Burnt out 60A ammeter shunt - ? causes

justincarter · February 22, 2016

I tried to collect my '67F from bare metal respray yesterday. I flew the aircraft into the paint shop a couple of months ago in working order. After I dropped it off , they have had it running to taxi it. Obviously, during the respray the tail has been off, nav lights, cowls, everything- and then all put back together. It has been stripped and water sprayed and then repainted- the works. Its had an engineering sign off for all the flight control work.

The paintshop folks handed over the aircraft keys, with a few items left to tidy up and commented that the battery is flat and needs charging. I thought that a bit odd after those weeks on the ground (unless something had been left on). I set about doing the most major preflight check I've ever done with the aircraft tucked away in the back of a hangar.

When checking the electrics, Sat in the P1 seat, upon switching the master on, I could hear the main solenoid in the rear click but I had no power to any nav lights, radios, panel items -nothing. I didn't try to turn the engine (a/c in the back of hangar).

I got a handheld voltmeter and checked..

across the battery -14v or therabouts

across master solenoid - 14v or therabouts

on the firewall at the starter solenoid - 14v or therabouts

I followed the high current cable from there, along the front of the firewall, through the R side of the firewall into the avionics bay, and then across the ammeter shunt to find a huge volt drop across it and on inspection the shunt is burnt out. You can see the shunt in the attached wiring diagram. Its a big baby. I guess it must have taken quite a load to burn that out. It'll need a new one like this...

http://www.sigmatek.com/pages/prod_description/PD_ACC_shunt.html

So my Qs...

Any thoughts on what might have caused it?

Ill be needing to get a mechanic to look at it but any preliminary thoughts on how to trouble shoot from here?

The output from this shunt feeds i) the main bus bars in the panel and ii) the Volt regulator and generator/alternator. Checking i) I suppose is easy enough - pull all the panel breakers, re-make the burnt connection then engage all the beakers one by one until one goes pop. But my amateur money is more on ii)- a Volt regulator / alternator issue.

Of course I am interested in how this happened. I suppose it wasn't like this when I flew it in there -but my main aim is to safely troubleshoot, fix and get it out of there!!

Any thoughts or tips /advice would help!

Mooney M20 Series Maintenance Manual M20F WIRING DIAGRAM.pdf

Edited February 22, 2016 by justincarter
Added photo

N601RX · February 22, 2016

It took a serious sustained short to ground to do that. The short would have had to be before any of the breakers or they would have tripped. My guess would be the large wire (-006 on diagram) that leaves the shunt and goes to the panel is rubbed and touching or there is a mechanic somewhere who has a melted screwdriver in his toolbox.

You will also likely find the 2 small inline fuses that go to the amp meter blown.

wpbarnar · February 22, 2016

Confirm the short to ground no longer exists before installing a new shunt and powering up. You will damage the new shunt if the ground is still present. Examine the buss bar(s) closely. You may find the tell tale signs of a short to ground in this area.

Bill

chrisk · February 22, 2016

Great picture!

As for debug, I would probably start with an ohm meter. Recall basic electronics. V=IR Resistance lower than 0.2 ohms would indicate a short sufficient to blow the 60 amp shunt. If you are lucky, you will get low ohms and find the short. If not, you may have to physically examine the wiring as other have said. I would expect you will find visual evidence on the wire, since it probably made a good sized spark if there was a power to ground contact.

Also, this reminds me of some recent electrical debug on my plane. The hot prop quit working. I got the manual and it indicated a complicated debug procedure. --a very thorough procedure that involved pulling the circuit breaker panel early in the process. I checked the switch, skipped a check of the circuit breakers, and went right to the slip rings. --That's odd, 12 volts on all 3 contacts. The ground wire had a broken connector at the frame connection. My mechanic was able to fix it in 10 minutes. --My point is: The manual gives a great solution. A step by step solution. A solution that will always give the right answer. However divide and conquer may save you quite a bit of time in getting to the bottom of the problem. Especially if you can avoid taking stuff apart.

N33GG · February 22, 2016

You might want to keep that fried shunt and frame it. I suspect it saved your aircraft from becoming M20Firestorm.

**carusoam** · February 22, 2016

Oddly enough...

What is the lock nut right near the shunt's melted part?

the shunt is connected to the battery at one end and is connected to the airframe through it's mounting hardware.

did something bridge that gap? (Tool, foreign object, or ...?)

where did the molten metal go? It may have left burn marks on the rug or on whatever (whoever) it fell on.

That was a serious short that probably melted whatever else was in the path...

If the path was the lock nut there isn't much else that was fried.

If the path was further away like the bus bar, more wires may have been affected.

The wiring diagram in the POH shows all the wires including the shunt.

Are the protective silicone boots pulled back by the photographer, or were they pulled back by someone else?

Best regards,

-a-

orionflt · February 22, 2016

57 minutes ago, carusoam said:

Oddly enough...

What is the lock nut right near the shunt's melted part?

the shunt is connected to the battery at one end and is connected to the airframe through it's mounting hardware.

did something bridge that gap? (Tool, foreign object, or ...?)

where did the molten metal go? It may have left burn marks on the rug or on whatever (whoever) it fell on.

