Bosch built-in oven – what have I done wrong?

[ProgRob]

Hello, I wonder if anyone can help.

I have a Bosch built-in oven (HBN432AGB/01). Recently the main oven stopped heating up. I have previously replaced the element successfully and opening it up it was clear I needed to replace it again.

I bought the generic replacement from eSpares which arrived yesterday.

Today I tried to fit the spare. Unfortunately, due to my incompetence I connected the earth and live to the element. When I turned the oven on there was a bang. I switched off immediately. Interestingly the light next to the dial (which indicates that the oven is on) stayed on.

I fitted the element across the correct two terminals and switched the oven on. The fan came on but the element did not appear to heat up. So I switched the oven off.

After some time (I don’t know how long) the light next to the dial went off.

I checked that the top oven still works (it does).

I used a multimeter to test the element and I could measure resistance (upper 20s of Ohms), so it seems OK.

I then tested the two terminals with no element connected and when I switched on the oven I could read a voltage across them (in the upper 20s of volts).

I attached the element to one of the terminals and switched the oven on. I could again read a similar voltage between the unconnected end of the element and the unconnected terminal.

However, when I attached the element to both terminals and switch on there is no heat and I cannot read a voltage across them.

Now I am probably being stupid (indeed I was stupid), but can anyone explain what is happening and what I have to do to fix the oven (in time for Xmas dinner)? Did I completely knacker the oven initially? Why did the light next to the dial stay on for a while then finally switch off? Isn’t that voltage too low (it is supposed to be a 1400W heater, my maths shows that 30V and 30 Ohms would only give 30W)? Most importantly, how do I get the new element to work?

Thanks,
Rob

[tiprat]

Re: Bosch built-in oven – what have I done wrong?

i would say get an engineer in before you do any more damage. circuit board may be fried if there is one, relay burned out etc etc….

[ProgRob]

Re: Bosch built-in oven – what have I done wrong?

Thanks for the advice but I would still like to see what I can do. I do, contrary to my actions, know what I am doing! My problem was having done it before so easily that I forgot the basics of checking very carefully what I was doing. The old story of check and double check.

Having looked at the diagrams of the cooker on the Bosch website it looks to me most likely that the temperature limiter has blown. There was a flash of light when I heard the bang and it was in the position of the limiter. This would explain why there is some current but not much. Also, it must be something simple gone as everything else is working including the fan for the main oven, the whole of the top oven and the clock etc.

My question is therefore how do I best get access to the rear of the main oven (behind the plate on which the fan sits) – is it best done by removing the whole cooker and removing a back plate?

My assumption is I will see a blown part, but if not, is there any way of testing the limiter?

[Penguin45]

Re: Bosch built-in oven – what have I done wrong?

You’ve most likely blown the selector switch fo the oven.

All access is by removing external panels, so oven out of housing.

Temperature limiter should read virtually zero ohms as it is normally closed circuit.

Stay safe – power off.

Penguin45.

[ProgRob]

Re: Bosch built-in oven – what have I done wrong?

Many thanks for the tip.

I’ve had a look on the Bosch exploded parts diagrams and cannot see the selector switch. I notice there is a part called ‘switch-selector’ but I cannot find it on the diagrams.

Where am I likely to find the part when I take the oven apart? Will I recognise that it has blown?

Thanks so much for your help,

Rob

[RocketMan]

Re: Bosch built-in oven – what have I done wrong?

That’s it, just look at it and you will find contacts have either melted or disentegrated – normally quite obvious normally.

It’s on the diagrame on page 2

https://www.bosch-eshop.com/eshop(bD1lbiZjPTAwOQ==)/bosch/gb/prodp.htm?prod=HBN432AGB{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d}2f01&ESHOPTYPE=S

Different ones for main oven and top oven though, watch out.

Would also suggest you don’t go removing any wires from the selector. You’re bound to forget. If it is the selector, wait until you have the part, make a good note of the wires then change.

As ever, power off and keep the kids away, small inquesitive little fingers don’t make for a good working environment.

[ProgRob]

Re: Bosch built-in oven – what have I done wrong?

Thanks so much – you guys are brilliant. Found the part on the diagram. Now just to persuade the Missus that I should attempt dismantling the oven before Xmas. Otherwise a New Year job.

[ProgRob]

Re: Bosch built-in oven – what have I done wrong?

Well finally the Missus got fed up with using only the top oven and so has let me start to take it all apart. I pulled the oven out a little and found I could get to the small housing on top quite easily. The selector switches look absolutely fine.

I am now, when I can get my son to help, about to pull the whole oven out to have a look behind. The reason is that when it went bang originally there was a flash in the top right (when looking inside the main oven) at the back (behind the back plate). Therefore I think it is something there that is fried. Any thoughts, please?

