Steering (PAS Pump): Description and operation

PAS Pump

A vane type pump supplies hydraulic pressure to the steering rack valve unit. The pump is driven by a Poly Vee belt from the crankshaft pulley. A self adjusting tensioner is fitted to maintain the correct belt tension.

The pump comprises a body and cover which house the internal components of the pump. A pressure relief valve assembly is installed transversely in the body. The relief valve also operates as a flow control valve. The pressure relief valve limits the maximum pressure to between 89 and 95 bar (1290 and₂ 1377 lbf/in ). The flow control orifice limits the maximum flow to between 5.0 and 6.0 l/min (1.32 and 1.58 gal/min). The body and cover are retained by four socket capscrews and sealed with a moulded seal plate.

A shaft runs longitudinally through the pump. One end of the shaft has a hub which accepts the drive pulley. On ’L’ Series engines, the opposite end has a ’D’ shaped drive coupling which mates with the drive for the water pump. On K Series engines, the opposite end is closed by an extended cover. The shaft runs on a ball bearing located in the body. Oil seals at each end of the shaft prevent the leakage of hydraulic fluid.

An oval cam is located in the body. Ten roller vanes are housed in a carrier and rotate within the cam. The carrier is mounted in the centre of the shaft and receives positive drive from the shaft via a drive pin. The carrier is seated against an end plate which is located in the cover. The front of the carrier is covered by a port plate which is located against a seal plate in the body. The port plate controls the fluid flow into and out of the roller vanes during their cycle.