That was a serious short that probably melted whatever else was in the path...

If the path was the lock nut there isn't much else that was fried.

If the path was further away like the bus bar, more wires may have been affected.

The wiring diagram in the POH shows all the wires including the shunt.

Are the protective silicone boots pulled back by the photographer, or were they pulled back by someone else?

Best regards,

-a-

that lock nut shouldn't even be on that side, it is way too close to the shunt. it should be the bolt head like the other side.

Brian

**takair** · February 22, 2016

Anthony is right, that nut sure looks close. Any chance they turned the bolt from the firewall side and caused contact?

justincarter · February 22, 2016

Its a fair point about the fixing to the firewall. On Installation, one nut and bolt goes one way (forwards through the firewall) and the other goes the other way. It seems unlikely they moved any of the fixings to cause a shunt but can't rule anything out. That said, the picture 'flattens the appearance' a bit, i.e. the nut is a decent distance from the shunt in the plane into the picture ( whilst appearing to be practically touching the shunt in the plane across the picture. I pulled the boots back for the picture.

Well here's what we did so far...

Looked for obvious burns to the fabric in the local area / scorched cables- couldnt see any.

Pulled all the breakers on the panel.

Used a 60amp inline ammeter across the poles of the shunt.

One by one, I closed the breakers while observing the current flow - ready to cut the power immediately there was a big draw.

With each panel item, I saw an appropriate current draw, so I'd pull the breaker and move onto the next.

Fuel pump, beacon, strobes etc.

Then got to the nav lights. Remember, in this 67F, the breakers on the Left bus are the throw switch type (whatever the posh name for that is).

As soon as the nav lights were on -immediate 30+amp draw. I don't know whether, had if I left it on, if the breaker would have 'popped' or not as I switched the nav lights off immediately.

So there seemed to be a short on that circuit. We then removed and inspected the three nav lights (one on each wing and third on the tail) and found that the high draw was associated with the tail light and were suspicious that on re-installation of the tail/ rudder assembly, the wires were reversed in polarity. Once these were swapped, all three nav lights worked with a 2A draw, as did all the other electrics.

What I don't get is, that for this tail light polarity switch to have fried that shunt, the Nav light Breaker must not have opened in overload. i.e. it would have to be suspect.

So another explanation is that something else fried the shunt during the works, and We just happen to have found another error coincidentally, without the fault we found having caused the shunt to fry. Its hard to imagine the wiring/bus bar/ breaker for the Nav light setup begin able to carry enough current to fry the shunt without being fried themselves.

Does anyone know where to source the modern day equivalent of a garwin 22-370-60 shunt (which is whats listed on the wiring diagram for the aircraft). I saw some RC Allen shunts which look similar on the A/C Spruce website like this..

http://www.aircraftspruce.com/catalog/inpages/rcallenshunts.php?clickkey=14951

but oddly they're all out of stock. So not sure what to go for next. And Ill need to have is shipped to the UK

and we might still not have found the actual cause of the problem!!

Edited February 22, 2016 by justincarter
grammar, detail

N601RX · February 22, 2016

I sold the one out of my plane last year after installing a primary engine monitor which came with a new one.. Several other members have recently installed these also so they should be some used ones available.

justincarter · February 22, 2016

Fingers crossed that someone will have one they can let go

**KSMooniac** · February 22, 2016

I have one from my 77 J which might be the same. The 77 shared a lot of parts with the E/F models. I'll see if I can find a part number. My old one is on the left. New one is JPI part.

Sent from my VS985 4G using Tapatalk

justincarter · February 22, 2016

i see form the sticker on it that the JPI unit is a 100A shunt.

Can you see from the J wiring diagram if the original is also a 100A unit or 60Amp?

I think (looking at the F wiring diagram- link above) that the one I have should be a 60A unit

I don't know if it would make much difference. Other than perhaps providing a little less protection in overload or ?? making the (crappy) ammeter over or under read a bit

**KSMooniac** · February 22, 2016

I'm sure you need to match the voltage rating to your instrument, otherwise you won't get a proper indication. I *think* my old one is 60A, but I don't know for sure. My schematic labels the shunt "21DA01A" for my '77 J, and I believe that is a Mooney part number so it doesn't have any meaning in terms of finding an equivalent off the shelf. I see your F schematic has a commercial part number...useful for shopping but not cross-referencing to mine, unfortunately!

My parts catalog only has a shunt for prop de-ice system (optional) and not the main one. The schematic has the Mooney number, but no other specs/details. I'm not sure where else to look. You might have to call an experienced Mooney shop or email the factory for support.

Worst case, you could install an aftermarket/certified ammeter to replace your factory one. (Or a complete primary engine monitor for that matter)

justincarter · February 22, 2016

these seem to be in stock

http://www.aircraftspruce.com/catalog/inpages/umaAmmetershunt.php?clickkey=14951

Sadly, engine monitor upgrade would probably be a paperwork nightmare in EASA-land.

At the moment, I just want to get flying again!

