Need advise - G3X peculiar ammeter reading

[MikeinPhoenix]

I just finished a new G3X install including a GEA24 and Garmin shunt. Initially, I was getting a negative amperage reading BOTH with the engine off as well as on. I later found that the 2 shunt wires from the GEA24 were backwards on the shunt and so I corrected that. Here’s my question: now the ammeter displays a positive amperage number with the engine off (which makes no sense to me). I’ve looked at the GEA24 config screen and all appears ok per the install manual.

Does anyone have any suggestions what I should check next?

Many thanks!

 

[mburch]

How much current does it display with the engine is not running, and is the value displayed when the engine is running fairly believable? It's not uncommon to have to adjust the calibration slightly to set the zero point.

 

[Walt]

You need to show where the shunt is located in the circuit. I always install in the B lead so it indicates alt output.

 

[MikeinPhoenix]

Matt, with the engine off, the ammeter shows +7 amps (and I'd expect about -7 amps with the G3X power draw). Unfortunately, I couldn't run the engine today, but will run it tomorrow and can circle back.

 

[dmattmul]

Matt, with the engine off, the ammeter shows +7 amps (and I'd expect about -7 amps with the G3X power draw). Unfortunately, I couldn't run the engine today, but will run it tomorrow and can circle back.

Sounds like shunt is not located on the B lead so doubtful engine running vs not running will make a difference. G3X power draw could be about 7 amps so shunt sounds like it's on that bus with it hooked up backwards. Always found it more useful to locate the shunt on the B lead to understand the health of your alternator unless you are using a Monk Works unit which has internal amp output sensing built in. Where exactly is the shunt sensing amperage?

 

[Draker]

My understanding is the ammeter is usually positioned either:

1. Between the Alternator B lead and the Battery contactor
2. Between the Battery contactor and the bus that the fuses or circuit breakers are attached to
3. Between the Battery contactor and the positive battery terminal

When it's at 1, you're measuring the output of the alternator only, so you'd expect it to be zero when the engine is off or the alternator field supply is not present, and positive when the alternator is powering your devices and/or charging the battery.

When it's at 2, you're measuring your electrical system's total load, regardless of whether the battery or alternator is powering it. It will be positive when anything is turned on.

When it's at 3, you're measuring current to or from the battery. It would be positive when the alternator is charging the battery and negative when the alternator is not charging the battery and your devices are discharging the battery. If it's showing positive with the engine off, the shunt may be reversed.

It looks like you have it positioned at #3, and probably just have to swap the measurement leads.

 

[Walt]

My understanding is the ammeter is usually positioned either:

1. Between the Alternator B lead and the Battery contactor
2. Between the Battery contactor and the bus that the fuses or circuit breakers are attached to
3. Between the Battery contactor and the positive battery terminal

When it's at 1, you're measuring the output of the alternator only, so you'd expect it to be zero when the engine is off or the alternator field supply is not present, and positive when the alternator is powering your devices and/or charging the battery.

When it's at 2, you're measuring your electrical system's total load, regardless of whether the battery or alternator is powering it. It will be positive when anything is turned on.

When it's at 3, you're measuring current to or from the battery. It would be positive when the alternator is charging the battery and negative when the alternator is not charging the battery and your devices are discharging the battery. If it's showing positive with the engine off, the shunt may be reversed.

It looks like you have it positioned at #3, and probably just have to swap the measurement leads.

#3 would have starter current going thru the shunt, (100 amp fuse), maybe not a good idea.

 

[Draker]

#3 would have starter current going thru the shunt, (100 amp fuse), maybe not a good idea.

Yea, I chose not to do it that way in my build--I went with two ammeters, in the other two positions I described, so I could measure alternator output and system load separately. But, Garmin's manual (21.3.9.1 Ammeter Shunt Installation) calls out "between the battery positive terminal and the battery contactor" as an option for installing their shunt.

EDIT: Looks like Dynon's manual advises a different way to achieve a battery ammeter: https://vansairforce.net/threads/ammeter-shunt-help-me-connect-the-dots.28092/ which does not get into the path of the starter circuit. Learned something today.