GM uses a positive trigger for the fuel pump relay. This is a bit of a safety thing, in the case of a collision and if the fuel pump trigger wire were to be pinched and shorted to ground, this would cause the fuel pump relay to be shut off, instead of being held on. Yes, it's an unlikely scenario, but it's a safety thing. GM has done this for many years, I'm not sure it's ever changed, except for maybe late model returnless systems that use PWM to control the pump, but that's a different control scheme entirely.I'm not sure on the real early trucks, but most have the fuel pump relay wired to either ignition or battery and the relay control is grounded by the ECM.
Looking at the diagrams it appears that in 84 they actually provide 12V from the ECM to the relay since the other side of the relays switch circuit is grounded. Guess it doesn't matter which way they do it since that side of the relay is such low amperage.
I image that the board in the ECM completes a path from B1 or C16 to A1 when it is needed. Most likely when the ECM receives a pulse from the pickup in the distributor or input from the oil pressure sender at B2.
Do you have power to and from the ECM B fuse?