**KSMooniac** · February 22, 2016

Looks like a good, cheap option. I'd try one of those first if I were in your shoes.

Sent from my VS985 4G using Tapatalk

**N201MKTurbo** · February 22, 2016

Here you go:

http://www.sigmatek.com/pages/prod_description/PD_ACC_shunt.html

**DonMuncy** · February 23, 2016

4 hours ago, justincarter said:

Its a fair point about the fixing to the firewall. On Installation, one nut and bolt goes one way (forwards through the firewall) and the other goes the other way. It seems unlikely they moved any of the fixings to cause a shunt but can't rule anything out. That said, the picture 'flattens the appearance' a bit, i.e. the nut is a decent distance from the shunt in the plane into the picture ( whilst appearing to be practically touching the shunt in the plane across the picture. I pulled the boots back for the picture.

Well here's what we did so far...

Looked for obvious burns to the fabric in the local area / scorched cables- couldnt see any.

Pulled all the breakers on the panel.

Used a 60amp inline ammeter across the poles of the shunt.

One by one, I closed the breakers while observing the current flow - ready to cut the power immediately there was a big draw.

With each panel item, I saw an appropriate current draw, so I'd pull the breaker and move onto the next.

Fuel pump, beacon, strobes etc.

Then got to the nav lights. Remember, in this 67F, the breakers on the Left bus are the throw switch type (whatever the posh name for that is).

As soon as the nav lights were on -immediate 30+amp draw. I don't know whether, had if I left it on, if the breaker would have 'popped' or not as I switched the nav lights off immediately.

So there seemed to be a short on that circuit. We then removed and inspected the three nav lights (one on each wing and third on the tail) and found that the high draw was associated with the tail light and were suspicious that on re-installation of the tail/ rudder assembly, the wires were reversed in polarity. Once these were swapped, all three nav lights worked with a 2A draw, as did all the other electrics.

What I don't get is, that for this tail light polarity switch to have fried that shunt, the Nav light Breaker must not have opened in overload. i.e. it would have to be suspect.

So another explanation is that something else fried the shunt during the works, and We just happen to have found another error coincidentally, without the fault we found having caused the shunt to fry. Its hard to imagine the wiring/bus bar/ breaker for the Nav light setup begin able to carry enough current to fry the shunt without being fried themselves.

Does anyone know where to source the modern day equivalent of a garwin 22-370-60 shunt (which is whats listed on the wiring diagram for the aircraft). I saw some RC Allen shunts which look similar on the A/C Spruce website like this..

http://www.aircraftspruce.com/catalog/inpages/rcallenshunts.php?clickkey=14951

but oddly they're all out of stock. So not sure what to go for next. And Ill need to have is shipped to the UK

and we might still not have found the actual cause of the problem!!

The replacement of the shunt must be exactly the same. The way the ammeter works is to read the voltage drop across the shunt. If it off just a little, the ammeter won't read correctly. If I'm wrong I'm sure someone will correct me.

**N201MKTurbo** · February 23, 2016

As long as it is a 60A 50mv shunt, you should be OK. Anything else will read incorrectly.

**carusoam** · February 23, 2016

Slow down big guy...

I am missing how your light bulbs have polarity and what would make a difference if they were wired backwards?

It sounds to me that You have a short and it is happening before the light bulbs. If it were happening after the bulbs it would not be noticed. The bulb's ground is a 'short' to the plane's air frame.

The tail's light has a long wire that runs the length of the tail cone. It often gets damaged when people open the small round access panel under the horizontal stabilizer. See if you can find if that wire had lost insulation or if it is showing no resistance to ground. This can be done without running any more electricity through the system while looking for a problem.

running power through a known yet unknown short is a bad way to perform problem solving. Don't do that. Something or someone can get hurt.

You may want to review another thread for similar info....

there was also a shunt being sold by someone dismantling a plane. The search box is pretty good for this...

Don't be afraid to contact your mechanic. These guys have the tools and knowledge to identify the short and fix it properly. A plane's painter is often, not a mechanic.

Best regards,

-a-

justincarter · February 23, 2016

The mechanics taking a look today. For interest - Ill let you know what he finds. 100% that the painters aren't very mechanically minded!

Two wires going to the tail light with push to fit unions in the lower right inspection panel appeared to have the live feed direct to earth with a large current draw. When reversed there is a low draw and the lights work. But i see your point that if both cables are live and neutral is just ground, can't see why that would make a difference (bulb doesn't care which way its wired!). Well take a good look for cable damage in the tail incase we've just nudged something live off an earthing point somewhere. Thanks for the other tips!

**larryb** · February 23, 2016

The only wires capable of carrying enough current to burn out the shunt are the thick cables leading to the master relay and starter. The thin wires to the tail light are protected by circuit breakers and would burn out before the shunt anyway. Follow the thick cables.

orionflt · February 23, 2016

I have one that is listed for a 67 C,D, and E, but the F is different

Brian

N33GG · February 23, 2016

Don't worry, if there is still a problem out there, it will let you know...

I would not want to fly that aircraft until I had a definite cause identified. Period.

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Burnt out 60A ammeter shunt - ? causes

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