Ammeter reading high

K

kujo806

Pre-takeoff checklist
Joined
May 6, 2013
Messages
178
Location
St. Charles, IL
Display Name
Display name:
kujo806
We just had our plane avionics upgraded with a PMA8000G, GTN650, and GTX345. We have flown it several times since the upgrade, and I noticed that the ammeter is almost fully deflected most of the time including when the avionics are switched off. The reading is over 70 amps! This is the Piper style ammeter with a 0 to 75 amp scale. We are trying to figure out if it is an alternator issue, a battery issue, or an ammeter problem. The plane starts fine, and I haven't seen any power issues with the avionics. When I add loads such as pitot heat, the needle moves higher, so it isn't quite fully deflected.

My thinking is that if a cell in the battery was bad, we would notice it in starting. These aviation batteries don't have a lot of extra cranking capacity, and I think we would have issues turning the prop over if part of the battery was bad. I have heard that some ammeters have a bypass shunt across them to allow most of the alternator current to bypass the meter. Because the meter is calibrated for the remaining current, if that shunt is broken, the higher current flowing through the meter side would show a high reading. Considering everything else seems to running fine, I am thinking the meter is most likely the problem. Anyone have a similar issue or suggestions?
 
I have a similar experience with a Piper ammeter after avionics work. There is an adjustment on the ammeter which can be used to try to bring it back in range.

As for the cause? I don't know but disturbing the shunt seems to make the meter go wonky. I call it an idiot meter now and mostly just monitor voltage now using a digital meter that plugs into the cigar lighter socket.

I should mention that my ammeter reads zero when the alternator master is off.
 
What is the voltage with the alternator on?
 
I believe it goes to zero, although I haven't directly observed that. Another guy in the club flew with the alternator off to avoid overcharging, so I assume he saw it go to zero with the alternator off.
 
I'm having the same problem with my Archer and it's driving me (and my mechanic) crazy. At startup, with just the beacon and engine monitor on, the ammeter jumps all the way to the right and stays there. If I wait a while (like after I do the runup) and turn off the beacon and then the master, then turn the master back on the ammeter goes to a normal level.

It's like something at startup is somehow stressing out the ammeter. The voltmeter on the engine monitor always shows between 13.8 and 14.0 volts.

Weird.
 
Make sure the terminal connections on both sides of the ammeter shunt are bright, shiney, and tight. These ammeters are merely sensitive volt meters that measure the voltage across a calibrated shunt. Any unusual resistance in this shunt circuit will create a high voltage reading on the meter.

-Skip
 
Skip Miller said:
Make sure the terminal connections on both sides of the ammeter shunt are bright, shiney, and tight. These ammeters are merely sensitive volt meters that measure the voltage across a calibrated shunt. Any unusual resistance in this shunt circuit will create a high voltage reading on the meter.

-Skip
Click to expand...

Yup. Bad cable, bad contacts, or a shunt that isn't the correct resistance.

The theory behind a shunted meter is pretty simple. Most of the current passes through the shunt and a small amount passes through the sensitive meter.

You just need to know what the meter wants for full scale reading and what the resistance of the bypass shunt (handles the majority of the current) is supposed to be. If you know one, you can calculate the other since you know the system voltage.

If the shunt circuit has developed even a little bit more resistance end to end (usually in the contacts or wiring, but the shunt itself could also be bad), more current flows through the meter and it reads erroneously high.


It doesn't take much. Shunts are usually designed to be very low resistance since they have to carry so much current. Otherwise heat and size would be a significant problem.

As you can see, the math is pretty easy, since it's just a touch of algebra and Ohm's Law. Measuring the shunt to see if it matches the resistance printed on it, shouldn't be too hard.

Any additional resistance at connectors or in cabling will screw it up.

Most piper meters I've seen are only measuring load. They are uni-directional meter movements. Very simple.

Cessnas tend to use bi-directional meter movements with a center zero and show charge/discharge.

Same theory, same type of shunt used to send most of the current around the meter.

