charging). When the battery is just sitting there and not supplying or absorbing current, it will be sitting at about 12 or so volts. The actual voltage will depend on temperature and how well the battery is charged. (typically - there are also 24 volt systems in a lot of aircraft)


The alternator converts mechanical energy into electrical current / power and is controlled by a regulator that tries to keep the system voltage at about 14.5 to 14.7 volts

And you have electrical loads that use the current / power to do things like let you talk over the radio, light things up, start the engine, etc.

The ammeter measures how much current is flowing into or out of the battery.

Lets walk through what should happen with a "good" system.

You get in, flip on the master and perhaps one or two other things (strobe, beacon, whatever). At this point, the battery is supplying all of the current that is being used by the loads and the ammeter should show a slight discharge. It might be hard to see because the current draw of things like modern radios / transponders is very small. The system voltage will be about 12 or so volts.

When you start to crank the engine, the starter motor takes a LOT of current out of the battery to start the engine. The starter motor has it's own wire from the battery so the current doesn't flow through the ammeter (it is too high and would burn the meter out) so you don't see much action on the meter. But as you probably know, it doesn't take long for a starter to suck all of the power out of the battery if the engine doesn't start. During the crank the system voltage will drop quite a bit.

Now, the engine starts. As it spins up, it turns the alternator which can now start generating electricity. The system voltage should jump up to about 14.5 volts and current will start to flow back into the battery to make up for what the starter had pulled out. Right after the engine starts, you look down at the panel to check that the oil pressure is coming up and glance at the ammeter as well. The ammeter should show a high charging current for a few seconds then back off to about zero. How high and how long it takes to back off will depend on how much cranking you did (or if you left the master on for a couple hours with the engine off...)

Now, at this point, the alternator has topped off the battery - so current is neither flowing in or out and the ammeter sits in the center. (The alternator is supplying any current necessary to keep the elecrical loads happy.) And the system voltage will sit at about 14.5

Suppose you turn on a load that takes a lot of current - say a landing light. Right when you turn it on, it will draw more current than the alternator is supplying so the battery will take up the slack and you will possibly see the ammeter needle jump. But, after a fraction of a second, the regulator will crank up the alternator enough to supply the necessary current and the ammeter will go back to zero. When you turn the light off, you might see the ammeter flick again.

Note: If you turn on everything electrical (lights, beacon, pitot heat, landing light, Super-Homer, etc.), and the engine is just idling, it is possible that you will be consuming more current than the alternator can supply and you will see some discharge, but if you speed up the engine a bit (say 1500 - 2000 rpm) the needle should (on a modern aircraft or automobile) move back to the center.

Now, lets think about what happens if the alternator fails.

First of all, once the engine starts, the alternator is not putting the cranking current back into the battery so you don't see the ammeter show any charging for a few seconds like you would expect (that's one reason to look at it right away). Instead, the ammeter will continue to show about the same discharge that it was before you started the engine (but depending on what is turned on, it may be hard to see on the gauge). Your system voltage will be about 12 volts and gradually decline...

Now, if you turn on the landing light, all of the current has to come out of the battery all of the time since the alternator is not supplying what is required - right? So, the ammeter will show a noticable discharge for as long as the landing light is on. And, speeding the engine up to 1500-2000 won't make any difference. Turn the landing light off and the current draw from the battery drops to what it was before and the needle moves back to about the middle (depending on what else is on). That's why turning the landing light on is a good check - it typically uses enough current to make the needle move a noticable ammount.

What happens with time? If the alterntor has failed, the more you have turned on, the faster you pull current out of the battery, the more likely you are to notice the discharge on the ammeter, and the faster it will go dead. But if you only have strobes and radios on, you are pulling very little current so you might not notice it on the ammeter and it will take a while to suck the battery dry.
 
Thanks a lot Geoffrey I'm pretty sure I fully understand now. I did not know that the ammeter should have returned to the 0 position without turning off the landing light.
 
Austin is 14 years old. He was wondering if it was even legal for him to own an airplane.

Austin, sure it is. You just can't legally fly it by yourself until you are 16.
 
Just so you know I am a dumb A&P mechanic with a Cessna 152 information manual dated 1982 in front of me. I know it’s not the one for your aircraft, but the electrical system is pretty close and not rocket science.

To start with as a pilot you find a problem (electrical) if it falls within the scope of Part 43 preventive maintenance you can fix it. If not you should ground the aircraft and contact a mechanic to trouble shoot and fix the problem, before something serious happens. Having said that lets move on.

