[Bigdav160]

I thought I responded to one of your transmission conversations but maybe not.

You can run the 700R4/4L60 in third forever. In third there is really no reason to lock the converter clutch.

If your 700R4 has good second and good third, there are very few reasons why there wouldn't be a 3-4 shift. Usually, governor valve or 3-4 shift valve line up in the valve body.

Your broadcast code began with a "Y"? That wasn't originally a truck transmission so it was from a fuel injected ECM controlled converter clutch vehicle.

IIRC, power from brake switch into A, ground to energize solenoid on D and 4th gear input to ECM on B