[JohnnieB]

The DCS controller contains about 1.2 Farads of filter capacitors (see attached image), which have a bleeder network attached for safety reasons and if the controller has been disconnected from the battery for more than a minute or so, that huge amount of capacitance will be discharged and whatever is the last cable connected will spark considerably, if the diode/resistor assembly is installed.

The capacitors are also charged via Pin-10, so a significant spark can occur when certain of the control wires are connected.