Bosch Exxcel 1600 - more help needed

[Space cat]

A week and a half ago I posted this:

//www.diynot.com/forums/appliances/bosch-exxcel-1600-express-heater-fault.293679/

and, thanks to zipper, I have a replacement element fitted - but I'm reluctant to put it all back together and switch on.

The old element came out with pieces hanging off and there's clear signs that the outer metal cladding has melted in several places. It looks as if it's burnt out in the absence of water. I've checked the temperature sensor. It measures about 7 kilohms, dropping as my hand warms it up, so that seems OK. The machine has been working happily for the last week on cold wash only so I don't think there's a problem with the level sensor.

My suspicion is that a fault on the control PCB has powered up the heater when it shouldn't and fried it and so I should investigate that. Alternatively, is it possible for an element to do itself so much damage without being dry? (Just in case it matters, there is no RCD on that circuit.)

 

[zipper]

A level switch prevents the heater from "dry cooking".
If the machine isn't rcd protected you could always use a plug in type http://cpc.farnell.com/1/1/12943-rcd-plug-rcd-1-pro-elec.html

 

[Space cat]

A level switch prevents the heater from "dry cooking".

Thanks zipper. You've got me thinking now. There is a level switch up on the right and it is controlling the water level on cold washes - but might it have an extra pair of mains rated contacts, dedicated to the job of protecting the heater? I'm going to trace some wiring to find out.

Meanwhile, I've had the control panel apart and the power PCB out. There's a fine covering of black in lots of places but no sign of any actual damage. The dirtiest components were in the vicinity of a switch mode PSU - but that must be working because the rest of the machine runs just fine. I specifically checked the heater switching relay for shorted contacts but found none. I even followed the coil connections back to a transistor but that gave reasonable diode test readings on my meter.

I'm putting the control panel back together now with no changes. I'll check that level switch for anything untoward then I'm going to reassemble the machine and give it a test run, listening carefully for any sound of a sizzling element.

Thanks for your help so far.

 

[zipper]

The level switch controls water fill levels during all washing phases (hot and cold). It also acts as an anti-boiling safety device when connected in series with the heating element. Carbon dust on the pcb is quite normal, it is especially noticeable on heat generating components. I wouldn't worry about it.

 

[Space cat]

It also acts as an anti-boiling safety device when connected in series with the heating element.

Spot on again! I found four wires: a pair of thin brown ones, going to the low voltage end of the PCB, and a pair of heavier grey ones which, on closer inspection, were part of the heater circuit. That pair of terminals was open circuit on the empty machine as expected. So that's a hardware interlock that no software bug (or operator error) could have bypassed.

I'm left with only one possible cause, a fault that killed an element in our old Hoover Logic 1300 many years ago. A blockage in the duct leading to its level switch fooled it into thinking the machine was filled when it was in fact empty. The thing is though, this level switch seems to have a wide open connection straight off the drum's drain outlet - unlike the Hoover's piddling little tube - so it's hard to imagine a blockage.

Edit: There's one puzzle though. There's a ball shaped section in the pipe connecting the drum to the pump and there's something hard in it. I can only think that it's a float valve but what's it doing there? Is it to stop water from a blocked sink running into the washing machine?

To conclude, I tested the new heater while also cleaning out the machine by running a 90 deg wash cycle - and boy was that water dirty! Then I had to open the front of the pump to remove a final piece of the old heater. It looks like everything's working again but I'll be watching like a hawk for the next few washes. Thanks again for your help.

 

[zipper]

The hard object in the sump hose is a plastic "Eco ball". It stops detergent charged water filling the sump-hose/pump & being wasted. On start-up the washer fills a little water through the pre-wash chamber which fills the sump hose/pump causing the ball to float up & seal off the drum. The machine then continues to fill through the main wash compartment (taking the detergent with it). A byproduct of the Eco ball is that it also doubles as a check-valve by preventing waste water entering back into the drum. Hope that answers your question

 

[Space cat]

It stops detergent charged water filling the sump-hose/pump & being wasted.

Now that's what I call smart. It also answers another question because I'd been wondering why the first fill water was squirting into the wrong soap tray. I thought I'd put the soap in the wrong one and was just about to scoop it across to where the water was but the water moved first. It all makes sense now. Award yourself another pat on the back.