89 240sx SE automatic transmision question

[89S13 Drifter]

I am tring to trouble shoot my over drive not working and why my car is performing like @$$. Is it controled by a fuse, if so where, like wise for a relay, also if their is a relay, what color so it is easier to find. And I have had mentioned to me that it may be in safe mode. What is safe mode and how do I check to see if it is in safe mode, and if it is in safe mode how do I get it out of it.

[240sxtkog]

the overdrive switch on the shifter can have a bad contact causing it to not shift. and about the safe mode..... my car's maf sensor connector fell off because it's missing the clip. and it would only rev up to around twenty five hundred