Ammeter failure?

[Cloudmirth]

What is it with Mooney ammeters?  My first Mooney was a ‘69 M20C which was one year old when I bought it.  The ammeter never really worked barely showing a reading positive or negative regardless of load.  Never solved the problem in 5 years of ownership.  Move forward 45 some odd years and I have another ‘69 M20C (go figure) and the ammeter barely registers.  The battery stays charged and a voltmeter reads 14 volts when the alternator is operating.  Ignoring the ammeter worked back in 1975 but I’m older (if not wiser) now.  It seems odd that two planes separated by 45 years would manifest exactly the same problem.  Any thoughts?

[flyingchump]

I'm a new Mooney (66E) owner and mine does the same thing.  Never shows positive.  It moves slightly negative when I turn the non-led landing light on.  Voltmeter always shows 14 volts.  I'm looking forward to what other folks say about this.

[PT20J]

Ammeters are pretty simple: It's a voltmeter bridged across a shunt. The shunt is just a simple resistor, and if it were to fail you'd have an electrical failure. This pretty much leaves the two wires from the shunt to the meter and the meter itself.

Skip

[carusoam]

Mirth,

How do you feel about new instrumentation..?

The ammeter is a simple voltage meter with two wires... (calibrated in amps)

The shunt is a giant bare resistor attached to the firewall...

The display is a simple device and may be sensitive to dirt, corrosion, or age...

The gauge can be OH’d.

Somebody builds replacement gauges that look very similar...

If your plane is a forever-plane... consider getting it some decent Primary gauges...

You are right... the ‘69 version of that gauge was crummy at best... it didn’t get any better adding 50 years to it... 

PP thoughts only, not a mechanic...

Best regards,

-a-

[Ragsf15e]

Just be careful about saying “it’s not working”... the way it’s connected in various type aircraft is different.  For older Mooneys it is reading amps to/from the battery, NOT total electrical load.  With the engine running, you’ll have just a few amps going to the battery which looks a lot like 0 on the old gages - no matter how much electrical load is on the whole system.  Also, when you’re starting, you have minimal electrical load before hitting the starter so it looks close to 0.  You turn everything on after the alternator kicks in and it just shows the amps going to the battery (only a couple amps).

I suggest you turn everything on (pitot heat, lights, etc) without the alternator/generator running (engine off or pull the regulator cb).  You should see a bigger discharge on the amp meter.

Edited January 13, 2021 by Ragsf15e

[Ragsf15e]

YGBSM!  I just got a “like” about an electrical explanation from a real live electrical expert!  This may be my best day ever because I’m generally EE challenged and have been trying hard to learn!

Edited January 13, 2021 by Ragsf15e

[PT20J]

Good point @Ragsf15e; perhaps you are not as EE challenged as you think. Just to make sure we are all on the same page:

An ammeter shows the current flowing to or from the battery. It has a needle that can swing either right (positive - current flowing to the battery) or left (negative - current flowing from the battery) from a zero center position. With the master switch off it should read zero. With master on and the engine stopped you will see it swing left showing a negative reading as you turn on equipment indicating that the battery is supplying current to the electrical load and thus discharging. Immediately after engine start, you should see the needle move to the right indicating that the alternator is charging the battery to replenish it of the energy used to start the engine. After a few minutes, a healthy battery should be recharged and the ammeter will only show trickle charge current -- usually a needle width or less to the right. A failed charging system will indicate a discharge with the engine running and the alternator switched on.

A loadmeter is different and shows the total current drawn by the electrical load and has a zero on the left and the needle only moves to the right. But let's not confuse things... 

[carusoam]

Skip,

Great points...

Wondering if Mirth is familiar with the needle width reading... being normal...

And why he has a similar experience on two different planes...

Best regards,

-a-

[PT20J]

7 minutes ago, carusoam said:

Skip,

Great points...

Wondering if Mirth is familiar with the needle width reading... being normal...

And why he has a similar experience on two different planes...

Best regards,

-a-

You know, sometimes I wonder why it seems we have to be part flight engineer. I get in my car and it just runs. I get in my airplane and spend too much time fiddling with mixtures and scanning engine instruments. Maybe I should just get a J3 with an airspeed indicator, altimeter, tachometer, oil pressure gauge, oil temp gauge and compass. No radio and a Esso road map! Ah, to be an aviator again!