The amp meter does not tell us much, but a voltmeter is the tool of chose here. All electrical problems start with the electrical source the battery. A voltmeter will tell you how much voltage is in the battery. If it checks within limits move on. The low voltage light will tell you if the battery is discharging below a certain point (idiot light like in your car) this goes through the alternator control unit (keep that in mind), as it is a source of many problems.

The battery feeds through a switch to the master switch to the alternator field circuit breaker to the alternator & ALT pull off to the primary bus.

If you have had electrical problems over the years your mechanic should disconnect the battery ground, clean it, reinstall per the manual and proceed on up the electrical system taking off all the ground wires cleaning and reinstalling with the proper stack up in accordance with the Cessna manual or AC 43.13-1b Change 3.

You might be surprised to find a ground wire or buss wire corroded causing the electrical problems. If this electrical problem has been going on for years I would change my A&P/IA and find one that will take a closer look as we mechanic are required to do and fix it.

This trouble-shooting is really basic A&P stuff and the problem should be found quite quickly. I would recommend you do not fly the aircraft until the problem is fix and properly with a proper record entry indicating what was found and fixed. Good records are a owners best friend when it comes to trouble-shooting stuff.

Just one man’s opinion.

 
Capjac said:
Don't know if this helps, but my 1973 172 does not have a low voltage light. It has an Over Voltage Light. (I have no idea if a 152 and a 172 would be the same in this respect) It is located above the co-pilot yoke. This is from my owners manual:

"In the event of an overvoltage situation, the over voltage sensor automatically removes alternator field current and shuts down the altenator. The red warning light will turn on, indicating to the pilot that the altenator is not operating and the aircraft battery is supplying all electrical power."
Click to expand...

Overvolt and low-voltage lights are the same thing. In older machines they called it an overvolt light, but it came on only when the regulator was taken off-line and voltage dropped. Confusing, huh?

Cessna has a little cylindrical sensor under the panel. Looks like an old paper-wrapped capacitor, tied to the wire bundle near the bus bar. It has three wires connected to ground, the alternator side of the master switch and the alternator field breaker terminal. If it detects too much voltage (around 18 volts in a 14V system or 32 in a 28V), it cuts off the supply to the regulator, shutting off the alternator relay within it. The field current to the alternator dies and the regulator, sensing the dropping system voltage, sends a signal from the "I" terminal to the "overvoltage"/"low voltage" light, which is basically the same circuit as the alternator idiot light in a car. In fact, Cessna's alternators and regulators, in the older airplanes, are all Ford stuff with Cessna part numbers on them to make them MUCH more profitable. Even the new alternators are the same Ford thing but the regulator is an electronic doodad that also has the overvolt sensor built in.

The overvoltage sensor is to prevent oversupplying the electrical system if the regulator sticks or shorts and sends the field voltage out of sight, which will make the alternator's output voltage go way up, too. Tends to let the smoke out of expensive electronics, see.

A rabbit trail: A "12 volt" alternator will easily produce 110 volts (DC) if the field gets full battery voltage, and you used to be able to buy a doodad to fit into your auto that switched the alternator's output away from the car's system to a 110V receptacle, and the field directly to the battery. You could drive series-wound motors (drills, skillsaws and the like) with the DC as well as heaters or lights. The engine RPM would have to be around 1500 or so. The battery would run down with the field current draw and you'd occasionally switch back to let the alternator recharge it. None of this hurt the alternator, since it is current that burns it out, not voltage (unless it has cheap 50V diodes in it). A 60-amp alternator, at 110V, produces 6600 watts. Not bad, huh? Just don't try to drive induction stuff with it, things that need AC to work. They don't listen to DC. They'll just get hot and burn out.

Anyway, back to alternator failures. If you have a split master switch, with separate battery and alternator switches, the overvolt light should come on if the battery side only is switched on. If it doesn't, the light is burned out or the connection to the "I" terminal is broken. Or the battery is totally dead.

If the alternator isn't charging the battery, several common failures should be chased:

1. A bad regulator. Check for voltage at the "F" terminal of the regulator, which should also be present at the "F" terminal of the alternator. Same wire.

2. A bad alternator. Most common thing here is worn-out field brushes. We pull the alternators off and apart every 500 hours to check them. The brushes will last about 1000-1200 hours, depending on how much time is spent on night flying or in dusty conditions. Much less common are bad diodes in the alternator.

