Introduction

I have a number of rain water tanks which I use to water my vegetable garden during the summer. I have a cheap water pump that has been working faithfully for the last 4 years but this year, when I turned it on for the first time, I got no pressure regulation, the pump was running flat out and water was pouring out of the electrical control box. Not cool!

The electrical control box is the size of, say, three decks of cards stacked up. It contains the electrical terminations for the power cable and the pressure switch. The pressure switch is used to set the operating pressure of the water pump.

To provide the capability of measuring the water pressure and switching the power to the motor there is a mechanical connection between the water and the electrical components. This mechanical connection is provided by a rubber diaphragm which allows the mechanical components to move but does not allow the water to escape. Well normally that is true. In this case the issue was that the diaphragm had perished and ruptured. Hence water flowing vigorously through the electrical switch gear.

I priced a replacement pump and a replacement pressure switch. The switch was the price I paid for the pump in the first place and a replacement pump was twice what I had originally paid. So I decided to have a crack at fixing the thing. This instructable is what I found and how I repaired it.

Step 1: Removal of the Pressure Switch

Having determined the there was a problem with the plumbing at the pressure switch it had to be removed. The main power cable comes into the pressure switch and then back out to the motor. The pressure switch is screwed into the outlet manifold and the only way to remove it is to disconnect the cable between the motor and the switch.

The photo below was taken, initially, to document where the wires were terminated.

Having recorded the electrical connections, those between the pump and the pressure switch were removed so that the pressure switch could be unscrewed from the manifold.

Maintenance activities were then relocated from the garden to the workshop.

Step 2: The Damaged Component

Once I had the unit on the work bench and I had removed the resident arachnids I disassembled the unit. The photos in this sequence show the removal of 6 Phillips head screws and the exposure of the problem.

Step 3: Replacement Diaphragm Manufacture

Having determined that the problem was in fact a damaged diaphragm I purchased some replacement diaphragm material from a local rubber suppliers. I had no idea what the correct material was but I figured at under $10 to repair the unit it was worth having a go and seeing if it could be done.

I used 1.5mm natural rubber sheet, with cotton reinforcing. I made a copy of the original diaphragm by placing it on top of the rubber sheet and giving it a quick spray with some spray paint. This perfectly marked out the shape I needed.
I cut the replacement out using a hobby knife. I didn't have any hole punches so I tried adding slots with a hobby knife and produced a star pattern as shown.

Step 4: Installation of the New Diaphragm

Not being sure of the potential chemical interactions between lubricants and natural rubber I did not add any sealants or lubricants to the diaphragm, but instead installed it dry.

The image below shows the new diaphragm installed just prior to reassembly of the pump.

Step 5: Reinstallation of the Pressure Switch

The switch was reinstalled with a little WD40 on the electromechanical components for good measure.

The pump was powered up and brought up to full pressure. I then left the pump pressurised for a couple of hours and inspected for any leaks. The seals were good, there were no leaks and my watering system was active again.

The final step was to put the cover back on the pressure switch so that the electrical components are not exposed for curious fingers or animals to find.

Step 6: Finished

The unit re-installed works a charm and the vegetables have thanked me for the effort.

To be perfectly honest, this is last years crop, but i'm hoping we get this result again.