Skip

[Cloudmirth]

Interesting comments but not necessarily helpful.  In this case we have a situation where the shunt is sound, the wiring to the meter is sound and regardless of what load is placed on the electrical system with the engine not running you can't get more than about a half needle negative deflection and upon startup when you should show a significant charge on the ammeter, again you maybe get half a needle positive deflection.  Now admittedly it would appear that the meter is the most likely problem but again I have seen the exact same situation in two '69 Mooneys.  Were the meters they used really that bad?  Ammeters are about the most simple meters going so it's hard to see how they could be that inaccurate in two planes of the same vintage.  Yes, going to a digital meter may turn out to be the way to go but really, there must be a more obvious solution.  I suppose the next step is to take the meter out and test it but again, similar situations with two separate aircraft makes me wonder.  Interesting how Stewart Warner can turn out inexpensive auto gauges that are accurate and inexpensive but we in the aircraft world live with hideously expensive gauges that fail with deadly regularity.  Glad I have a voltmeter in the plane that at least tells me the alternator is putting out 14 volts.

[N201MKTurbo]

Old meter movements usually fail because of a mechanical issue. I have seen the tape around the armature come unglued and plastic parts melt in the heat. The forces moving the needle are tiny and it doesn’t take much to stop it. 
 

you can trouble shoot it with volt meter. Put the meter across the shunt, turn on something and see if you are getting the correct reading. The transfer function for the shunt is written on it somewhere. It is usually something like 50mv/A. If you are getting the correct reading, then it is the meter.

 

 

[PT20J]

1 hour ago, Cloudmirth said:

Interesting comments but not necessarily helpful.  In this case we have a situation where the shunt is sound, the wiring to the meter is sound and regardless of what load is placed on the electrical system with the engine not running you can't get more than about a half needle negative deflection and upon startup when you should show a significant charge on the ammeter, again you maybe get half a needle positive deflection.  Now admittedly it would appear that the meter is the most likely problem but again I have seen the exact same situation in two '69 Mooneys.  Were the meters they used really that bad?  Ammeters are about the most simple meters going so it's hard to see how they could be that inaccurate in two planes of the same vintage.  Yes, going to a digital meter may turn out to be the way to go but really, there must be a more obvious solution.  I suppose the next step is to take the meter out and test it but again, similar situations with two separate aircraft makes me wonder.  Interesting how Stewart Warner can turn out inexpensive auto gauges that are accurate and inexpensive but we in the aircraft world live with hideously expensive gauges that fail with deadly regularity.  Glad I have a voltmeter in the plane that at least tells me the alternator is putting out 14 volts.

Sounds like you already knew the answer. Maybe 1969 was a bad year for ammeters.  I think there is a complaint department at the new Mooney website.

[David Lloyd]

Once I had the ammeter in my airplane repaired, it works the same way.  It is pretty insensitive.  Right after startup, it will show about a quarter scale charge.  After a minute or so, about a needle width charge, another few minutes about half a needle width charge.  Next to no information!  The ammeter being set up as a loadmeter in older airplanes is much better for providing useful information.

[RJBrown]

Test with an inductive ammeter.

they clamp over the wire to or from the ammeter or master switch.

compare numbers.
 

Fluke sells good electrical test equipment. 

[carusoam]

For additional not necessarily helpful info...

Consider getting an O1... they were built in the mid 90s..

Its amp meter is an analog gauge similar to a 60s amp meter...  it’s also a voltmeter if you push a button... like magic... 

I’m pretty sure it is intended to show a deflection when the alternator has gone off line...

Suddenly, the gauge makes more sense...

Somebody above gave you every method other people have used to solve their similar gauge challenges... 

What else can we do for you?

Now I have to go look... do any of my cars actually have an amp meter in them?

I typically look to see if the voltage is correct... if it isn’t neither are the amps...

Alternators don’t usually fail in a partial way... when they do, they give off enough noise in the audio system to ask for help...

If you want to experiment with an additional gauge... we have a few resources around here that specialize in selling these old things...

Or skip the old crusty nostalgic gauges... and pick up some fancy digital devices... or one big color screen...     EI, JPI, or Garmin...

PP thoughts only, not a mechanic...

Best regards,

-a-

[Floyd]

Get a USB adapter that plugs into the lighter plug.  Sporty's sell one for about $20 that has a voltmeter built in.  If the volts are correct, no need to worry about the amps.

[zaitcev]

I have a 69 E with a dead ammeter too, in a block of Rochester gauges. I heard from someone that it's fairly easy to clean them. and most likely the coils of the metering unit itself are fine, just the needle is stuck. But I never got around to taking the Rochester block apart, even though I have an old upright windshield and a hatch.

[MikeOH]

Personally, I have very low requirements out of my ammeter.

Just looking at the thing is enough to realize accuracy was WAY down the design importance list, and cost was clearly number 1.

