On my 81SD the switchover (also called the crossover) is mounted on the left side firewall, above the brake master. Best way to locate is to trace the rubber line from the banjo bolt on the very rear of the intake. It should be in a tight area right between the intake and firewall.
Follow that line to the switchover. It should have an electrical plug into it, and then another small plastic line running out and to the ALDA.
That line carries boost pressure to the ALDA and in turn the ALDA dumps more fuel into the engine when running a boost. If you note weak accleeration and little or no black smoke on take off, you will likely find that line, or the banjo bolt, or even the valve is plugged up with goop. Clear it and find a lot more power.
The electical wires running to that valve serve to relieve an overboost pressure to the ALDA. To check it run 12v power to it and it should open and relieve the pressure on the incoming line.