That diode is in the memory protect/battery circuit. It does one of two things (I don't have schematics handy....and I can't remember which)......it blocks the batteries from getting a charge from the 5VDC line.....or it prevents the batteries from trying to power the entire 5VDC line, "routing" the battery voltage to only power the CMOS RAM when power is off.
Your thrust issue is, more than likely, on the small interface board.
Edward