• If You Are Having Trouble Logging In with Your Old Username and Password, Please use this Forgot Your Password link to get re-established.
  • Hey! Be sure to login or register!

Volts constant but amps will fall off and run negative

I will. Should have the alternator removed tomorrow and will start tracing wires for shorts, connections, etc.

Thanks for the help everyone.
 
What happens when a starter is fully engaged and being spun by the engine. I remember this causing something similar.
 
Could you please explain this? I was thinking that the starter motor would be acting as a too-fast generator, creating a reverse EMF on the system.

Actually there are two scenarios of this. If the start relay stays engaged, the starter just keeps spinning. When the engine starts, it tries to spin even faster. It has no magnets or the like to generate a voltage and current. But it is extremely low resistance so the high current flow just keeps flowing. It can cause a fire in some cases. The other scenario is when the start relay opens, stopping the current flow, but the drive stays engaged with the flywheel or drive gears. Not as prone to starting a fire but can still 'disassemble' the starter.

Starter/generators used on items like turbine engines use a switching relay when changing from the start mode to the generate mode.

Web
 
Oh. And even if it did generate power, it wouldn't be able to force a current back through the shunt and into the alternator to cause a reverse current reading.

Web
 
It doesn’t sound like the shunt is the issue, but I’ll share something to be aware of.

If a builder mis-wires the shunt, and attaches the sense wires to the buss(es), the display you are using to indicate both voltage and amps can interpolate that value in strange ways.

IMG_7991.JPG

IMG_8010.JPG

IMG_8016.JPG

IMG_8026.JPG

Rest assured, I corrected the issue.

IMG_8039.JPG

IMG_8044.JPG


Transmitted from my FlightPhone on fingers...
 

Attachments

  • IMG_7991.JPG
    IMG_7991.JPG
    141.9 KB · Views: 203
  • IMG_8010.JPG
    IMG_8010.JPG
    303.2 KB · Views: 243
  • IMG_8016.JPG
    IMG_8016.JPG
    120.7 KB · Views: 201
  • IMG_8026.JPG
    IMG_8026.JPG
    141.4 KB · Views: 182
  • IMG_8039.JPG
    IMG_8039.JPG
    179.1 KB · Views: 186
  • IMG_8044.JPG
    IMG_8044.JPG
    146 KB · Views: 193
Update: had the alternator tested by AutoZone. They used testing parameters from a similar alternator (as they do not sell this particular alternator): 12V 40A. The alternator passed, tested 3 times.

Talked with Dynon, shunt is not current flow directional, can be installed either way.

Started up the plane before all this just to check; when either alternator breaker is pulled (field and main) the volts drop to the battery voltage (~13V) and amps go to zero. Re-engage breaker and volts go to ~14.5V and ~7amps.

Now tracing wires.
 
Yes, the shunt is bi directional. Works well connected either way. It's just a resistor with a very small but very precise value. As current flow across a resistance creates a specific voltage drop, this voltage drop is used by the ammeter to show the amount of current flow on the output circuit. Volts = Amps x Resistance (in ohms). Resistance is fixed so any change in current flow will result in a change in voltage drop across the shunt resistance.

But, again, the diodes in the alternator rectifier (output) only allow current to flow OUT of the alternator, never IN. And the shunt is connected in series with the output (meaning there is only a single path of current flow through the shunt and alternator output. Therefore, there should never be a reverse current flow on the alternator output circuit.

I'm a little suspicious of the testing done. I'd have left it at max current out for a good 20 to 30 minutes. If a diode is breaking down it will need to get hot first. Let us know if you find any shakey wiring on the alternator output.

Web
 
Agree with your comments on the testing, but the store was busy and the guy was kind enough to test it even though they did not have testing parameters for this particular alternator. I also tried Napa but they wouldn't even try to test it. Since the alt is still out of the plane, I will try a couple of other places.

I also spoke with a Dynon Tech Rep, and he was focused in on the two small guage wires that are across the shunt to measure volts and amps. Dynon recommends to fuse the two leads either with a 1 amp fuse or by butt soldering a 26 gauge wires as a fusable link. I choose to butt solder and he suggested that might be a likely area since the fault is intermittent.
 
Those small wires are always fused in accordance with all the installation diagrams I've used on the certified stuff. Fusable links leave exposed wire ends to cause further trouble. Remember that the shunt is connected to the alternator output at one end and the bus (through a large breaker) at the other end. If the 'link' burns through, do you really want the bare ends of the wires under your panel? A fuse keeps the wires covered even if the fuse blows.

