referring to fig1 a washer , such as a dishwasher 10 , includes a wash tub 12 for containing objects to be washed . a lower part of the tub 12 defines a sump 14 for collecting washing liquid , such as a solution of water and detergent . a pump 16 driven by a motor 18 is located in communication with the sump 14 . the pump 16 is adapted for delivering liquid to an upper spray arm 20 and a lower spray arm 22 through suitable conduits 24 . the spray arms 20 , 22 spray liquid on the objects in the tub 12 . a valve 26 is disposed between an outlet of the pump and the conduits 24 . the valve 26 includes a ball 28 for alternately blocking each of the conduits 24 to limit liquid flow to one of the spray arms 20 , 22 at a time . such a valve is described in detail in u . s . pat . no . 4 , 741 , 353 to milocco . interruption of a flow of liquid from the pump 16 through the valve 26 causes the ball 28 to change position from blocking one conduit to blocking the other conduit . when flow resumes , liquid is delivered to the other spray arm . thus , as described below , proper control of liquid flow from the pump 16 causes alternating flow from the spray arms 20 , 22 . a controller 30 is connected to the pump motor 18 to achieve the desired liquid flow . referring to fig2 the controller 30 is connected between the pump motor 18 and a power source 32 such as a household alternating current supply . the controller 30 includes a door switch 34 that is normally closed when a door closing the tub 12 is closed and locked . a rinse cycle selector switch 36 and a wash cycle selector switch 38 are connected in parallel with each other and in series with the door switch 34 . the rinse cycle selector switch 36 is closed for a &# 34 ; rinse & amp ; hold &# 34 ; and a &# 34 ; pots & amp ; pans &# 34 ; washing operation . the wash cycle selector switch 38 is closed for a &# 34 ; normal wash &# 34 ; and the &# 34 ; pots & amp ; pans &# 34 ; washing operation . a first bus switch 40 and a second bus switch 42 are connected in series with respective cycle selector switches 36 , 38 . the bus switches 40 , 42 are normally open . the first bus switch 40 is closed during a pre - rinse period and the second bus switch 42 is closed during the remaining period of operation . other combinations of cycle switches and bus switches for controlling other cycles are also suitable and well known in the art . a motor switch 44 is connected in series with the bus switches 40 , 42 . the motor switch 44 is normally closed and is opened to deenergize the motor and also during interrupted operation of the motor to achieve alternating flow of liquid to the spray arms 20 , 22 , as discussed below . a drain switch 46 is connected in series with the motor switch 44 . the drain switch 46 is normally closed and operates the pump motor 18 during a drain phase . the drain switch 46 is opened during phases other than draining . a wash switch 48 is connected in series with the motor switch 44 and in parallel with the drain switch 46 . the wash switch 48 is normally closed and operates the pump motor 18 during washing and rinsing operations . unless stated otherwise , it is assumed that the wash cycle switch 38 , second bus switch 42 , and wash switch 48 are closed during operations discussed below . a motor interval switch 50 , is connected in parallel with the motor switch 44 . the motor interval switch 50 is normally closed , but is held open during all operations except during interrupted operation of the motor 18 . the following description relates primarily to the operation of the motor interval switch 50 and corresponding operation of the pump 16 during a washing operation during which flow alternates between the spray arms 20 , 22 . the combinations and connections of other switches can vary . fig2 shows the status of the other switches during such a washing operation . referring to fig3 the controller 30 includes a cam wheel 52 driven by a timer motor 54 . the cam wheel defines a plurality of coaxial cam disks 55 . plural spring arms 56 are located above the cam wheel 52 and electrically connected to connection lugs 58 . the lugs are adapted to be connected to electrical circuits of the dishwasher 10 . referring to fig4 the timer motor 54 rotates an eccentric cam 60 on an axis to oscillate a finger 62 having an eccentric opening 63 . the finger 62 engages teeth 64 on the cam wheel 52 to drive the cam wheel stepwise about an axis of the cam wheel 52 . the finger 62 is biased toward the teeth 64 by a spring 66 . the spring arms 56 are arranged in sets of three , each set including a top arm 56t , a center arm 56c , and a bottom arm 56b . the center arm 56c is provided with a pair of electrical contacts 68 adapted to create an electrically conducting path with a contact 70 on the top arm 56t or a contact 72 on the bottom arm 56b . the bottom arm 56b is biased to normally close its contact 72 with the bottom contact 68 of the center arm 56c . in a conventional manner , contacts 68 , 70 , 72 on different sets of spring arms 56 are used as the switches 40 , 42 , 44 , 46 , 48 discussed above . preferably contacts 68 , 72 on the bottom arm 56b and center