Q: 1994 GMC Sonoma not getting power to fuel pump

The problem is somewhere in the fuel pump relay control circuit. There are five relay terminals: battery power, ignition power (controlled by the ECM), ground, output to the pump and connection to the fuel pump prime connector. What the prime connector does is bypass the control circuit and apply power directly to the relay output pin. The ECM supplies power to the fuel pump relay to energize it and deliver power to the fuel pump. You’ve already replaced the relay and ECM, so we will assume those are good. You’ll need a digital multimeter to test the circuit and see what’s going on. Check the following:

-There should be power going to the relay at terminal C5 (orange wire) -There should be a good ground at terminal C1 of the relay (black wire) -There should be power going to the relay at terminal C2 (dark green/white wire) when the engine is cranked -There should be power coming from the relay at terminal C3 (gray wire) when the engine is cranked

When you find what the problem part of the circuit, you’ll need to consult the factory wiring diagram to pinpoint the cause. Electrical diagnosis can be difficult, so I suggest you have a professional, such as one from YourMechanic, address the problem for you.