by Phil Nielsen
The interval wiper control used on Ford trucks in the late 1970s and early 1980s is easily adapted to control the single-speed Mopar wiper motor used in early A-body vehicles. This article describes two fairly simple adapter circuits, one utilizing a single relay, and one utilizing two relays.
1) Ford dash-mounted interval wiper control switch. The correct control will have two fixed positions (low and high) when turned clockwise, and a variable control when turned counter-clockwise.
2) Ford interval wiper control module.
3) One or two 12VDC relays with contacts rated at 3 amps or higher.
The single-relay circuit needs only an SPST relay. The two-relay circuit requires an additional SPDT relay.
We recommend getting the harness connecting the Ford module to the Ford motor to simplify wiring. You do not need the entire harness, but if you can, get the connector and at least 6 inches of the wiring. If a harness is not available, you can use spade terminals to connect to the module.
If you have access to a Pick-N-Pull or similar yard, these items are relatively inexpensive. We purchased ours for $8 for the switch and $5 for the module. The additional relays (12VDC DPDT 5 amp contact rating) were $3 each at a local electronics surplus shop.
A schematic of the Ford module, control, and external wiring is shown here.
In interval mode, wire "58" is energized, causing the motor to operate at low speed. When the motor's internal cam switch connects wires "28" and "63" ("63" is +12 volts), wire 58 is switched inside the module to be driven by wire "28." At the end of a sweep, the cam switch disconnects wires "28" and "63", which de-energizes wire "58." In the Ford motor, wire "28" is then grounded, dynamically braking the motor.
This picture shows the standard Mopar single-speed wiper circuit. The internal cam switch (indicated by the hashed line from the motor to the switch) ensures the motor will park after the RUN wire is de-energized.
The simplest adaptation uses a single relay:
Instead of applying power to the motor "park" terminal, this circuit uses the Mopar cam switch to indicate motor position, and controls the motor using only the Ford wire "58" (the "run" wire). When the motor is sweeping, the relay is energized, which connects Ford wires "28" and "63". When the motor reaches park position, the cam switch opens, the relay de-energizes, and wires "28" and "58" de-energize.
This circuit has a minor shortcoming. If the driver turns off the ignition with the wipers not parked, then turns off the wiper switch (while the ignition is off), the wipers will not self-park when the ignition is turned back on. However, simply turning the wiper switch on and off while the ignition is on will force the wipers to park.
This shows a complete solution, which overcomes the shortcoming of the simple adaptation. This circuit uses two relays. The "lower" relay acts as a "mode" control. If the Ford control switch is "off," the Mopar motor "park" terminal is connected to switched +12 volts, ensuring automatic wiper parking. When the Ford switch is in any "on" position (low, high, or interval), the Mopar "park" terminal connects to the second relay, which operates exactly like the relay in the simple circuit.