I have the same Problem after changing battery as discussed in this thread:
And I’d like to calibrate the HSCM according to the procedure:
http://static.garmin.com/pumac/G1000:CessnaNavIII_LineMaintenanceManual_CessnaNavIII_.pdf (Ammeter calibration)
The procedure states in step 2 & 3:
2. Disconnect the J2 connector at the J-Box.
3. Using a power supply with an ammeter apply 28.5V to any pin on the J2 connector (to apply power to the current shunt inside J-Box). Reference the power supply to aircraft ground (GN002).
I know J-Box is the electric box on the left side under the cowling but where’s the J2 connector in that box ?
You need input from an avionics shop which does the G1000. There are several on EuroGA; whether they will post is a good question 
