These DRACs have been modified with a 6 pin DIP switch to enable calibration for different tire sizes and gear ratios.  This style works with the '87-92 style ECMs like the 1227747 and is also used in 1993 TBI trucks.  Instructions are included.  They work great with our TBI harnesses that have 700R4 control.  If your setup has the DRAC built into the gauge cluster, this probably isn't a good replacement.  If you need the '94-95 style VSSB instead, let us know.