arm 56c nearest the front of the controller 30 are used as the motor interval switch 50 , for reasons discussed below . a contact spacer 74 positions the top and bottom arms 56t , 56b to maintain a proper air gap between the contacts 68 , 70 , 72 . a cam follower 76 is provided at an end of the center arm 56c . the cam disk 55 nearest the front of the controller 30 corresponds with the spring arms 56 nearest the front . the cam disk 55 engages the cam follower 76 to maintain the center arm 56c in a neutral position wherein the contacts 68 , 70 , 72 are open , as shown in fig4 . a tooth ( not shown ) on the cam disk 55 will raise the center arm 56c to close the center contact 68 with the top contact 70 . this is not utilized in the embodiment shown for the spring arms 56 nearest the front . a gap 78 in the cam disk 55 permits the center arm 56c to lower and close with the bottom contact 72 , thereby closing the wash interval switch 50 . the front cam disk 55 is arranged with plural gaps 78 . the gaps 78 are separated by lobes 80 that raise the center arm 56c and open the wash interval switch 50 to deenergize the pump motor 18 . as the cam wheel 52 rotates , gaps 78 and lobes 80 on the cam disk 55 open and close the contacts 68 , 72 to control operation of the pump motor 18 and pump 16 . similarly , other dishwasher functions are controlled by other contacts operated by gaps , lobes , and teeth of other cam disks . referring to fig6 a first timing diagram shows the spacing of gaps 78 and lobes 80 and the corresponding operation of the pump 16 . according to a preferred construction of the invention , the lobes 80 are spaced at approximately 180 second intervals and open the wash interval switch 50 ( fig2 ) for 7 ± 3 seconds . referring to fig5 the timer motor 54 drives an auxiliary cam wheel 82 that is preferably concentric with the eccentric cam 60 . the auxiliary cam wheel 82 has a generally smooth surface with a single lobe 84 . the auxiliary cam wheel 82 is sized to rotate once in approximately 180 seconds , corresponding with the period of the gaps 78 on the cam disk 55 . a lever 86 mounted on a pivot 88 has a cam follower 90 that engages the auxiliary cam wheel 82 . an end 92 of the lever 86 opposite the cam follower 90 is positioned to operate an appendage 94 extending from the cam follower 76 of the center arm 56c . when the tooth 84 operates the lever 86 , the center arm 56c is lifted to open the switch 50 ( fig2 ). the auxiliary cam wheel 82 , thus , overrides the cam disk 55 to deenergize the pump motor 18 regardless of the cam disk 55 position . referring again to fig6 the lobe 84 opens the switch for 1 ± 0 . 5 second about every 180 seconds . the lobe 84 is positioned to open the wash interval switch 50 about midway between the lobes 80 on the cam disk 55 . therefore , as shown in the bottom timing diagram , during a 180 second cycle of pump 16 operation , the pump will stop twice , once for 1 second and once for 7 seconds . the stopping of the pump 16 permits the ball 28 of the valve 26 to change positions . when the pump starts again , liquid flows to the opposite spray arm , thereby alternating the flow of wash liquid to the spray arms 20 , 22 . according to another embodiment of the invention ( not shown ), the auxiliary cam wheel can operate another switch separate from the motor interval switch 50 . the other switch would be connected in series with the motor interval switch 50 . thus , the motor interval switch would not need to be nearest the front of the controller . according to operation of a preferred embodiment , the wash switch 44 is closed during rinsing and draining operations to operate the pump 16 continuously . during washing operations , the wash switch 44 is open and the wash interval switch 50 is operated as discussed above . at the beginning of a wash operation , the cam disk 55 closes the switch 50 to energize the pump motor 18 and pump 16 . liquid flows through the valve 26 to the lower spray arm 22 . flow to the upper spray arm 20 is blocked by the ball 28 . after about 60 seconds , the auxiliary cam 82 opens the wash interval switch 50 to deenergize the pump motor 18 and pump 16 for about 1 second . the ball 28 changes positions , the switch 50 closes again , and liquid flows to the upper spray arm 20 . operation continues for about 60 seconds until the cam disk 55 opens the motor interval switch 50 for 7 seconds . water flows out of the conduit 24 and the ball 28 again switches positions . the motor interval switch 50 closes and water flow to the lower spray arm 22 resumes . operation continues with alternating flow to the spray arms 20 , 22 until the wash operation is completed . at the end of the wash operation , the cam disk 55 opens the motor interval switch 50 for the remainder of the dishwasher operation . subsequent operation of the pump is controlled by the motor switch 44 , drain switch 46 and wash switch 48 . the present disclosure describes several embodiments of the invention , however , the invention is not limited to these embodiments . other variations are contemplated to be within the spirit and scope of the invention and appended claims .