Hotpoint WMF740 Intermittent Random failures

I’m troubleshooting a neighbour’s WMF740 which is exhibiting intermittent and random failures.

Trouble began three days ago with an H2O error as soon as a programme was started which suggested a water inlet issue. When the programme started the initial water inrush could be heard for a few seconds then it’d stop, spurt again, stop, then the machine would sit with the timer counting down until failing after a couple of minutes with “H2O”. No other error codes have ever been displayed.

However, after being left powered off for a while the machine would start the programme correctly but fail later in the programme.

The owner also reported other random failures such as the drum not turning when expected.

The owner replaced the motor brushes approximately 3 years ago.

Due to the random failure I suspected an electrical connection may be loose, solder dry joints, or similar.

When I inspected the machine the first thing I noticed was a very heavy deposit of brush carbon in the back of the machine on the drum pulley wheel and other places. I blew out all the carbon from the motor and the rest of the machine using an air compressor and wiped clean. This included removing the control module, uncasing, and brushing away the carbon from its components and traces. Both brushes look to be worn at least 50{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} and one is 4mm more worn and has rough and broken edges.

At the same time we took the opportunity to remove the drain hose, detergent drawer, and door gasket and thoroughly cleaned them to remove the build up of grease and solidified detergent.

Once reassembled the machine behaved properly and several loads (more than four) went through over 24 hours without a problem.

Then it failed again, simply stopping part way through a programme with the drum full of water.

Earlier we had noticed a metallic ticking noise which sounded like vibration causing something to tap and this was reported again prior to this new failure. Investigating further it turned out this was caused by arcing across the commutators which turned out to be caused by residual carbon dust in the insulator grooves between each pad of the commutator. I cleaned those out with a stiff tooth brush.

The machine returned to normal operations and the ticking noise no longer occurred.

A couple of loads later it failed again with different symptoms. Due to the intermittent nature of all these failures it feels to me like a power or thermal issue – if the power supply is rippling or under-voltage briefly it could cause the controller to error in random ways.

I inspected the control module carefully and noticed that the 33uF 400V electrolytic capacitor looked like it might be starting to gas (the top was ever so slightly convex rather than concave as I’d expect).

I decoded the power circuitry to get a feel for what might be going on and tagged the BTB10-800BW Triac as another possible culprit if overheating may be the cause.

I replaced the capacitor in case it was the culprit. The machine did a couple more loads then failed randomly again.

At this point we’ve removed the machine and I have it for diagnostics, repair if possible, or else scrapping.

I’m wondering if this could all be down to the worn brushes causing power fluctuations and if simply replacing them might the simple solution.

I’ve been trying to find a service manual for this model but not had success. I was wondering if the diagnostics port on the control module (which is exposed externally on the machine) might provide useful data such as an error log – no idea if it is a simple UART, I2C, SPI or similar serial interface or whether it’d need expert software to decode errors though.

Any assistance would be appreciated.