Thanks,
Rob

[ProgRob]

Re: Bosch built-in oven – what have I done wrong?

OK – have taken the back off the oven. There are various burn marks over the back of the oven and on the panel that was removed. Where the panel was removed the burn mark is where the heater element parts go through the back of the oven.

There appears nothing burnt out.

By following the wires through I can see why you might think it is the switch selector, but that looks fine.

So, any way I can test the switch selector?

Is there anything else it could be?

By the way I have photos if that would help answer this but I have no idea how to post them.

Thanks,
Rob

[cornflakes]

Re: Bosch built-in oven – what have I done wrong?

rob clearly you have done damage to other components in the oven heating circuit.

WITH THE POWER TURNED OFF!!!!

1] TEST FOR CONTINUITY OF CIRCUIT FROM THERMOSTAT TO CONNECTION ON HEATING ELEMENT..YOU WILL HAVE A FEED IN TO ONE SIDE OF THE THERMOSTAT AND THE OTHER GOES TO THE ELEMENT.

IN CIRCUIT COULD WELL BE AN OVER HEAT CUT OUT A ONE SHOT ONE ONCE GONE YOU NEED TO REPLACE)

ALSO TEST FOR CONTINUITY ACROSS CONTROL THERMOSTAT TERMINALS WHEN CALLING FOR HEAT( SELECT HEAT SETTING)

IT MAY WELL PROVE YOU HAVE BLOWN MORE THAN ONE COMPONENT WHEN YOU PUT PHASE DIRECT TO EARTH

THE JOB IS REALLY AN ENGINEERS ONE..SOMEONE WHO CAN TEST CIRCUITS WITH the CORRECT TESTING INSTRUMENTS AND A FULL UNDERSTANDING OF WHAT THE FAULT YOU INSTIGATE..What damage it would produce.

TO GET HER INDOORS BACK IN FAVOUR..I WOULD CALL A LOCAL COOKER REPAIR SPECIALIST OR IF YOU HAVE LOADS OF CASH BOSCH DIRECT as a new appilance is not cheap

CORNFLAKES

[ProgRob]

Re: Bosch built-in oven – what have I done wrong?

Thanks for the advice so far. With the power switched off I have used the multimeter to check for a current on various components and traced the circuit as follows:

1. The switch-selector and temperature regulator are plugged in together, almost as one unit. By tracing the circuit back I could identify where the live that leads to the element comes out. I put a multimeter over between the live and what appears to be a neutral terminal (from the colour coding on the wire) and turned the knob. Switching on showed current could flow from over between the live and the neutral terminals. Also the resistance changed as the temperature was varied – so looks OK.

2. Between the live terminal on the temperature regulator and the temperature limiter and current flowed, so the wire between them looks OK.

3. Between the terminals of the temperature limiter – nothing. This looks like it might be the problem as no current flowed. Is this a safety element in the circuit which is designed to blow to stop overheating?

4. The short wire between the limiter and the oven element looks OK and the multimeter agrees

5. Putting the meter between the terminals on the new element shows that is OK.

After that it looks like the neutral all the way and no further components.

When power is back on again, the fan itself will start by switching on the oven, so probably nothing wrong there either.

So, am I right in thinking that no current between its terminals means that the temperature limiter is kaput and is the most likely culprit? Is this the correct way of testing the temperature limiter?

I did also notice that there appeared to be some blackening just above the temperature limiter, but I have not unscrewed it yet to look closer. Also, the temperature limiter is in the right position for the flash I saw when I switched everything on originally.

Any help to confirm (or not) my diagnosis would be appreciated!

Cheers,
Rob

[ProgRob]

Rereading the earlier comments in this thread I see that Penguin45 said that “Temperature limiter should read virtually zero ohms as it is normally closed circuit.
“

Does this mean that I would get absolutely no reading across its terminals or just a very high resistance?

Thanks

[ProgRob]

I have read elsewhere that I can test whether the temperature limiter is the problem by simply bypassing it. I guess (with power off) complete the circuit avoiding that component. Power on. if the oven heats up it is the temperature limiter. If not, it is something else (and could be the temperature limiter, as well).

Thoughts, anyone, please?

[nomadPaul]

Hi Rob , we can’t really condone the test that you’re suggesting. What i will say is that the resisitance value of the temp limiter should be a very low resistance value in ohms i.e practically zero ( depending on the accuracy of the meter you’re using ) when its in its working condition

Temp limiters have 2 states – closed or open circuit

Hope this helps

[ProgRob]

I knew I was right to ask here before trying that test! So thanks.

My meter goes down to measure in 0-200 Ohms (to one decimal place, I think). Would this be sufficient to measure the resistance?

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