GFCI Receptical Issue

[GWH]

I installed a 120 vac circuit in a battery room about a year back (which included a brand new a GFCI receptical) to supply power to a battery watering system. The system ran w/o issue until just recently when the GFCI started to trip when the pump was turned on. I pulled the pump that was tied to the watering system and checked it for a short or anything that might cause the GFCI to trip. Finding nothing wrong, I then replaced the current GFCI with a new one. I tested the pump and it again tripped the GFCI.

I then plugged the watering system into a non GFCI protected outlet and the unit ran w/o issue. I went back to the GFCI and plugged a drill into the receptical and again no issue. What am I missing?

 

[WorkSafe]

The pump is defective. Maybe the pump windings shorting out.

 

[kwired]

Make sure you do not have a neutral to ground fault.

 

[don_resqcapt19]

...I pulled the pump that was tied to the watering system and checked it for a short or anything that might cause the GFCI to trip. Finding nothing wrong, I then replaced the current GFCI with a new one. I tested the pump and it again tripped the GFCI. ...

How was this testining done and what meter was used?