the converter unlocks after it has locked up under a couple conditions.
the fuel is let off.this saves fuel as you coast downhill.
you apply the brakes.
right before an od to 3rd downshift.the duration time before it locks up again is controlled via sensors (the tps being a big player,and why it's important to have it set correctly.)
if you had a failed converter,you wouldn't have any gears.
if you had a failed lockup disc in the converter,she wouldn't lockup or if she did would slip.you could get code 62 (62 can be a results of improper lock state due to something else too though.) and you would likely see a lot of deep black metallic shavings in the pan.
keep focusing on the solenoid pack plug,the solenoids themselves,the mlps connector,and the pcm plug.
you must have the correct voltages/resistance values to get 1st gear in the D position,and finally od after 3rd gear shift and lockup.
making sure your not leaving anything out;
why did you replace the tach sensor?
also,this transmission was recently working correct? this isn't a recent replacement that you haven't got working yet?
adding couple snapshots for hardware diagnosing info.
if push comes to shove,you could drop your pan,remove the solenoid pack and take just that to a trans shop to have tested for you.
you can advance search here using my screen name.iv posted full solenoid testing info before.
also you still haven't told us about the mlps replacement yet.
be it was old,have replaced with new.the new was the old or updated.you re-pinned it.etc.etc.etc.
remember we are not there,we don' know whats going on,when it was last working etc.you can't leave anything out.