Page 2-2 of the FlightDEK-D180 Installation Guide says, "Because of the current drawn by the FlightDEK-D180, even very small resistances between battery ground and instrument ground can cause voltage differences which adversely affect
engine sensor readings. An easy way to test the quality of the instrument?s ground is to measure voltage between the ground pin at the FlightDEK-D180 and the ground lead at your aircraft?s battery. With the FlightDEK-D180 powered on, connect one lead of your voltmeter to a free ground lead coming from the FlightDEK-D180. Connect the other lead of your voltmeter to the ground terminal of your battery. The voltage between these two points should measure very close to 0 mV (within 5 mV). If it does not, you must improve the ground connection between the ground of your battery and that of your avionics bus."
In the above test, the D-180 case can be used in leu of a ground lead.
Make sure that the battery cable is well grounded to the airframe at the battery box. Make sure that the control board is well grounded to the airframe with a wire and screw per the plans. Make sure that the Dynon D-180 is well grounded to the airframe. There should not be any paint on aluminum at the ground connections. An additional grounding wire could be added to the D-180 EMS 37-pin connector at location 3 or 5 or 13 if desired.