First thing Monday morning is the best time to savour a good cup of coffee in our household, after everyone else has gone out for the day.
Unfortunately, none was to be had. The pump just kept running, but no flow out of the portafilter. Had I left the blind basket in place, it would be easy to do, but no.
Looking in the water tank, it was empty, but why had the machine not cut-out and lit the low water light on the front panel like normal?
Inside the water tank, the water softner cartridge was sitting on its side, so not drawing the last inch of so of water from the tank. No problem, I filled the tank and prepared for it all to work. But no.
The pump had been running for a few minutes by now, so I thought I might have damaged it, although I could occasionally see air bubbles in the return pipe to the tank, so it appeared to be working. They are only supposed to run for a minute, before cooling for 1.5 minutes. The best thing was to let the machine cool and then investigate again.
Some short time later. I turned the machine on, and the pump ran for a few seconds, as it normally does to top up the water tank after the machine has cooled and the water has slightly contracted on cooling as it becomes more dense. So the pump does appear to be working.
But still no coffee flow.
The other likely culprit is the solenoid. This sits on the side of the head and directs the water flow which comes from the tank, through the pump, and through the boiler heat-exchanger to reach the coffee. But it should only do this when you draw a shot. Otherwise, it lets any water above the puck run to the drip tray, so you don’t get splattered with hot water, coffee and steam when you remove the portafilter.
Each time the pump operates when you pull a shot, the coil is energised on the solenoid, and a metal slug inside moves to reroute the water flow. You should hear the coil click and if you hold an insulated screwdriver to the side of the coil you should feel a slight buzz of the magnetic field changing. These two effects are quite distinctive, so if they are not present, the solenoid is probably not working.
To confirm the coil is faulty, check for mains voltage at the coil terminals when pulling a shot. The multimeter is set to AC volts. In the UK the mains supply is nominally 230V AC, so seeing 243V is okay, the solenoid should be switching.
The next thing to do is to check the coil resistance. The multimeter is set to ohms. The display of OL (overload) on this multimeter shows the coil is open-circuit. Not good.
Comparing to a replacement coil, we see a resistance of 0.896k ohms (896 ohms). We are now certain the coil is faulty.
First I removed the drain pipe from the top of the solenoid.
Here you can see it free. The other end connects to a piece of black hose which ends up in the drip tray.
It is not possible to remove the coil from the solenoid without removing the steam tap. Loosen the pipe connection to the tank. Two spanners are required to avoid damage to the pipe. The boiler must of course be cold.
Pull the tap handle off and remove the chrome nut.
Guide the steam tap free.
Here is the replacement coil I hoped to use (right) and the existing coil (left). The existing coil is a Sirai ZA34A. It has the same internal diameter as the Parker ZB09 I planned to replace it with, but as you can see the depth is different.
I did not have a spare Sirai coil in stock, so I replaced the whole solenoid. Note the pipe connection is flat at the top above the screw thread. You need this sort of pipe connection, not the alternative conical one which is more commonly found.
The base dimensions of the valve are pretty much common, so the two valves shown are interchangeable. Although not pictured, you need to remove the top nut of the solenoid and take the coil off to allow access to the four fixing bolts (3mm allen key required).
Orientate the new valve correctly before fixing back into place.
Reconnect the wiring. The closest terminal has a single wire connection from the switch.