My garage is not connected to my house and it's on a circuit that contains a few outlets inside the house as well. I found out the first time I tried to charge my PHEV that the entire circuit isn't properly grounded. I had an electrician come look at it, and found some old wiring that he replaced and it worked! Until it didn't... All of a sudden it wasn't grounded anymore.
The electrician came back over and when he went to test it, it was grounded properly?! Turns out this was just after a rainstorm. Now I've found this to be true that every time it rains, the garage circuit is grounded properly and when it's dry, it is un-grounded again.
Now the electrician want to tear up the underground conduit connecting the garage to the house, but I'm not sure why that would fix it? If anything I would think that it would be un-grounded when it rains, or trip the breaker every time it rains?
Also, there are no outside outlets and the underground wiring is the old metal sheathing style that runs through a 1.5" dia pipe into the house. He has already tried pulling the wire through one end with a stringer but apparently it didn't work.