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AOG-in my hangar!

Plummit

Well Known Member
Yup, pulled the plane out for breakfast run this morning, hit the starter and it did it's usual thing where it couldn't get past the compression stroke of the first cyl. I released the starter then hit it again. Same thing so I released it and hit it one more time - Everything, including the fuel pump shut off. Dead, nada nothing.

Talked to 2 A&P friends and the builder and they all said "Dead Battery".

It just didn't feel like a dead battery to me, but I had kept my old battery and it was on an approved maintainer so I swapped it out. Same thing, nothing. Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero.

Hmmm. good voltage on the battery terminals but nothing when using the airframe. I've got and open ground.

Started tracing the #2 welding cable all the way to the panel and then I saw it: a shunt mounted to the cabin side of the firewall that let the smoke out.

I had never seen a shunt on a negative cable but hey, at least it happened at home! I'll upgrade the 60 to a 100 amp when I can get one next week.

-Marc
 
Yup, pulled the plane out for breakfast run this morning, hit the starter and it did it's usual thing where it couldn't get past the compression stroke of the first cyl. I released the starter then hit it again. Same thing so I released it and hit it one more time - Everything, including the fuel pump shut off. Dead, nada nothing.

Talked to 2 A&P friends and the builder and they all said "Dead Battery".

It just didn't feel like a dead battery to me, but I had kept my old battery and it was on an approved maintainer so I swapped it out. Same thing, nothing. Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero.

Hmmm. good voltage on the battery terminals but nothing when using the airframe. I've got and open ground.

Started tracing the #2 welding cable all the way to the panel and then I saw it: a shunt mounted to the cabin side of the firewall that let the smoke out.

I had never seen a shunt on a negative cable but hey, at least it happened at home! I'll upgrade the 60 to a 100 amp when I can get one next week.

-Marc

If you have the light weight FlyTech starter I suspect the starter solenoid on the starter has failed. You can either buy a new starter for megabucks or replace the existing one with a Ford starter solenoid for around $55. There are several threads on VAF that discuss this issue. It’s not a hard job. Good luck.
 
Sounds like a short somewhere, I think I'd do a bit more investigating before putting in a bigger fuse! :)
 
Should not have any fuse in the starter main cable. The sky Tec can pull 150 amps easy. Ask me how I know
 
A shunt has a certain amount of resistance. Any resistance in the starter circuit is going to slow the starter down.
Remove that shunt.
If you still want to measure current, use a hall effect sensor which is usually placed around a positive wire, not the grounded cable.
 
So the shunt is in-line when you hit the starter?
Very unusual.

The shunt is on the ground wire before the forest of tabs so yes, all of the current for the starter goes through the shunt. It's worked flawlessly for over 1000 hobbs hours. I find it strange that the the shunt is on the ground too. The instructions say to put it on the positive cable.

I'm running a 70 amp PP alternator so a 60 amp shunt seems too small anyway.

-Marc
 
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"It just didn't feel like a dead battery to me, but I had kept my old battery and it was on an approved maintainer so I swapped it out. Same thing, nothing. Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero."

I wonder how is it possible for the VOM to read 13 V+ between the battery positive terminal and the ground on the airframe, when the battery ground connection is broken due to the failed shunt?

There's a couple of options for correctly locating the current sensing shunt, depending on whether the aim is to measure what's going into the battery, or, what is the equipment load (excluding the starter). If the latter then a 60A shunt is still appropriate.
 
I wonder how is it possible for the VOM to read 13 V+ between the battery positive terminal and the ground on the airframe, when the battery ground connection is broken due to the failed shunt?

It was probably back feeding through one of the light or device circuits as they are not LED.

paulvS said:
There's a couple of options for correctly locating the current sensing shunt, depending on whether the aim is to measure what's going into the battery, or, what is the equipment load (excluding the starter). If the latter then a 60A shunt is still appropriate.

Yes I agree. The problem I have now is relocating the shunt to the engine side of the firewall so I can put it on a positive cable after the starter. There are 2 shunts there already.

I have a PP alternator and a B&C back-up alternator. I don't know what the failed shunt was measuring. I have a 900X panel with an informative EIS.

-Marc
 
Yup, pulled the plane out for breakfast run this morning, hit the starter and it did it's usual thing where it couldn't get past the compression stroke of the first cyl. I released the starter then hit it again. Same thing so I released it and hit it one more time - Everything, including the fuel pump shut off. Dead, nada nothing.

Talked to 2 A&P friends and the builder and they all said "Dead Battery".

It just didn't feel like a dead battery to me, but I had kept my old battery and it was on an approved maintainer so I swapped it out. Same thing, nothing. Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero.

Hmmm. good voltage on the battery terminals but nothing when using the airframe. I've got and open ground.

Started tracing the #2 welding cable all the way to the panel and then I saw it: a shunt mounted to the cabin side of the firewall that let the smoke out.

I had never seen a shunt on a negative cable but hey, at least it happened at home! I'll upgrade the 60 to a 100 amp when I can get one next week.

-Marc

This #2 cable was grounded first to the firewall brace on the engine side before it went to the panel... RIGHT?

