The switching unit from OpenInverter relies on a 5V signal from a sensor in line with the pump. These sensors again can be picked up second hand from eBay. Mine appears to have two part numbers:

• 0 261 230 061
• 036 906 051 G

The sensor also features a check valve, so you place it in-line immediately after the pump (facing the right way round).

The connector for the sensor is a four pin unit with the OEM part number 1J0973704. You can buy these in packs of five for little money. The pinout took a little research though.

Left to right when looking at the connector on the sensor, with the curved end of the connector to your left, the pinout is as follows:

• Pin 1: Signal
• Pin 2: 5V
• Pin 3: Not used
• Pin 4: GND