* Under the hood in the fuse box: This is the most common location. The fuse box itself might be located near the battery or on the fender apron (the area between the engine and the front of the car). Look for a relay labeled "Fuel Pump," "EFI," or something similar. Sometimes the relay is unmarked, and you might need to consult your owner's manual or a wiring diagram to identify it.
* Under the dash: Less common, but possible, especially if the car has a separate under-dash fuse box or relay center.
How to determine the location and test:
1. Consult your owner's manual: This is the best place to start. It should have a diagram of the fuse and relay box(es), indicating the location of the fuel pump relay.
2. Use a wiring diagram: If you don't have the owner's manual, a wiring diagram specific to your 1990 Oldsmobile Cutlass Supreme 3.1L is your next best bet. These are available online from various automotive repair websites (some are free, others require a subscription). The diagram will show you the relay's location and the wiring associated with it.
3. Visual inspection: Carefully examine the fuse boxes under the hood and (if applicable) under the dash. Look for relays that are the same size and shape as other relays known to control electrical components (they're usually square or rectangular).
4. Testing the relay: Once you *think* you've found it, you can test it. This usually involves removing the relay and checking for continuity using a multimeter. There are many online tutorials explaining how to test relays; search for "how to test a fuel pump relay". Caution: Always disconnect the negative battery terminal before working with electrical components.
You don't "turn on" the fuel pump relay directly. The relay is activated by the car's computer (PCM) as part of the starting sequence. When you turn the ignition key to the "ON" position (not necessarily starting the engine), the PCM should energize the fuel pump relay, sending power to the fuel pump for a few seconds (to prime the system). If the pump isn't priming, the problem could be the relay, the pump itself, the wiring, the PCM, or a fuse.
Important Note: Working with car electrical systems can be dangerous. If you're not comfortable working on your car's electrical system, it's best to take it to a qualified mechanic. Incorrectly working with the fuel system could lead to fire or other hazards.