In automotive charge/discharge, usually the shunt is hidden inside the meter case. It makes the meter physically bigger, but also brain dead easy to use and little chance of corrosion or other problems in connectors or wiring to change the resistance value -- unless the meter case is regularly dunked in water and isn't sealed well -- like... oh... the one on my tractor.

Guess which direction it reads? LOL. :)

Just physically moving the shunt shouldn't change resistance. If it does, that indicates there's something going on in the circuit that's causing different resistance as it's moved. Probably dissimilar metal corrosion at a contact, but can be other physical wire problems at these low resistances.

All of this is measurable with a high quality VOM, but really the usual fix is to clean all the connectors up, get a nice shiny ground to the airframe, and the meter should work a heck of a lot better. ;)
 
P.S. @Clark1961 if you can find out what the max scale current is on your Piper ammeter, you should be able to make a table in Excel showing your clueless avionics guy exactly what the resistance is of the shunt circuit for all values of the meter! Hehehe.

You're doing trig in Excel in the other thread... this ain't even trig! ;) ;) ;)

Would be fun to hand it to him, anyway. "If the meter is working correctly, there's an additional 75 milliohms of resistance somewhere in that stupid external shunt circuit."

He'd look at you like this... if he's a box swapper and not a classically trained electronics tech...

o_O

;)
 
Last edited:
Could a shunt issue explain what's happening in my case (high reading at startup, but returning to normal after resetting the master after some time)?
 
bcool said:
Could a shunt issue explain what's happening in my case (high reading at startup, but returning to normal after resetting the master after some time)?
Click to expand...
That could be high resistance in the shunt circuit or it could be calibration of the meter. Watch the ammeter response as load is added when the needle is in the normal range.
 
When the needle is back to normal, it does rise with each addition of a load.
 
denverpilot said:
P.S. @Clark1961 if you can find out what the max scale current is on your Piper ammeter, you should be able to make a table in Excel showing your clueless avionics guy exactly what the resistance is of the shunt circuit for all values of the meter! Hehehe.

You're doing trig in Excel in the other thread... this ain't even trig! ;) ;) ;)

Would be fun to hand it to him, anyway. "If the meter is working correctly, there's an additional 75 milliohms of resistance somewhere in that stupid external shunt circuit."

He'd look at you like this... if he's a box swapper and not a classically trained electronics tech...

o_O

;)
Click to expand...
I'll meet you at the hangar and yer welcome to crawl around under the panel to locate the contacts and stuff. I have a $15 not made in the USA or Europe VOM that will give good enough readings I'm sure. Hey the seats are out right now for the annual inspection so getting access is as easy as it gets. Hangar should be pretty toasty by 5 p.m. See ya then.
 
Clark1961 said:
I'll meet you at the hangar and yer welcome to crawl around under the panel to locate the contacts and stuff. I have a $15 not made in the USA or Europe VOM that will give good enough readings I'm sure. Hey the seats are out right now for the annual inspection so getting access is as easy as it gets. Hangar should be pretty toasty by 5 p.m. See ya then.
Click to expand...

Oh man, your airplane is STILL in annual?! Yikes. What came up?
 
Clark1961 said:
It's just waiting in line. Low priority in a sorta backed up shop.
Click to expand...

Ahh ok, was worried you ran into something ugly. And yeah, all the shops around here are constantly backed up, but nobody is opening any new ones...
 
All you gurus who pointed to the shunt were right (at least in my case). My mechanic worked his wizardry on it & my overvoltage indication went away. Thanks for all the input!
 
bcool said:
All you gurus who pointed to the shunt were right (at least in my case). My mechanic worked his wizardry on it & my overvoltage indication went away. Thanks for all the input!
Click to expand...

If you watch that video you see just how little extra resistance it takes to drive the meter wrong. The shunt thing is great but that's the Achilles heel of the idea generally. A little corrosion on a terminal and the meter goes insane.
 
You must log in or register to reply here.
Back
Top