If the shunt was mounted to the cabin side of the firewall, was the cable also connected (grounded) to the firewall? If it is, then the shunt is not the starting problem unless you don't have a ground cable from the firewall or battery - to the motor. And if that is the case, your starter has been grounding through any metal between the airframe and the motor....not good! I don't think this is just a case of just replacing the shunt, you need to confirm the ground path from the battery to the starter.

Look for ground cables going from the engine to the firewall... There should be at least one..... and if the builder was smart, there will be two.
 
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This #2 cable was grounded first to the firewall brace on the engine side before it went to the panel... RIGHT?

Nope. If the battery was grounded to the airframe first I wouldn't have had a problem -and- I don't believe the shunt would have worked. Actually, I'm not convinced the shunt was being used for measurement purposes anyway. I found the wires were not secured tightly to the shunt!

gasman said:
If the shunt was mounted to the cabin side of the firewall, was the cable also connected (grounded) to the firewall? If it is, then the shunt is not the starting problem unless you don't have a ground cable from the firewall or battery - to the motor.

I'll get back over to the hangar this morning and check it out. I've never considered that I had a "starting" problem just because the engine would come up on compression and the starter wasn't powerful enough to drive the crank through it. The engine has 9/1 pistons, and it's difficult to pull the prop through by hand let alone the little starter.

I will confirm the ground path today. So far I've just been looking for the cause of the electrical failure. Even with this poor excuse for an electrical system, it's worked for 10 years and 1000 hours....

-Marc
 
It just didn't feel like a dead battery to me, but I had kept my old battery and it was on an approved maintainer so I swapped it out. Same thing, nothing. Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero.

It is not uncommon for a bad battery to show normal voltage if no load is applied. But when a large load (like a starter) is applied, the voltage can go down to zero.

I wonder how is it possible for the VOM to read 13 V+ between the battery positive terminal and the ground on the airframe, when the battery ground connection is broken due to the failed shunt?

A VOM only needs microamps to work so he might be getting this ground to the airframe via some other wire (not just the big wire) which would be just enough for the VOM to show voltage. But not being able to see this particular installation it is hard to say.

PLUMMIT: Put your airplane battery in your car and see if the engine turns on. If the car engine turns on, then your battery is good and you have a bad starter. If the car engine doesn't turn on, then your battery is bad.

:cool:
 
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It is not uncommon for a bad battery to show normal voltage if no load is applied. But when a large load (like a starter) is applied, the voltage can go down to zero.

Yup, I'm aware of this but as I stated, the problem was the poor wiring path the builder chose. I will fix it and get it right. I'm not going to replace the fried shunt. I'm going to correct the ground wiring and then I can diagnose any problems I *may* have with the starter or battery. I suspect (hope) that once the wiring is correct everything (the starter and battery) will work even better.

I'm sure glad I didn't wire this; I'd be too embarrassed to post about it!

-Marc
 
Was it a shunt or possibly one of these current limiting fuses? They can look very similar when installed.
 

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So today I joined the shunt lugs together, tested the electrical system and everything worked. It was raining so I didn't check the Skytec 149NL starter. I'll try it tomorrow weather permitting. Fortunately I have another brand new starter if the old one is bad.

-Marc
 
... Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero.

Hmmm. good voltage on the battery terminals but nothing when using the airframe. I've got and open ground.

Started tracing the #2 welding cable all the way to the panel and then I saw it: a shunt mounted to the cabin side of the firewall that let the smoke out.

I had never seen a shunt on a negative cable but hey, at least it happened at home!

-Marc

Are you saying that you initally measured the voltage from the positive terminal of the battery to aircraft structure (ground) on the fuselage, and that voltage dropped from 13V to zero when the master was switched on?

That would seem to indicate either a dead short is being placed across the battery, or else the VOM reference point (the airframe itself) had jumped up to 13V!

There should be a short, direct ground connection between the battery and the airframe back at the battery. Is there not? That connection establishes aircraft structure as ground.
 
Are you saying that you initally measured the voltage from the positive terminal of the battery to aircraft structure (ground) on the fuselage, and that voltage dropped from 13V to zero when the master was switched on?

That would seem to indicate either a dead short is being placed across the battery, or else the VOM reference point (the airframe itself) had jumped up to 13V!

There should be a short, direct ground connection between the battery and the airframe back at the battery. Is there not? That connection establishes aircraft structure as ground.

There is no airframe ground at the battery. The #2 welding cable runs directly to the forest of tabs ground at the cabin side of the firewall. On the engine side there is a smaller forest and a flat braided ground cable that attaches to the accessory case of the engine.

-Marc
 
. . . or else the VOM reference point (the airframe itself) had jumped up to 13V!
Yes, that is what happened. With the master switch open and with the red
voltmeter probe touching the battery positive terminal and the black voltmeter
probe touching the airframe, the voltmeter will display 13 volts. Even though
the shunt (in series with the battery ground cable) was burned open, it
connected the negative battery terminal to the airframe through its high
resistance. When the master switch was closed, it connected battery positive
to the airframe through the coil of the battery contactor. Since the battery
positive terminal and the airframe are now at the same voltage, the voltmeter
indicates zero volts.
 
