Herm wrote:I assume is there for safety in the Leaf, what does it actually do?
The car signals to the EVSE to turn on power by placing a specified resistive load on the +12v/-12v pilot signal. It is theoretically possible to get the same load from dropping the handle into a puddle of dirty water. The diode allows the load to only affect the positive side of the signal, and the EVSE is supposed to check for this to be sure that it's really a car that's loading the signal before turning on the high voltage. Some EVSE manufacturers figure that their GFI circuitry is sufficient protection in this admittedly rare happenstance, and don't bother to implement the diode check.
Not having a functional diode in the car doesn't create a safety hazard, but does create the described scenario where the car doesn't work with some EVSEs.