Ammeter reads ZERO. is this correct - C172?

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JasonM

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I was doing my pre-takeoff checklist and noticed the Ammeter reads zero with the engine at 1700 RPM and the Master and Avionics turned on. CFI said it was good. I want to believe that but I guess i am questioning it now to be sure. just seems like it should be showing something other than zero.
 
Flip the pitot heat or landing lights on while watching the ammeter. Let us know what you see.
JasonM said:
I was doing my pre-takeoff checklist and noticed the Ammeter reads zero with the engine at 1700 RPM and the Master and Avionics turned on. CFI said it was good. I want to believe that but I guess i am questioning it now to be sure. just seems like it should be showing something other than zero.
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Is it an ammeter or loadmeter? I've seen both in airplanes, and they look similar.

An ammeter shows the current coming out of the battery. Once recharged from starting, it should read zero unless you run off the battery (alternator turned off). Typically, these are labeled "charge" and "discharge" with zero in the center.

A loadmeter shows the total current coming through the power bus. It should always be positive.

To test an ammeter, turn the alternator off. It should discharge. Then turn it back on and it should charge (until it has replenished what you used -- won't take long). To test a loadmeter, do something power intensive like lower the flaps (if electric) and you should see it go up.
 
Normally, my ammeter appears to be in the middle, but the needle is just barely on the positive side. As mentioned, you will probably see it move when you turn something on. The strobe gives it a little kick.
 
If i remember correctly it looks like this. Its on zero with the plane turned off and when its running. I'll have to take a closer look and see what happens when setting the flaps etc. Am I to assume this is correct?

ammeter.jpg
 
Read your POH. That's very likely an ammeter, but your POH is the authority. Get in the habit of doing that; your examiner will expect you to follow the POH even if your instructor trains you differently.
 
JasonM said:
If i remember correctly it looks like this. Its on zero with the plane turned off and when its running. I'll have to take a closer look and see what happens when setting the flaps etc. Am I to assume this is correct?

ammeter.jpg
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An ammeter, as someone already said, measures the current flow in and out of the battery. If the battery is carrying a load, like the flaps or landing light, it will indicate a discharge. The ammeter will indicate a charge only when the alternator is recharging the battery. The current used by flaps or lights or anything else, while the engine is running and the alternator is working, does not pass through the ammeter and it will only give a small flick when you switch something on. That flick will be a brief discharge, very brief, because the voltage regulator senses the voltage drop in the whole system as the amperage increases and it sends more field current to the alternator and brings everything back to normal. The battery carried a bit of the load for a tiny bit before the voltage regulator caught up.

A loadmeter, on the other hand, measures everything the alternator is putting out and will increase as you add loads like flaps and lights. Different system. It is between the alternator and bus, while the ammeter is between battery and bus.

Your 172 has an ammeter. If it indicates zero in the runup, and you actuate the flaps, it should stay at zero except for that tiny flicker of the needle. If you turn everything on--all the radios, all the lights, and hit the flap switch, it should still read zero. If it indicates a discharge, the alternator needs work. Alternators are sized to carry all the loads at once in a light aircraft at such RPM, which is why the regulator doesn't have a current limiter like the old generator regulators did.

Dan
 
Ammeter will read 0 if there is no load large enough to bring the battery down and require charging at a rate that shows on simple instruments. As Wayne said, hit the pitot heat or some large draw item and see if the needle says +, if it does, all is normal. What you don't want to see is that needle in the - very long.
 
Dan Thomas said:
An ammeter, as someone already said, measures the current flow in and out of the battery. If the battery is carrying a load, like the flaps or landing light, it will indicate a discharge. The ammeter will indicate a charge only when the alternator is recharging the battery. The current used by flaps or lights or anything else, while the engine is running and the alternator is working, does not pass through the ammeter and it will only give a small flick when you switch something on. That flick will be a brief discharge, very brief, because the voltage regulator senses the voltage drop in the whole system as the amperage increases and it sends more field current to the alternator and brings everything back to normal. The battery carried a bit of the load for a tiny bit before the voltage regulator caught up.