The only way the small wires could cause a negative reading is if they were connected in reverse. Not really likely as the same wires are used to sample voltage, which you said was stable.

Web
 
aeroaddict
Sounds like you wired this system yourself. Is there ANYTHING connected to the shunt or alternator output besides the gauge wires?

Web
 
Yikes, I'll replace with fuses! Yes I did wire the plane, according to diagrams supplied by CC and adjustments made for a Dynon panel, they use Garmin.

I'll try to explain the wire path from the alternator:
From the B pole: to OV relay to 50amp breaker to shunt. From shunt to aircraft load.
From alt field connection: to OV relay to 5 amp breaker to master switch.

And as someone mentioned about the breakers before, the 5 amp is the typical button type and the 50 amp is a switch.
 
There is a Over Voltage relay that is between the alternator and 50 amp breaker (alternator has a built in voltage regulator). It "looks" just like a regular automotice relay that you can buy if wiring extra acsessories (lights, USB chargers).

Update: just had another auto parts store test the alternator. Again, they had to use a similar alternator so they could tell the testing machine what it is. Again, it passed. These are quick tests. BUT the operator noticed a sound and said the bearing was failing! They do not sell these alternators, so nothing for them to gain.

Maybe my plane has a virus?

Anyhow, as the saga continues, Cub Crafters is sending a replacement/new alternator!

I will be changing the fusible links to mini blade fuses.

I REALY appreciate all the help. I will install the new alternator, replace the fusible links and go flying. I will report back.
 
Could you have wire rubbing somewhere on buss and it actually is drawing 50+ amps? A control cable?


Sent from my iPhone using SuperCub.Org
 
Thanks for the idea, but unlikely as most of the wiring is not close to control cables. I will spend the next couple of days going thru the plane.
 
Quick update: recieved the new alternator this afternoon, will install tomorrow morning and the weather looks to be perfect for flying.

I have not replaced the fusible links as I wanted to keep the variables to a singular. So the only change will be the alternator.

Will keep everyone posted.

Dan
 
I havent read every post in the thred so this might have been said already, I just spent a month troubleshooting a what I thought was a ground somewhere causing my high draw items to not work and my ammeter to swing full negative. Voltage was constant and good, but as soon as the battery switch was turned on the ammeter pegged to the negative wall... I am no wireweenie so I had to look over diagrams and learn about my system, I was stubborn and didnt want to give in and pay someone to fix it.

Eventually I figured it out, there was no ground and no negative draw causing the ammeter to indicate a negative draw. The electric wizzes here will know right away what can cause this, I did get the tip from an avionics tech to look at what wound up being the problem, and replaced it.

It was simple and no moving parts... now I understand my electical system better, and that an ammeter signal will show a huge negative draw if your shunt is open.

I would look to see if the large battery cable size gage wires from the battery to the ammeter shunt are not tight and causing more than calibrated voltage to run through the ammeter tricking it to thinking it is drawing more than it really is.
 
Last edited:
Thanks for the heads up. In a call to Dynon, they did mention that the shunt could have a crack causing the amp fluctuation. This is on my check list if the alternator doesn't fix the problem.

I did fly for a hour today and no issues. But time will tell.
 
How does one test the shunt to see if it is cracked or if the cable is not well connected?

Look at it.. this one took a hard ground and was sending all battery current to the main bus through the ammeter and back to the other side of the shunt than to the bus.



https://mooneyspace.com/topic/17852-burnt-out-60a-ammeter-shunt-causes


image.jpeg.f39af2089dc6570a32178f25f5b6d579.jpeg
 
How does one test the shunt to see if it is cracked or if the cable is not well connected?

I’ve heard if you tap them with something metallic and they don’t “ring” then they’re cracked. I give mine a gentle tap every time I’m under the panel and have always heard a ring. Mine is working fine, but I’m not the smartest electrical guy. That test may not be worth doing.
 
I’ve heard if you tap them with something metallic and they don’t “ring” then they’re cracked. I give mine a gentle tap every time I’m under the panel and have always heard a ring. Mine is working fine, but I’m not the smartest electrical guy. That test may not be worth doing.

Never heard that one. It's usually visual or olfactory. Look it over on installation and it should be good for life. If it does crack at a later date, you'll smell it.

Web
 
Never heard that one. It's usually visual or olfactory. Look it over on installation and it should be good for life. If it does crack at a later date, you'll smell it.

Web

Awesome, thanks for the feedback. That will save me some wasted time, haha!
 
Back
Top