I moved to the famously damp environs of western Washington a while back. I moved here from Phoenix with a car that did not have intermittent wipers – a mistake that could be undone with some effort.

A google search for a circuit I could build landed me on this page, which included this diagram.

 

That was a good start, but it took some head scratching to figure out how to add it to my car. I got this circuit off of the Auto Zone website (a great place to go looking for wire diagrams for your car).

It matched the existing circuit in my car except for the circuitry associated with the intermittent wiper feature. Note that the GRN/BLK wire is a live connection to the positive terminal of the battery (at least when the ignition circuit is on). This means the wiper switch and the integrated control unit control the wiper by (dis)connecting the motor to (from) ground. This is known as “sinking” control – as opposed to “sourcing” control.

Both circuits had to be integrated correctly. The circuit for the intermittent switch had to be modified to fit the application. I had to use a double pole – double throw relay to break the connection with the blue wire. I also had to use a 47 k-Ohm resistor in place of the 10 k-Ohm resistor on the leg that charges the capacitor – the intervals between swipes was way to short – even with the 1000 k-Ohm variable resistor that I had to use since I didn’t find a 500 k-Ohm variable resistor. This may be because the transistors I found were not an exact match to what was indicated on the diagram.

The numbers in this diagram correspond to the connection points shown in the next diagram, indicating where I splice the new circuit into the existing wiring.

The installation wasn’t pretty. Surprisingly, the little circuit I soldered together worked on the first try (other than the 10k – 47k resistor swap), even though this was the first time I built a circuit. I barely managed to cram the switch, variable resistor, and relay behind the dashboard. It looks clean in the end though and has worked fairly well. The mechanical relay causes some issues on rough roads. Hitting the turn signal often causes the circuit to trigger early if the circuit is turned on.

One thing that could be improved on: there’s a long wait for the first cycle as the capacitor initially charges. Finding some way to keep the capacitor charged while the circuit is not in use would eliminate this problem.