A loadmeter, on the other hand, measures everything the alternator is putting out and will increase as you add loads like flaps and lights. Different system. It is between the alternator and bus, while the ammeter is between battery and bus.

Your 172 has an ammeter. If it indicates zero in the runup, and you actuate the flaps, it should stay at zero except for that tiny flicker of the needle. If you turn everything on--all the radios, all the lights, and hit the flap switch, it should still read zero. If it indicates a discharge, the alternator needs work. Alternators are sized to carry all the loads at once in a light aircraft at such RPM, which is why the regulator doesn't have a current limiter like the old generator regulators did.

Dan
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Excellent description of an Ammeter operation.:yes::thumbsup:

On my plane I use a non shunted ammeter.. and not a voltmeter, which is the current gauge of choice.....

I grew up with ammeters and with a quick glance at it I can tell if the alternator is keeping up with the electrical demand, or not...
 
Thanks for all the good info. So how exactly am I supposed to tell if it is broken? Am i looking for it to be in the negative or not flicker at all?
 
JasonM said:
Thanks for all the good info. So how exactly am I supposed to tell if it is broken? Am i looking for it to be in the negative or not flicker at all?
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If it reads in the negative for extended durations, you have an alternator problem. If it reads in the positive for extended durations you have a battery problem. The electrical system runs off the battery, the alternator recharges the battery.
 
N801BH said:
Both....:yes:
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ok.. So if its negative, then most likely the alternator is not functioning properly and if no flicker, the ammeter may be broken? is this a good assessment of what the check is telling me?
 
JasonM said:
ok.. So if its negative, then most likely the alternator is not functioning properly and if no flicker, the ammeter may be broken? is this a good assessment of what the check is telling me?
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See Hennings post......
 
Henning said:
If it reads in the negative for extended durations, you have an alternator problem. If it reads in the positive for extended durations you have a battery problem. The electrical system runs off the battery, the alternator recharges the battery.
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So if its positive to much, is it a risk of over charging the battery?
 
JasonM said:
So if its positive to much, is it a risk of over charging the battery?
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Yup.. but.. running in the positive all the time tells you the battery is not charging properly and NOT holding that charge...
 
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JasonM said:
So if its positive to much, is it a risk of over charging the battery?
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It means you likely have a shorted cell in the battery and it can't make voltage because it's missing 2.2 volts worth of cell. Hopefully your charging system will keep you from exploding the battery.
 
Henning said:
It means you likely have a shorted cell in the battery and it can't make voltage because it's missing 2.2 volts worth of cell. Hopefully your charging system will keep you from exploding the battery.
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On a different note, what's a trianle?
 
JasonM said:
Doesnt that attitude fall into some kind of wreckless category? :dunno: :rofl:
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Ha......

If there were no test pilots, then there would be NO certified planes to fly today....

it is a dirty job................ but someone has to do it..:yes:;)
 
JasonM said:
Doesnt that attitude fall into some kind of wreckless category? :dunno: :rofl:
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I haven't been wreckless since I was 9 years old and crashed my first 3 wheeler. I am not IMO reckless though, not in a long time.;)
 
JasonM said:
ok.. So if its negative, then most likely the alternator is not functioning properly and if no flicker, the ammeter may be broken? is this a good assessment of what the check is telling me?
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Another check is to look at it right after starting up the engine. The starter motor is the largest current draw item in your plane. Immediately after startup, watch the ammeter. It should show a charge as the alternator restores to the battery the power used up during the start. This assumes a single switch that turns on the master and the alternator field current simultaneously. If you have a "split master" you should see this charging current after start as soon as you turn on the alternator field switch.

-Skip
 
Henning said:
. The electrical system runs off the battery, the alternator recharges the battery.
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The way it's wired, the battery only runs the system if the alternator isn't keeping up. The alternator otherwise runs everything.

6-34.jpg


See how the ammeter is between the primary bus and the battery. See how the alternator's output is connected to the primary bus, too (the "B" terminal). See how everything except the starter (and any clocks or hourmeters) runs off that primary bus.

