Change Grundfos PM 2 Pressure Monitor Cable

The Grundfos PM 2 pressure control unit is used to automatically switch water pumps on and off. The electrical connection to the pump is made using a Schuko (type F) plug connection. In my case, this connection had burned out and I had to replace it. This blog post describes how to do it.

If you enjoy messing around with pipe connections, you can remove the pressure control unit to replace the cable. However, this is not strictly necessary.

Before starting work, the pump, the pressure control unit, and any other electrical equipment in the vicinity must be isolated, i.e. disconnected from the power supply and secured against being switched on again. The absence of voltage must be verified by measurement. Depending on the circumstances, it may also be appropriate to short-circuit or ground the system.

The first step is to remove the black plastic cover with the indicator LEDs and switches. To do this, four Phillips-head screws have to be removed. The cover must then be carefully removed because it is connected to the pressure control unit by a short ribbon cable.

The two plastic clips above and below the connector must be carefully folded out of the way. The cable can then be disconnected.

Next, the two beige-colored inner plastic covers have to be removed. They are not screwed in place and can simply be pulled off.

First, pull off the left cover.

Then remove the right cover.

I had to remove the cable connecting the pressure control unit to the pump. It is connected to the lower of the two green terminals. Just in case, it is a good idea to take a photo of the wire configuration.

In my case, the neutral conductor was connected at the top, the live conductor in the middle, and the protective conductor at the bottom. This could theoretically be different, of course.

The terminal is soldered to the circuit board and cannot be removed!

Next, unscrew the old wires. The cable is secured against being pulled out by a plastic nut on the right-hand side. This can normally be loosened by hand.

You should remember the order of the individual parts, or take another photo.

Normally, you will not use a rigid cable but a flexible cable with stranded conductors. This means that the stripped ends should be fitted with ferrules to ensure proper contact without sparking. This is something you should always do anyway. However, in the damp environment around the pump, this measure is absolutely essential.

The ferrules I used did not fit into the terminals in their original state. After crimping, however, they fitted relatively easily. I did have to remove a small section with pliers, though, because otherwise too much bare metal protruded from the terminal.

It is a good idea to test this beforehand using a single conductor from a piece of old cable.

Before tightening the new cable in the terminal, check once again that all the parts of the cable fastening have been placed on the cable in the correct order. The cable can then be secured again with the plastic nut. It should be sufficient to tighten the nut by hand.

Now reassemble everything in reverse order:

1. Fit the right inner plastic cover.
2. Fit the left inner plastic cover.
3. Connect the control cable.
4. Secure the control cable with the plastic tabs at the top and bottom.
5. Attach the cover with the LEDs and pressure switches using the four Phillips-head screws.

The control cable connector is keyed and cannot be inserted the wrong way around:

The raised parts of the connector must face outward.

The Schuko plug connection can be housed in a waterproof box, which is available at many DIY stores:

However, I simply connected the cable coming from the pump directly to the terminal of the pressure control unit because I initially suspected that the box had not been properly watertight and that this was why the cable had burned out. It turned out, however, that the plumber who originally installed the pump had protected the pump supply with a 50 A (!) circuit breaker. The protective conductor was not connected either. No wonder the plug connection eventually overheated and melted.

This has of course been fixed now. And presumably, the plug connection would have been just as safe inside the waterproof box.