Old school ammeter not working. I'd like to service it myself if that's reasonable?

[Mooneydreamr]

Here's a picture of it. Anyone have advice on how to fix it? Maybe a schematic or manual exert that a novice could understand? Thanks in advance for your inputs MS. By the way, if this is not something a novice should tackle please tell me that too.

 

[PT20J]

Looks like a load meter. Ammeters have zero in the middle and read amps into (+) or out of (-) the battery. Load meters read total current draw from the electrical load. 

Either way, the meter actually reads the voltage drop across a shunt (the difference is where the shunt is placed in the electrical system). So, if the electrical system is working properly,  it’s most likely either a bad meter or a broken wire between the meter and the shunt. 

[MikeOH]

@PT20J

That meter looks mechanically broken to me... not just broken wire. If a load meter it should read 0 with a broken wire; it looks mechanically pegged well below that.  Personally, I think the meter itself is a goner. Maybe some specialist instrument repair shop could fix...but definitely not a DYI project IMHO.

[Utah20Gflyer]

Probably a professional job to repair the actual gauge but I would check the electrical connections and just make sure there isn’t a problem other than the gauge before sending it out. FYI if the gauges are all together in one unit they ALL have to be overhauled at the same time.  I understand they won’t do them piece meal.  
 

It may be a good time to consider a primary engine monitor.   If you are going to spend a bunch of money I would want something with new senders, wiring and display.  
 

I checked the required equipment list for my 1968 Mooney and the ammeter was not on the list.  I think you may still be legal to fly without it functional but check your POH and the applicable FARs.  
 

Personally I would want a voltmeter rather than ammeter because I think it’s better at conveying the condition of your battery and alternator.  If you have a cigarette lighter socket you can get plug adapters that display voltage until you get it fixed permanently. 

[Kelpro999]

1 hour ago, Mooneydreamr said:

Here's a picture of it. Anyone have advice on how to fix it? Maybe a schematic or manual exert that a novice could understand? Thanks in advance for your inputs MS. By the way, if this is not something a novice should tackle please tell me that too.

 

Looks like you have master “on”. If so, does it return to “0” with master “off”? 

[Andy95W]

2 easy things to check first- a broken wire at the shunt or burned out fuse(s) at the shunt.  The shunt is on the cabin side of the firewall in front of the co-pilot seat.  It is near the top and toward the outboard of the firewall.

[MikeOH]

33 minutes ago, Andy95W said:

2 easy things to check first- a broken wire at the shunt or burned out fuse(s) at the shunt.  The shunt is on the cabin side of the firewall in front of the co-pilot seat.  It is near the top and toward the outboard of the firewall.

@Andy95W

Please explain how a blown fuse or a broken wire (no signal to the meter) results in the meter pegged below zero.  Depending upon how things are wired, I can see that with the master switch on reverse current might flow through the shunt and peg the meter like the photo. But, broken connection?  It doesn't make sense. I'm open to being educated, however

[PeteMc]

I was just going to say what @MikeOH said.  Unless it is normal for the needle to drop all the way down on the side when everything is turned off, it looks like a mechanical issue.  There may be some shops that *could* fix it, but parts are becoming a big issue.  

[MikeOH]

@Mooneydreamr

Has it always been broken like this, or did this just happen?  If it has always been like this, it is possible the meter got wired to the shunt backwards at some point in the past.

Either way, with the master off it should read zero.  If not, I'm maintaining my "it's mechanically broken' diagnosis

[MikeOH]

3 hours ago, PT20J said:

Either way, the meter actually reads the voltage drop across a shunt ...

@PT20J

I'm going to be a pedant and point out that these old school mechanical meters ALL read current, NOT voltage (whether they are labeled Fuel, Amps, Volts, Oil Pressure....). Their motion is driven by CURRENT flowing through a coil of wire creating a magnetic, not electric, field.

The shunt really lives up to its name as it diverts (shunts) most of the current through itself while a small, proportional, fraction goes to the meter coil.

[Andy95W]

45 minutes ago, MikeOH said:

@Andy95W

Please explain how a blown fuse or a broken wire (no signal to the meter) results in the meter pegged below zero.  Depending upon how things are wired, I can see that with the master switch on reverse current might flow through the shunt and peg the meter like the photo. But, broken connection?  It doesn't make sense. I'm open to being educated, however

Nope, can’t explain it.  It’s all black magic to me.  Maybe something about where the voltage is measured across the shunt versus to a ground through the ammeter itself. (?)

