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Following successful replacement of seized pump motor bearing, the machine will now complete rinse, and 35 deg C Quick wash cycle, but during the latter, the chamber and contents are heated to way over that temperature, probably to greater than 60 deg C in my estimation.
I would like to understand how the heating element is controlled.
I see that an NTC 85 deg C sensor (Bosch part number 165281) is mounted on the heater body. Has anyone a test method for this component?
I assume the above sensor controls the heating element, but via the PCB and a relay?
Any pointers would be very welcome
Regds
547
Re: Overheating – Bosch SGS43E02GB/35
OK, after studying a pic of the RHS of machine, this is how I think it works:-
The part 165281 is a combined NTC thermistor sensor and 85 deg C cut-out switch.
One terminal of the heater element is in series with the 85 deg C cut-out switch.
The other terminal of the heater is in series with a Thermal overload fuse.
So, my guess is that the NTC sensor measures a water temperature below set point demanded by the programmer. The programmer turns on the heater, via a relay located on the programmer PCB. Whe the measured water temp reaces set point, the heater relay is switched off.
If the measured water temperature reaches 85 deg C, the 165281 cut out switch deploys. If that fails, and the heating continues to an extent where the water is boiled off, then the Thermal overload fuse is melted, taking the heater out of circuit.
So when i next get to the machine, my investigation will begin with checking for any loose wiring connections / terminations around the heater area that I could have disturbed.
Then i will re-check the programmer PCB and heater relay for sticking.
However, it would be re-assuring at this stage to know if I am on the right lines with the above , or not!
Re: Overheating – Bosch SGS43E02GB/35
Well, all above was accurate apart from the anticipated thermal fuse. This was indeed a water flow or pressure switch.
The high resistance of the NTC sensor can be put down to the design of the sensor, in particular the construction of it. The thermistor pill is electrically connected to the terminals via no less than mechanical 5 interfaces, including 2 springs. All interfaces are silver plated. There is no hermetic sealing of the internals, the can being crimped to the terminal body. Thus over the years oxidation of the silver plating occurs, and with excess temperature caused initially by the seized pump, this probably increased the oxidation enough to give a higher resistance.
Strangely enough the genuine Bosch replacement sensor failed open circuit/very high R, as soon as it was fitted, and before the machine was switched on.
As the wash pump seal was still leaking, I have replaced the dishwasher with a new SGS45EO2GB. It will be interesting to see how long this one lasts.
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