Diaphragm Fuel in Fuel out Fuel in Fuel out Electrical connection Solenoid coil Strainer Calibrated spring Check valves Steel plunger Chamber C Chamber D Magnet Pivot A B Electrical contacts Figure 14-52.  A pulsating electric auxiliary fuel pump is used on many light reciprocating engine aircraft. In A, the pump is shown with  its solenoid coil energized, which draws the plunger down between the coil. This opens the breaker points allowing the calibrated spring  to push the plunger upwards, thus pumping fuel out the outlet B. This cycle repeats at a speed related to the fuel pressure buildup at the  pump outlet.  As with all vane pumps, an eccentric rotor is driven inside  a cylinder. Slots on the rotor allow vanes to slide in and out  and be held against the cylinder wall by a central floating  spacer pin. As the vanes rotate with the eccentric rotor, the  volume space created by the cylinder wall, the rotor, and the  vanes increases and then decreases. An inlet port is located  where the vanes create an increasing volume space, and fuel  is drawn into the pump. Further around in the rotation, the  space created becomes smaller. An outlet port located there  causes fuel to be forced from the cylinder.  [Figure 14-53]  The fuel metering device for the engine delivers more fuel  than it needs to operate. However, the constant volume of  a vane pump can be excessive. To regulate flow, most vane  pumps have an adjustable pressure relief feature. It uses  pressure built up at the outlet of the pump to lift a valve off its  seat, which returns excess fuel to the inlet side of the pump.  Figure 14-54  shows a typical vane type fuel pump with this  adjustable pressure relief function. By setting the relief at a  certain pressure above the engine fuel metering device air  intake pressure, the correct volume of fuel is delivered. The  relief pressure is set via the pressure adjustment screw which  tensions the relief valve spring. During engine starting, or if the vane pump is inoperative, fuel  must be able to flow through the pump to the fuel metering  device. This is accomplished with the use of a bypass valve  inside the pump. A lightly sprung plate under the relief valve  overcomes spring pressure whenever the pump’s inlet fuel Rotor Vane Sleeve Spacer Inlet Outlet Cylinder Figure 14-53.  The basic mechanism of a vane-type fuel pump. 14-32