I still maintain that it is the easiest thing to check, and better than removing the cluster, sending it off, paying $1000 for overhaul, and having it still not work.

The wires that attach to the shunt are flimsy and one of mine broke.  I don’t remember how the ammeter reacted, but it was very easy to find just by taking a look at the shunt area.  The fuses tend to last forever, but might as well be checked since they’re right there at the shunt also.

Edited October 20, 2023 by Andy95W

[PT20J]

2 minutes ago, MikeOH said:

@PT20J

I'm going to be a pedant and point out that these old school mechanical meters ALL read current, NOT voltage (whether they are labeled Fuel, Amps, Volts, Oil Pressure....). Their motion is driven by CURRENT flowing through a coil of wire creating a magnetic, not electric, field.

The shunt really lives up to its name as it diverts (shunts) most of the current through itself while a small, proportional, fraction goes to the meter coil.

Current measurement shunts of the type used in this instance are rated in millivolts/amp. The current through the meter is so small that it doesn’t affect the measurement appreciably, so the macro effect is one if measuring voltage. But you are of course correct that current through the meter coil creates the magnetic field which creates the force on the armature to rotate the needle. 

But this esoterica probably doesn’t help the OP much 

[Echo]

Just replace the whole gauge pack.  There are several for sale on Ebay and one in the classifieds here on MS.  They are reasonably priced.  ESPECIALLY in comparison to a primary replacement that is suggested by MANY here on MS.  CB card in tact.

[MikeOH]

^^^^^^ THIS. ^^^^^^

[MikeOH]

11 hours ago, Andy95W said:

I still maintain that it is the easiest thing to check, and better than removing the cluster, sending it off, paying $1000 for overhaul, and having it still not work.

No argument there!

[vik]

Looking at the other gauges I think the picture is taken with the master switch on, means they are powered. First thing to check if the wires are reversed at the shunt. This might be the cause why the gauge shows below zero. Chances that this is the cause are small, but the check is easy. I would start here.

It is definitely possible to fix the gauge if it is broken mechanically, but one has to remove it from the cluster first and look inside to tell what needs to be done. They are simple, but delicate. Sometimes cleaning with alcohol and adjusting the limiters is all that is needed to prevent pegging. The limiters get dirty with age and the needle tends to stick to them.

I see people started to talk about currents and voltages, electric and magnetic fields etc. I can talk about Maxwell's equations all day long, but this is not the best place. Mooneydreamr asked how to fix a simple broken gauge, not why (nabla)xB ~ j + epsilon0dE/dt  ...

[Mooneydreamr]

Thanks for the input guys. For more background, the ammeter never worked in the 3 yrs I've owned her. It just stayed stuck on 20 amps and never moved no matter how much load I put on it. So, at last annual I had the AP look at it. He repaired it and it worked for for a few months, then this. By the way, in the pic the master is on. Also by the way, I was a finance major, so only the most basic technical talk is comprehended by yours truly. I thank you all.

[cliffy]

Granted that it "looks" to be a broken gauge BUT its proper trouble shooting to check all the connections and fuses BEFORE firing

the parts cannon.

You have two wires coming down from the shunt  each has a fuse in line. 

Connections can corrode even when tightly connected for 60 years making the connection bad

Fuses are connected to the wiring by springs or slide connectors, they too can corrode

The standard rules apply - Check and clean all connections (even if it means disconnecting to do so) and then

check the end point for power (amps or volts) 

Most( even cheap) volt/ohm meters have a current check up to about 10 amps built in.

This is where your A&P comes in to check the connection at the back of the instrument to see if the millivolt signal is even getting

to the instrument. If it is then its a gauge exchange or repair job.

Mooney used two types of clusters, older planes had an all in one 6 pack that has to be serviced as a unit (6 gauges)

Newer Mooneys had individual gauges that can be serviced separate. 

If to fix yours takes all 6 then look to the STC'd after market  gauges or just pick up a cluster from a junk yard dealer

Now to be technical- this is not a DIY job for the owner legally. 

Nor is going inside the gauge unless its in a certified instrument repair shop

Not passing judgement, just saying what the rules say.

Also if its not required (?) then it should be placarded as 'INOP" per FARs

(Why was it in there from certification if it was not required?)

I too like a volt meter better. 

[MarkW]

I hope you can get it fixed, because I too like to tinker and learn. 
 

Only if you're interested in doing otherwise, I just this past month pulled a Garwin into like that out of my 1962 C as part of a panel upgrade. Everything working when it came out. Send me a DM if you're interested...?