Remove the shunt

Did that and rechecked the system today. The starter spun the engine effortlessly. Much better than any time in the recent past. All electrical systems worked as they did before. Problem resolved, but I'm still going to chase down the wires that were landed on the shunt. The only thing I didn't check is the Alt out light. It's supposed to glow if the main alt quits alt-ing.

-Marc
 
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Plummit,

In that position in the circuit, the shunt will sense TOTAL current into or out of the battery, which includes starter current. Definitely not a good idea unless it is a 200+ amp shunt. (If I'm interpreting the info provided correctly - as you know, it's tough to do remote diagnosis)

I'm surprised that it has been working for this long. But it certainly explains the weak cranking. And, yup! that's what a way-overloaded shunt looks like. (it could have started a fire). It has been cooking for months/years and finally gave-up-the-ghost.

I have seen similar situations where the starter was downstream of the typical 60 Amp shunt due to a wiring error. In a couple of cases, the aircraft was under construction and we were able to fix the error before the shunt got vaporized.

I will be curious to know where the little wires on the shunt go? Please let us know. I wonder what the electrical system designer was trying to accomplish.

Hopefully, you can just eliminate it!
 
Update

I got into the G-900X manual today and I'll be darned! The install manual shows this pic:

UMA Shunt.jpg

Guess I'll order a new one but I'll use the right amp rating (100 instead of 50)

-Marc
 
Plummit,

Something doesn't seem right. The existing 50 Amp shunt should have never seen starter current and I don't think that replacing it with a 100A is gonna be the right answer.

Is it possible that there is a loose/bad connection somewhere in the big wires in the starter wiring? Maybe in the engine block grounding wire/strap? Might be worth a thorough inspection/cleaning of those terminations.

My theory is that the normal, high-current ground path for the starter is flakey, so when you are cranking, the power is finding another path to ground - possibly thru the 50 amp shunt, which is not good.
 
Plummit,

Something doesn't seem right. The existing 50 Amp shunt should have never seen starter current and I don't think that replacing it with a 100A is gonna be the right answer.

Pretty sure this is a correct statement.
 
Do you have the latest revision of the Garmin manual?
A shunt should NOT be in series with starter current.
Below is a quote from a Garmin manual that I found online.
https://www.manualslib.com/manual/508847/Garmin-G900x.html?page=107
Installing a shunt in the negative battery terminal is only meant for a standby battery, not an engine cranking battery.

5.6.2.6.2 Alternator Ammeter Sensor
Sensor Description:1C4: 100 Amp / 50 mv shunt
General Installation Guidance:
Given most alternators are case grounded, the primary alternator ammeter shunt should be installed inline in the alternator output ("B" terminal). Depending on the location of the alternator relative to its supported electrical bus, it is typically desirable to install the shunt on the firewall near where the alternator output would normally penetrate the firewall.
-
5.6.2.6.3 Auxiliary Ammeter Sensor
Sensor Description:1C1: 30 Amp / 50 mv shunt
-
General Installation Guidance:
Standby Battery: The standby battery ammeter shunt should be installed inline between the negative battery terminal and the airframe ground.
-
Standby Alternator: The standby alternator ammeter shunt should be installed inline in the alternator output ("B" terminal).
 
Do you have the latest revision of the Garmin manual?

I have revision F

A shunt should NOT be in series with starter current.
Below is a quote from a Garmin manual that I found online.
https://www.manualslib.com/manual/508847/Garmin-G900x.html?page=107
Installing a shunt in the negative battery terminal is only meant for a standby battery, not an engine cranking battery.

I looked at the page you provided the link to but I didn't see the sentence you quoted. Everything I've seen says the shunt is for MAIN BATTERY current sensing and it is to be installed inline between the battery and ground. If it's on the main battery ground it will be on the cranking battery.

I called Garmin but the tech on duty told me to call back Monday.

-Marc
 
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While you are on the phone with Garmin, ask if the GRT HALL EFFECT SENSOR will work with your Garmin system.
You won't have to worry about exposed shunt terminals shorting out or a shunt burning open or degraded starter cranking.
 
My shunt got fried but was on the Pos not Neg, not sure a battery shunt is allowed on Neg in certified planes. Mine took a hit when I was working on my engine driven feul pump.. the alternator was in the way so I took it out and than the darn positive charging cable kept poking me in the face as I was trying to see where my pump was leaking.. so I did what anyone else would have done shved that dam alternator cable out of the way right to a place I would forget about and grounded it really good against the engine case.. forgot all about it when I was ready to turn the battry on to flip my electric boost pump on to pressure the fuel system and see if the engine fuel pump was still leaking…. BIG BANG when I turned the master on.. took out both 70amp alternator breakers and the next component inline the battery shunt melted.. took a few days of troubleshooting to figure that out.. the low power items would work but the high items dead.. and the battry ammeter was slammed to the discharge side.. will let that sit for a couple days and see if some sharp person here can explain why those things were happening with the shunt burned apart.
 
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