code 51 definition should be gear ratio error in 1st. the first qustion i
have is did you clear the codes from the ecm? the second question is did you
replace the gasket under the low reverse piston housing during your rebuild?
with the snap on scanner connected go to data and watch the input and output
speed sensor readings normally this unit has a high failure rate of these 2
sensors which are accessable from outside the trans.input sensor just above
solenoid block and output on the end of the trans the input speed sensor
should have a reading in park but read 0 when trans is engaged into gear.the
output speed sensor will read 0 till you actually start the car moving..if
either of these sensors is not working correctly the trans wont shift.
nextquestion did you open the valve body up? since this is early model 604
it should have 4 checkballs, but 5 checkball holes. also on the valve body
is an accumulator that was plastic in some units and is known to crack. if
you have plastic piston in accumulator it will directly affect the 1-2
shift...this should be enuff info to make you scratch your head.if you need
more help email me directly
Ken ase certified transmission tech 25 yrs exp.