1973-1991 Dual fuel tank systems theory of operation

[hatzie]

The stock TBI relay on the firewall is wired inline with the incoming power to the NL2 option rocker switch in the dash.
The relay GM used is a Metripak sealed 5 terminal relay. Electrically it's the same as your garden variety Bosch mini ISO relay but it's more compact and the terminals are not open to the elements.

You have an electromagnet on terminals D & F that pulls a Common terminal E switch reed from the Normally Closed C terminal to the Normally Open A terminal.
The electromagnet receives power from the ECM and ground from the chassis.
The relay switches power to the fuel pump to protect the ECM driver circuit from the fuel pump motor startup inrush current, motor running current, and shorts in the fuel pump wiring.
I believe terminals D & F are Metripak 150 and A C & E are higher current Metripak 280 terminals.

The relay, oil pressure switch, and the ECM perform an important safety function. When the engine is running the ECM gets pulses from the ignition module and switches on the fuel pump relay along with enabling injector pulses. As you can see in the diagram the oil pressure switch has open contacts to power the fuel pump once the oil pressure rises and cut power to the fuel pump(s) when the engine stops rotating. If you have an accident, and the engine shuts off, the fuel pumps will stop running as well rather than feeding a fire. Not all early fuel injected vehicles had this safety feature. The accident scenes were extremely unpleasant.

The RED wire test terminal allows you power the fuel pump(s) without actuating the relay.