TRANS.FAILSAFE-can be driven in this mode for how long?

[g300d]

I've had a power steering fluid-covered alternator on the 3.0 with the GM trans for a long time and it never caused a transmission failsafe situation (now that I've said that I've probably jinxed it).

I experienced steadily worsening trans failsafe for a time on the 4.6 with the ZF. It was the transmission selector switch on the side of the transmission, losing contact inside, confirmed by pulling the switch, actuating it manually, and checking for continuity at the pins. Opening the switch, cleaning and bending the spring contacts for a better connection, and resealing the switch resolved it in my case.

Credit to Edwin (IIRC) of the E38 forums for his writeup on this which IIRC included the testing procedure, pinouts, and lots of pics.