What keeps a dual-element constant displacement fuel pump operating if one gear seizes?

• A. The shear section of the driveshaft
• B. The backup gear
• C. The relief valve
• D. Crankcase pressure

Answer: (A)

The protection comes from a shear section in the driveshaft that links the two pumping elements. In a dual-element constant-displacement fuel pump, both elements are driven by the same shaft. If one gear seizes, the overload causes the shear section to fail in shear, uncoupling that seized element from the drive while the other element stays driven and continues to pump. This lets the pump keep delivering fuel instead of locking up entirely. The relief valve, crankcase pressure, or a supposed backup gear don’t provide that immediate isolation and continuity—the shear section is the specific feature designed to prevent a total pump failure under that fault.