G3X question

[Glen Davis]

Glass is new to me. I’m an old steam gauge guy (47 years). On the G3X in my 1989 J, volts show correctly at 28 but amps show minus 7 with a full electrical load.  Is that normal? 

Edited August 22, 2023 by Glen Davis
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[PT20J]

1 hour ago, Glen Davis said:

Glass is new to me. I’m an old steam gauge guy (47 years). On the G3X in my 1989 J, volts show correctly at 28 but amps show minus 7 with a full electrical load.  Is that normal? 

Since you are reading 28V, I presume the engine is running and turning fast enough that the alternator is supplying current to the electrical system. Therefore, the current cannot be negative and the sense leads on the shunt are likely reversed. Whether or not 7 amps is reasonable depends on whether this reading is taken shortly after a start when the alternator is recharging the battery, or during cruise when the battery has been full recharged. In the former case, it is reasonable. In the latter case, you should be seeing very close to zero (zero with a fairly new battery) and so the shunt calibration was probably not entered correctly. If this is a new installation, I would take it back to the dealer and have them fix it.

[Ragsf15e]

2 hours ago, Glen Davis said:

Glass is new to me. I’m an old steam gauge guy (47 years). On the G3X in my 1989 J, volts show correctly at 28 but amps show minus 7 with a full electrical load.  Is that normal? 

Just to add something to slightly simplify the good details @PT20J gave you, the amps (when wired correctly) are going to show only the amps passing to/from the battery.  So if the alternator is working and the battery is charging, you should see a low number of positive amps that are charging the battery. If your alternator isn’t working (engine not running, alternator broken, etc) you will see negative amps which are coming from your battery to power your electrical system.  This negative number is dependent on what you’re trying to power.

[Pinecone]

It depends on whether it is wired as an typical ammeter or as a load meter.

A load meter shows the total current draw of the system.  An ammeter typically shows the current to/from the battery.

[PT20J]

27 minutes ago, Pinecone said:

It depends on whether it is wired as an typical ammeter or as a load meter.

A load meter shows the total current draw of the system.  An ammeter typically shows the current to/from the battery.

True, but M20J came with an ammeter, so the G3X should have been connected as an ammeter to match the certified instrument configuration. I suppose it’s possible it was installed as a loadmeter, but that would take a bit if rewiring.

[Dennis Pendergast]

As correctly mentioned above, ammeters in small GA aircraft can come wired from the factory as showing the amount of current flow going into or out of the ship's battery or the current flow coming out of the alternator. If an analog ammeter is wired to the battery this is sometimes indicated by an ammeter with a zero in the middle and a plus sign to the right/ minus sign to the left.

When we install Electronics International ammeter shunts in our shop, we ask the customer which way they would like the ammeter to be wired, as the STC permits you to choose. I am not familiar with the G3X system, but perhaps it is similar.

On a well equipped J model, with everything turned on (all lights, radios, transponder, pitot heat... ) you should be showing quite a bit more current flow than 7 amps, regardless of the state of charge of the ship's battery.

Provided the battery is well charged, a simple test would be to leave the engine off and turn on all electrical equipment that can be safely operated on the ground and read the current flow indicated on the ammeter.

[PT20J]

23 minutes ago, Dennis Pendergast said:

I am not familiar with the G3X system, but perhaps it is similar.

The Garmin STC allows use of the originally installed shunt. The installation manual contains the following note, "Shunt installation is outside the scope of this STC and must be approved using other means."

[Pinecone]

6 hours ago, PT20J said:

True, but M20J came with an ammeter, so the G3X should have been connected as an ammeter to match the certified instrument configuration. I suppose it’s possible it was installed as a loadmeter, but that would take a bit if rewiring.

Understood.  Was just pointing out the difference.

[81X]

On 8/21/2023 at 8:13 PM, Glen Davis said:

Glass is new to me. I’m an old steam gauge guy (47 years). On the G3X in my 1989 J, volts show correctly at 28 but amps show minus 7 with a full electrical load.  Is that normal? 

the picture won’t show, but If you are at idle, this is probably “normal” because let’s say your alternator produces 20 amps at idle and a full load is 27 amps.  
 

if this is at cruise RPM (or at least say 2000 RPM) this is not normal, you’re draining using all of your alternator output and are drawing 7 amps from your battery, and thus not charging or maintaining your battery.   

your alternator should be able to produce 50-60 amps above 2000rpm or so  if you’re -7 there, something is wrong.  
 

This commentary above is predicated on it being configured as an ammeter to/from the battery, not a load meter of total electrical load.   

 

[Dennis Pendergast]

On 8/22/2023 at 11:31 AM, PT20J said:

The Garmin STC allows use of the originally installed shunt. The installation manual contains the following note, "Shunt installation is outside the scope of this STC and must be approved using other means."

Thanks for the info.