I’ve got a gen 3 Hemi 6.4 engine with an a518 46rh behind it in my 70’s Satellite. The 46rh has overdrive and lockup.

When coming to a full stop the engine wants to stall. Then I throw the trans in Neutral and after the full stop I put it back into drive.

The 3 pin connector is hooked up but the Patc vacuum switches aren’t. The vehicle speed sensor isn’t used.

What can be wrong?

It sounds like you are staying in lock up.Check your wiring and hook up the switches.

https://transmissioncenter.net/shop/727-to-518-transmission-swap-information/

Agree, sounds like the car doesn't know your coming to a stop.

i put a lockup switch on my first Cummins diesel powered pickup truck, 1998 1/2 24 vavle with the weak automatic, if I didn't turn the switch off when stopping it would drag the motor RPM way down and make me afraid to not unlock the converter
Sounds like you're having the same thing

Simple solution is to use a toggle switch(es) to control lock up and o/d. Could use the same switch for both wired as a ground interrupt if you do not care about running them independently from one another.

The fluid that is used to lockup the converter originates at the 1-2 shift valve...so there can be no lockup until after the 1-2 upshift occurs.

In normal operation, as the vehicle slows to a stop, the trans downshifts directly from 3rd to 1st bypassing 2nd. If there is a governor problem and the trans doesn't downshift to 1st just before the stop, the converter can remain locked up. Shifting to Neutral restarts the sequence.

Other problems could be the sealing rings on the input shaft allowing line pressure to lockup the converter un-commanded.