Our microwave worked fine, but the convection oven part would not stop at pre-set temperatures and it would just keep getting hotter.
When I looked at the circuit board, found pair of wires identified as ‘oven temp’. I figured that there was a sensor at the end of the wires that was not working.
As it turns out, the wires should have a high resistance reading as they are connect to a thermistor, which is resistor that changes resistance as it gets warmer.
The wires measured zero resistance so I figured the thermistor is broken.
Found a place to buy the $20 thermistor and proceeded to remove our microwave to replace the thermistor, not knowing what was in store for us.
Our microwave oven is a Sharp model R-1830B and is original on our coach.
Thermistor part number is Sharp FH-HZA011WREC.
To remove oven from wall:
1. Remove two small bolts in plastic round protectors from the bottom back. These are wall latch safety locks.
2. The microwave ‘sits’ on a side to side ‘ledge’ of the wall mounts and will be secure until wall latch levers are pulled down.
3. Foretravel put two extra wood screws in the upper left and right through microwave into above cabinet corners.
4. These wood screws are visible after removing top vent by pulling forward. Remove the wood screws.
5. The next step will release the microwave from the wall latches allowing the microwave to swing forward from the top.
6. Using a flat screw driver, pry down the two wall latch levers noted in step #1&2, being sure to hold microwave so it does not swing off wall.
7. Microwave weighs about 70 pounds so four hands are needed for safe removal.
8. After tilting the microwave forward a little, reach up and pull the power cord and plug through the hole in the floor of the above cabinet.
9. The floor of the above microwave cabinet is a single piece of wood, unlike the other cabinets that have false floors.
To reinstall microwave:
1. Place rear on wall ledge
2. Feed power plug and cord up through cabinet hole.
3. Push hard to swing microwave back to wall and lock on to the wall latches.
4. Screw back the two small bolts with plastic round protectors to prevent the latches from being accidently being pulled down.
5. Put two wood screws back in to cabinet corners.
Thermistor is on left side of microwave and is visible after removing metal microwave cover, which is fastened with side and rear screws.
Remove two bolts holding thermistor in the convection heating element air flow. Unplug two wires and reverse to install new thermistor.
Microwave front – panel removed and cover off
Microwave left – cover off
Microwave left close – thermistor mounting hole visible below motor. Original thermistor has been removed.
Thermistor box – showing part numbers on label
Thermistor in – thermistor visible. This is the part that fits inside microwave.
Thermistor out – showing mounting holes and wire connectors
Wall latch release – view of inside of microwave cabinet. Wall latch fits in the rectangular hole.
Wall mount – mount is two pieces of steel firmly fastened to wall. Bottom of mounts have ledge that carries the vertical weight of the microwave.
Wall mount close – note the 9 screws and the fixed wall latch.
Wiring diagram – this is a schematic diagram from inside microwave cabinet.