Hey there -
I've had the same issue with my '91 240 automatic transmission. Sometimes it starts with the arrow on the dash lit up, sometimes not. I seems to spontaneously shift in and out of the 4 gear disengage.

It is particularly annoying when on the highway and it lights up, downshifts into 3rd and stays there...

I pryed out the button from the shift knob and disconnected the two leads that go to the button...no effect.

I looked at the button, and it doesnt seem to have an on or off position (like a click pen might). Looks like all it does is close the circuit between the two leads every time you depress it...so does that mean that you close the circuit, it turns on, you close it again, it turns off?

Anyways, it seems as though its not the button...

Hoping someone out there knows how to disconnect this option b/c I don't mind living without it. Can I remove a fuse? Something easy, anything to keep my tranny shifting into 4th when it gets up to speed.

Thanks,
Marc