The point of grounding the transmission and engine to the frame is so the electric will flow through the ground straps and not the transmission itself.
I still would still like to know where this supposed electricity flow through the transmission originates from (I won't argue that a slight potential
you have a short in the transmission's internal harness - but I think it is a very
low potential). You must have a (+) connection for electricity to flow to the (-) connection. Jack is showing that there is no connectivity between the frame, or the driveshaft for that matter, to the transmission.
All (+) electrical supply to the transmission controls has the circuit completed through the harness ground wires to the PCM, not through a transmission case ground (pages 29-1, 2, and 3 of Wiring Diagram Manual).
You can't have it both ways. Electricity is either intended to flow (return) through the transmission or it isn't. Clearly I am in the camp that it isn't intended to flow through the transmission (as are most of the people posting in this thread I assume
With no practical source of a (+) connection, adding a ground strap (frame to the transmission body) will only increase the likelihood of some electrical flow through the transmission ... ESPECIALLY if the vehicle frame to block grounding is in bad shape.
It seems clear to me that this is why
Ford doesn't want a second body unit grounded to the transmission, AND
that is also why they didn't provide a ground between the transmission and frame from the factory.