Before 1995 Ford interval wiper systems mainly consisted of a switch, the motor and the interval governor (also called the wiper control module or 'WCM'). The basic wiper systems just parked the wipers at the end of their travel at the base of the windshield and involved just one park switch.
The more complex systems not only stopped the wipers at the base of the windshield, but would actually recess them below the windshield to park them. This involved using two park switches to turn the wiper motor in a reverse direction. These were installed on the Crown Victoria, Grand Marquis, Town Car, Taurus and Sable.
The wiper switch (named 'Multi-Function Switch' on most models) had a common return or ground circuit and two input or signal circuits to the WCM. Basically, the switch was two modified potentiometers that varied the resistance to ground for the two signal circuits. When the wiper switch was placed in the low or high position, the WCM looked at only one of the circuits- the 'mode input' -and controlled the wiper motor accordingly. If the wiper switch was placed in any of the interval positions, the mode input told the WCM that the switch was in an interval position. The WCM then looked at the second input - the 'delay input' - and used it to determine the amount of delay between wipes.
1) To run the wipers on low or high speed, the WCM provided constant power to the wiper motor on the proper circuit to provide the requested speed. Dual park switch motors
1.1 The first park switch operated basically in the same way as the single switch systems.
1.2 The second park switch worked in the opposite manner- when the wipers were off the park position, the second switch provided a ground for the wiper motor. Power and ground were now provided by the park switches.
1.3 When the wipers needed to park, the polarity of the switches changed, and thus provided reversed voltage to drive the motor in the opposite direction.
2) To run the interval wipers, the WCM applied voltage to the wiper motor only momentarily to get the wipers off the parked position.
3) The electro-mechanical park switch made a circuit between the power feed in and the output to the WCM. Power was provided from the park switch through the WCM and back to the wiper motor.
4) When the wipers reached the park position, the park switch made a circuit between the ground and the output to the WCM, and the motor stopped running. The wiper motor and the wiper switch have remained much the same as the single park switch motors of the past.
The main difference is that the switch has become an input to the GEM instead of the WCM.
1) When the wiper switch is placed in the low or high position, the Run-Park relay is grounded by the GEM to provide constant voltage to the High-Low relay, which then provides voltage to the proper circuit of the wiper motor.
2) High-Low relay is in the low speed position unless grounded by the GEM.
3) The park switch not only feeds the Run-Park relay but is now also an input to the GEM. This tells the GEM when the wipers are in the parked position so it can de-energize the Run-Park relay.
4) When any of the interval positions are selected, the Run-Park relay is momentarily grounded to get the wipers off the parked position
5) The park switch then provides voltage to the other set of contacts in the Run-Park relay.
6) When the GEM de-energizes the Run- Park relay, voltage is still provided from the park switch to the High-Low relay and the wipers complete the stroke.
Click over diagrams for zoom:
Hope this helps.