So, if our battery has 13 volts, it will run the system if the alternator is off or producing less than 13 volts, and the ammeter will show a discharge. If the alternator is producing it's normal 14.5 volts or so, it takes over the loads and the ammeter will show that current is now flowing the other way, recharging the battery, as well. The bus is always positive, the ground always negative, and the loads don't know anything different between battery and alternator except for the small voltage change during charge or discharge.

Dan
 
Henning said:
I haven't been wreckless since I was 9 years old and crashed my first 3 wheeler. I am not IMO reckless though, not in a long time.;)
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Ben said: "I am crazy and he is downright nuts..." :) , just sayin..:nono::rofl:
 
JasonM said:
Ben said: "I am crazy and he is downright nuts..." :) , just sayin..:nono::rofl:
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There is a 180° difference between wreckless and reckless, just sayin'...;):D

One who is reckless will not be wreckless for long.:yesnod:
 
I keep reading this as Altimeter, not Ammeter (whatever that is.)
 
The problem with an ammeter is that, in a perfectly functioning system it's not going to appear to be indicating anything. It only indicates off of zero (+ or -) for any significant length of time when something is wrong. As noted earlier, the way to test it is to turn on a load such as landing lights or pitot heat then switch off the alternator half of the master switch. You should then see a noticeable discharge indication and if you leave it off for a short time you will see a noticeable charge indication for a short time after turning the alternator switch back on.

So to answer your question: an indication of zero is normal, except when the indicator or it's circuit is broken. ;)
 
Silvaire said:
The problem with an ammeter is that, in a perfectly functioning system it's not going to appear to be indicating anything. It only indicates off of zero (+ or -) for any significant length of time when something is wrong. As noted earlier, the way to test it is to turn on a load such as landing lights or pitot heat then switch off the alternator half of the master switch. You should then see a noticeable discharge indication and if you leave it off for a short time you will see a noticeable charge indication for a short time after turning the alternator switch back on.

So to answer your question: an indication of zero is normal, except when the indicator or it's circuit is broken. ;)
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Or shortly after starting, when the battery need a good charge.
 
Silvaire said:
The problem with an ammeter is that, in a perfectly functioning system it's not going to appear to be indicating anything. It only indicates off of zero (+ or -) for any significant length of time when something is wrong. As noted earlier, the way to test it is to turn on a load such as landing lights or pitot heat then switch off the alternator half of the master switch. You should then see a noticeable discharge indication and if you leave it off for a short time you will see a noticeable charge indication for a short time after turning the alternator switch back on.

So to answer your question: an indication of zero is normal, except when the indicator or it's circuit is broken. ;)
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That's not really a problem. If the ammeter is broken, the primary bus is dead. It's not too hard to figure out that nothing at all works.
 
Takeoffs in the Cardinal RG are normally 10 degrees flaps so as part of my pre-departure run up flow I move the flap switch to 10 degrees while looking at the ammeter. The expected momentary blip to the negative side indicates that the flap motor is working normally and the return to 0 means the alternator and voltage regulator is working normally. Then I look left and right to confirm that the flaps are in the correct position.
 
spinfire said:
Takeoffs in the Cardinal RG are normally 10 degrees flaps so as part of my pre-departure run up flow I move the flap switch to 10 degrees while looking at the ammeter. The expected momentary blip to the negative side indicates that the flap motor is working normally and the return to 0 means the alternator and voltage regulator is working normally. Then I look left and right to confirm that the flaps are in the correct position.
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Good point.......

Another use of mine is when I am changing pitch on my prop..... I have a IVO , electric inflight adjustable unit and when the blades get to the end of their travel the motor draws ALOT more amps.. ie, locked rotor... The ammeter clearly shows that event..:yes:;)
 
If you know anybody who flies a King Air, ask them to show you the gages in their plane. They are marked in both volts and load, with a push-button to toggle between the two indications. Other planes have similar multi-function displays as well.
 
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