I view it as pretty much a 'digital' device to tell me, at a glance, in conjunction with the voltmeter, that my charging system is performing properly.  After startup I first verify that I have around 14V, then a quick glance to see the ammeter is on the charge side.  At runup I look again to see it's at zero.  During flight I periodically scan the ammeter to make sure it is staying at zero.

There's really no need to accurately quantify amps when, if everything is working properly, it should read zero.

[carusoam]

We kinda get three options...

-1) Needle indicating wild discharge...  needle points to the left.  Alternator failure

0) Needle indicating near normal operation... needle near centered.  Nominal

+1) Needle indicating wild charging... needle points to the right.  VR failure

On my IP, I would have to stare at the amp meter to see if it was charging after start-up...

Mirth has made a great point...

To get better amperage information requires something more expensive... with a downloadable memory... like an instrument from EI, or JPI...

For an actual amperage reading without using a shunt... measuring the magnetic field around the main wire would be modern!

Shunts were nice back in the day... they are yesteryear’s technology now... where induction could be used... 

From a safety point of view... Shunts have been responsible for some interesting in flight arc welding experiments around here...

PP thoughts only, not a mechanic or electrician...

Best regards,

-a-

[MikeOH]

While more modern than a shunt, measuring a DC magnetic field requires a more complex solution, for example, a Hall effect sensor since a static (DC) magnetic field will NOT induce a voltage.  Most clip-on ammeters will only respond to AC current flow.  Hall devices are a much more expensive solution than a shunt which, if calibrated carefully, is very accurate, and Hall effect devices require careful circuit design and temperature compensation to achieve high accuracy.  Not really a good engineering solution for the aircraft being discussed here.

PP thoughts only, but I am an EE

[carusoam]

There were two devices brought up recently that would be nice if widely adopted... to keep prices down...

1) Tire pressure monitoring...

2) Decent amperage gauging without the exposed shunt...

I think there would be thousands of plane owners and builders that would want this...

Kind of like those Cies fuel level sensors... Way better than the original equipment that we have, but not really sci-fi either... and really well executed by the Cies guy...

He is an engineer as well... 

Probably worth inviting him to stop by to see the conversation...

Wondering if @Cloudmirth will be stopping by...  see if he is familiar with Cies...

Best regards,

-a-

[PT20J]

I’ve flown planes with both ammeters and load meters and I prefer an ammeter. A load meter tells me what current the alternator is supplying, but I don’t really need to know that. The ammeter tells me what’s going on with my battery which I find more useful. Over time if the charging current progressively increases and takes longer to recover after a start, I know the battery may be getting tired. In flight a slight positive reading tells me all is well. If I see a significant positive reading, I suspect that something has gone wrong inside the battery. If the alternator fails, I see how much current I am drawing and can see the effect of shedding load and can make a rough calculation of the time remaining before the lights go out.

Skip

[MikeOH]

8 minutes ago, PT20J said:

I’ve flown planes with both ammeters and load meters and I prefer an ammeter. A load meter tells me what current the alternator is supplying, but I don’t really need to know that. The ammeter tells me what’s going on with my battery which I find more useful. Over time if the charging current progressively increases and takes longer to recover after a start, I know the battery may be getting tired. In flight a slight positive reading tells me all is well. If I see a significant positive reading, I suspect that something has gone wrong inside the battery. If the alternator fails, I see how much current I am drawing and can see the effect of shedding load and can make a rough calculation of the time remaining before the lights go out.

Skip

^^^^ THIS ^^^^

I just don't see a need for a 'better' ammeter for our needs.  It would just be a gadget.  There is no reason a shunt couldn't be enclosed if it really is a hazard.

TIre pressure monitoring is an interesting idea and likely somewhat more useful.  My reasoned opinion is that a certified system for aircraft would be pretty pricey, however.  Honestly, with leak stop tubes I've only had to fill the tires once a year.  It's pretty easy to look at the sidewalls during preflight and know if there's an issue.  Every couple of months I'll go to the bother of checking pressure with a gauge.

The cost would have to be pretty low before I'd even consider such a system.  I'm not holding my breath.

[David Lloyd]

The ammeter in our Mooneys is pretty simple.  The needle might drag on the faceplate.  More likely, it is electrical, not in in the gauge.  Two fused wires from either side of the shunt connect to the meter.  If either fuse has failed, no indication. They are 1 amp, I believe, pretty fragile. Also, each connected point is an opportunity for a little corrosion over the last 50 years. Any one connection or fuse will cause the ammeter not to work.

[fuellevel]

Interesting   we are being taxed by an OEM for a lot of new things at the moment.  An ammeter with a simpler shunt  or another method