* Under the hood in the underhood fuse box: This is the most likely location. The fuse box is usually located near the battery. You'll need to consult your owner's manual or a wiring diagram to identify the specific relay; it's often labeled, though not always clearly.
* Inside the cab, under the dash: Less common, but possible. This would typically be within the interior fuse panel. Again, your owner's manual is crucial for this.
To find it definitively:
1. Consult your owner's manual: This is the best first step. The manual will have a diagram of the fuse box(es) showing the location of all relays and fuses.
2. Use a wiring diagram: A wiring diagram specific to your year and model GMC Sierra 2500 HD will show the exact location and circuit details for the fuel pump relay. These can be found online (often for a fee) through auto parts websites or repair manuals.
3. Search online forums: Searching online forums dedicated to GMC Sierra trucks (e.g., on sites like GMC Sierra forums) might reveal the location based on others' experiences with the same model year.
Important Note: Before working on any electrical components, always disconnect the negative battery cable to prevent accidental shorts.
