The OD solenoid would be the first thing to check for sure.
If the pan was clean (only normal crap in it) the OD unit is probably ok.
Since OD gets it’s apply fluid from the 3rd gear circuit you know thats there.
Good chance it’s the solenoid.
As far as the electrical goes I used relays to do the ground switching for the solenoids and the hot is a key switched supply.
Also used adjustable pressure switches so with my cabin switches in Auto mode it shifts 123 OD at 60 and lockup at 70 fully automatic.
When slowing lockup turns off when below 70 and OD turns off at 60 and the trans returns to 3rd.
With both switches in the cabin in the off position the trans acts just like a 904 with the low geared first.
With Nitto 245/40-18 high performance tires in the back and 3.55 rear gears it ripped a stock driveshaft in half off a light.:D
