Amp meter doing odd things, voltage steady.

[bmcconnaha]

Video attached.  Amps are all over the place, but voltage remains relatively steady.  Any ideas? 

[toto]

For whatever it’s worth, that’s what my ammeter looks like nearly 100% of the time.

Right after engine start, it does show a minor deflection to the plus side, and if the battery was run down for any reason, it will stay deflected for a bit as the battery charges. But when everything is charged and operating normally, it looks exactly like yours does. 

[bmcconnaha]

7 minutes ago, toto said:

For whatever it’s worth, that’s what my ammeter looks like nearly 100% of the time.

Right after engine start, it does show a minor deflection to the plus side, and if the battery was run down for any reason, it will stay deflected for a bit as the battery charges. But when everything is charged and operating normally, it looks exactly like yours does. 

Interesting.  Mine was rock solid until yesterday.  I landed, picked a friend up and took off.  On climb out it started doing that.  

[toto]

2 minutes ago, bmcconnaha said:

Interesting.  Mine was rock solid until yesterday.  I landed, picked a friend up and took off.  On climb out it started doing that.  

I really don't have any idea, but it's always like that for me. I do leave my strobes on at all times, and I've had in the back of my mind that it might be the very quick, sharp current draw that causes the ammeter tick. 

Hopefully someone with more technical knowledge of the gauge can weigh in. 

[EricJ]

On many of them it's really a little voltage meter that measures the voltage across the shunt.   I think the usual way to test it is disconnect it from the shunt and put a 1.5V battery across it both ways and see what it does.

It may just be the connections at the shunt got loose or dirty or something, too.

 

[bmcconnaha]

Just now, EricJ said:

On many of them it's really a little voltage meter that measures the voltage across the shunt.   I think the usual way to test it is disconnect it from the shunt and put a 1.5V battery across it both ways and see what it does.

It may just be the connections at the shunt got loose or dirty or something, too.

 

Thanks, Eric.  I’m going to have the cowl off for an oil change tomorrow, any idea where to find the shunt? 

[EricJ]

Just now, bmcconnaha said:

Thanks, Eric.  I’m going to have the cowl off for an oil change tomorrow, any idea where to find the shunt? 

It probably varies between models, serial nos, etc.    It might be possible to just follow the wires from the ammeter.   It's probably the only thing they connect to.

[Mooney in Oz]

Mine is much the same.

Sometimes minor left and right deflections as you have and other times it's close enough to solid. 

[buddy]

This happened to me when I had my M20J, the connections in back of your master switch are all single wire spade connectors. I had to get under the instrument panel with a long thin screw driver and push each individual female spade connector in back of the master switch in (they sometimes start to get loose or a little corroded over the years). This should solve your problem, your not the first one this has happened too. If you want PM me with your phone number and I can probably explain it to you a little better.

 

[A64Pilot]

Amps can’t fluctuate without a corresponding voltage fluctuation. In theory if you had a large amp draw and a super fast voltage regulator they could but in reality no VR can respond that fast.

‘So one of these possible things is going on, well several actually, but either the amperage is fluctuating widely and the volt meter isn’t showing it, or everything is good and your meter is swinging from either an outside influence like a vibration that happens to correspond to the needles natural frequency, or something wrong with the meter or it’s contacts.

‘My bet is the voltage is actually fluctuating but your voltmeter isn’t showing it, most digital instrumentation has a lag or an averaging or “smoothing” to keep the display from constantly changing, even important turbine isntruments, Piper has what they call a snap function that when the meter is close. it snaps to the set reading and won’t change until it gets a set amount away, pilots like seeing dead nuts steady readings,so manufacturers give them what they want to see. Electronics International calls it smoothing I believe and a good example would be fuel level, you don’t want to see your fuel gauges swinging wildly when taxing, so they don’t.

Get an old fashioned needle voltmeter and connect it and see, they aren’t usually excessively dampened so if there is a fast voltage change, the needle will swing.

‘I bet you will find it swinging.

 

Edited May 31, 2021 by A64Pilot

[PT20J]

The shunt on my 1994 M20J is located behind the circuit breakers. It's 70A/100mv so the output voltage is pretty small. The wire from either side of the shunt passes through a fuse on it's way to ammeter. The speed and range of motion of your needle indicate an instrumentation error of some kind as it's unlikely the real load could change that much so rapidly. What I think you are seeing is a glitch in the voltage across the meter and the resulting underdamped mechanical response of the meter.

Skip

[A64Pilot]

Yes, but a amp meter is actually a volt meter, one that measures tiny voltage changes across a very slight known resistance, so it’s normal to see voltage fluctuations in a amp meter.

‘Plus as voltage changes the amps into and out of a battery will change, because voltage is the “pressure” that pushes current into a battery.

‘Our Amp meters don’t measure load or I don’t believe they do, they only measure current into and from the battery. this is why they read zero on a fully charged battery and a electrical system under a steady load.

To measure load, you need a shunt connected to the alternator output.

[carusoam]

Summary-ish...
 

Shunts...

Are physically big resistors...

That have very low resistance...

Often, bolted to the firewall...

All of the ship’s continuous power goes in one end, and out the other.... on large diameter wires...

 

The ammeter measures voltage drop across the Shunt...

both small wires, to and from the Shunt ends, will have fuses on them...

 

So... the ammeter is technically a voltmeter... calibrated to show amps...

Be on the look-out for things that can cause a noisy signal to the ammeter...

Use caution poking around the shunt... very easy to cause a large spark to ground... 

Often metal shavings and/or loose metal bits falling on and near the shunt have generated big sparks...  there is not a lot of insulation on parts of the shunt...

PP thoughts only, not a mechanic...

Best regards,

-a-

[JimB]

Mine is doing something similar with a JPI 900. Volts are rock steady at 14-14.1 volts and amps fluctuating. Mostly fluctuating negative but some positive as well. It is 100% an indication issue verified by using a clamp on Fluke amp meter. During a ground run, the clamp on meter was rock steady and the JPI started fluctuating after about 10 mins. The connections are good on the shunt so in my case it is either the shunt or wiring to the JPI. I guess it could be the JPI but I doubt that. It also gets worse the longer the flight or the run. Now that I know it is just indication, I'll get around to fixing it ....... sometime....

Edited May 31, 2021 by JimB

[EricJ]

BTW, don't forget this one:

https://www.donmaxwell.com/fluctuating-ammeters

[A64Pilot]

You should be able to verify the master switch and the voltage regulator etc by measuring the voltage at the field terminal on the alternator itself.

‘If you have a good steady field voltage with the engine running, then the master switch and VR etc and all wiring and connections are doing their thing, if it’s fluctuating, then you have a problem.

I connected a long wire with a alligator clip and ran it back through the little window on the pilots side, so no body was up there near the prop.

Edited June 1, 2021 by A64Pilot

[flyboy0681]

Been there twice, with two different causes. The first was already mentioned, the terminals on the master needed to be cleaned. The second time turned out to be the voltage regulator.