3. Bad alternator field or output breakers. The contacts in these get oxidized with age and can quit conducting. The output breaker might try to warn you by popping off when it shouldn't, since the resistance of the oxidized contacts generates heat that fools the bimetal trip strip into thinking there's too much current flowing in it. Landing light breakers do this, too.

I took a chunk of plywood, about 4" by 12", and put four screws into it, spaced well apart. Four long wires soldered to the screws go to alligator clips which get connected to ground, battery +, field and regulator "A" terminals.

The regulator's "A" terminal senses alternator output voltage and adjusts the field voltage ("F") up or down to get what it wants at the "A" terminal. More out means more in, see. An investment system. If we turn on a light, the electrical system's overall resistance drops, the current flow goes up, causing a voltage drop in the system, the regulator sees this and fixes it by pouring more into the F to get more out of the alternator's A. The reverse happens when we turn the light off. When the RPM goes up or down the alternator will produce more or less, too, and so its field is adjusted constantly to keep the system voltage where it's supposed to be.

Sitting in the airplane with these long wires hooked up, I can use my voltmeter to find where the problems are. The ground screw is for negative reference, and the battery and A terminals should show about 13v before start, 14.5 or so after start. With most everything turned off the field should show around 2-3V, maybe, up to 8 or 9V or more if the battery is sucking up a lot or the landing lights and everything else are on. Those long wires let me take vital measurements without fooling around near that propeller.

Zero field voltage is usually a dead regulator or bad overvolt sensor. Good voltage at the A terminal but only 11 or 12 volts at the battery says that the alternator output breaker is bad or the ammeter's shunt is shot or its connections are corroded or burnt. Further measurements can be taken at the bus to determine which.

Replacement regulators tend to have an electronic circuit to control field current rather than the old buzzer-type make-break relay. The electronic control is smoother and makes less system noise but they burn out with distressing frequency. I think maybe they don't want to be switched off and on with the engine running. (I wish I could still buy those old mechanical regulators.) That voltage spike can fry some radios anyway and you shouldn't fuss with the alternator switch unless you need to.

Easy to do once you study the schematics in the service manual and see what should be where.


Dan
 
Last edited:
Dan Thomas said:
Overvolt and low-voltage lights are the same thing. In older machines they called it an overvolt light, but it came on only when the regulator was taken off-line and voltage dropped. Confusing, huh?


Dan
Click to expand...


Dan, thanks for the explaination. (I always thought it odd to not have a low voltage warning) I agree on the confusing part but your spelling out the "how it works" makes sense. The checklist of the light coming on is to cycle the switch, I am assuming this would (hopefully) bring the alternator circuit back on line?
Tim
 
Capjac said:
Dan, thanks for the explaination. (I always thought it odd to not have a low voltage warning) I agree on the confusing part but your spelling out the "how it works" makes sense. The checklist of the light coming on is to cycle the switch, I am assuming this would (hopefully) bring the alternator circuit back on line?
Tim
Click to expand...

That's the intent. Most overvoltage protection "latches" the overvoltage condition once it's detected and turning the alternator (actually the regulator) off and back on, resets the latch. If the problem was temporary (aka a glitch) everything will continue to work normally. If the regulator has failed and the overvoltage immediately recurrs you should shut the switch back off and leave it off. It's also possible that no overvoltage event occurred, just a loss of alternator output in which case cycling the alternator switch won't do anything useful.
 
Capjac said:
Dan, thanks for the explaination. (I always thought it odd to not have a low voltage warning) I agree on the confusing part but your spelling out the "how it works" makes sense. The checklist of the light coming on is to cycle the switch, I am assuming this would (hopefully) bring the alternator circuit back on line?
Tim
Click to expand...

Yup. Turning off the alternator' switch resets the overvolt detector.

Something else I should have mentioned when I was talking about worn alternator brushes: If those brushes (which are just little oblong chunks of graphite that carry the current to the alternator rotor's slip rings, which feed the field) are allowed to wear too far, they'll pop out of their holder and the springs (which are behind them and pushing them against the slip rings) will gouge the slip rings and ruin the alternator. Gets expensive. Those brushes should be checked occasionally rather than letting things fall apart. Goes for your car, too.

Alternators that sit a lot also suffer bearing dryout and corrosion unless they're cleaned and relubed, especially that rear bearing. But use very little grease on it or it'll get on the slip rings and screw things up.

See this page (it's automotive but close enough):
http://www.procarcare.com/icarumba/...car_resourcecenter_encyclopedia_starting1.asp

Dan
 
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