Constant Amp Discharge with Steady Voltage

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My 172 partner has noticed on the last two flights that the JPI is showing a constant -2/-3 amp discharge with no change when you pull various CB. The voltage is holding rock solid at 28v and the Voltage Light remains off. The battery minder is showing 3/4 of a charge at the end of the flight.

We recently replaced our voltage regulator for the four time in two years and the alternator was replaced two years ago.

Any ideas?
 
Has it always shown this way, or is this a recent change? Do you have the EDM data downloaded showing the change? If you switch the alternator offline, does the voltage start dropping and the discharge increase? It's possible that the ammeter shunt sense leads are installed backwards leading it to sense the current incorrectly, but I wouldn't expect that to just happen one day and it seems doubtful you'd need to disconnect them to access the VR since it's very accessible on the firewall. And what is going on with your charging system that you've needed to replace the VR 4 times in 2 years?
 
The EDM data shows good amps for the first five to ten minutes. However, once the run up happens it falls to -2/-3 the entire flight. This is new development with the engine being reinstalled back on the airframe after being sent back to Lycoming.
 

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Recently worked on a 172 that had a battery drain issue. Ended up that the clock was drawing too much, and it’s on all the time. Disconnected it and the problem went away.
 
I suggest you connect a temporary second ammeter to double check the JPI.
 
I have seen a dirty shunt show a false discharge.
 
Plano Pilot said:
I have seen a dirty shunt show a false discharge.
Click to expand...
I wondered about that, but to get a negative reading, even dirty, you need current flowing in the discharge direction in the shunt. Dirty shunt connections will usually under-read if the leads to the meter are dirty, and overread if the shunt's connections are dirty. In the latter case, some of the shunt current gets redirected into the meter leads, raising the current there and making the higher reading.

This is a tough one. Voltage steady at 28V, which is fine, yet showing enough discharge that voltage should be falling. Reversed meter leads are the most obvious, and yet apparently they haven't been disturbed. And it indicates just fine up until the runup. The fact that this is a fresh rebuild, which means that the engine was off and back on, suggests that some wiring is short or poorly tied up and is being pulled as the engine moves forward on its shockmounts and something is getting shifted.
 
Dan Thomas said:
I wondered about that, but to get a negative reading, even dirty, you need current flowing in the discharge direction in the shunt. Dirty shunt connections will usually under-read if the leads to the meter are dirty, and overread if the shunt's connections are dirty. In the latter case, some of the shunt current gets redirected into the meter leads, raising the current there and making the higher reading.

This is a tough one. Voltage steady at 28V, which is fine, yet showing enough discharge that voltage should be falling. Reversed meter leads are the most obvious, and yet apparently they haven't been disturbed. And it indicates just fine up until the runup. The fact that this is a fresh rebuild, which means that the engine was off and back on, suggests that some wiring is short or poorly tied up and is being pulled as the engine moves forward on its shockmounts and something is getting shifted.
Click to expand...

The shunt on my boat is in a place where it can get stuff on it when I am working on the engine. Most of the time, when dirty, it will show a large draw even the batteries are charging. I have also seen it show a small draw. I spray the shunt with electrical cleaner and blow the stuff out and after a few minutes it starts working correctly. It has happened many times over the years.
 
Plano Pilot said:
The shunt on my boat is in a place where it can get stuff on it when I am working on the engine. Most of the time, when dirty, it will show a large draw even the batteries are charging. I have also seen it show a small draw. I spray the shunt with electrical cleaner and blow the stuff out and after a few minutes it starts working correctly. It has happened many times over the years.
Click to expand...
Strange. I wish I could visualize what's going on there.
 
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