What’s happening?
The microwave does not heat well and cooking an item takes twice as long as it once did. The problem is likely a faulty microwave circuit. These are the steps to troubleshoot.
Step 1: After first checking the voltage supply, and it tests out OK, unplug the oven to remove the outer case, then plug it back in. Place a dish of water in the oven, and start a short cooking cycle.
Step 2: Take an Amp reading of the white wire on the front transformer, which is for the top microwave circuit. If the Amp reading is good, the top microwave circuit is drawing the correct amount of current.
Readings: Front transformer – white wire = ~ 7 Amps
Step 3: Test the rear transformer (for the bottom microwave circuit) for Amps by clamping your meter around the black wire. If the Amp reading is inadequate, the next step is to find the source of the problem in the bottom microwave circuit.
Readings: Rear transformer – black wire = ~ 7 Amps
Step 4: Test for VAC between the Common terminal (white) and the 230-volt terminal (red). The meter should read line voltage. If not, continue troubleshooting to identify which side of the terminal has the problem.
Readings: Red to white terminal = Line voltage
Note: Re-fill your container of water in the microwave so that it does not all boil away during testing. If the oven times out, start another cycle.
Step 5: Test for VAC between the Common terminal (white) and the 230-volt terminal (red). The meter should read line voltage. If not, continue troubleshooting to identify which side of the terminal has the problem.
Readings:
- HV Transformer 230-volt terminal to Ground: 120 VAC from the control board
- Com terminal to ground = 0 VAC indicates a problem on the high voltage side
Step 6: Upstream of the transformer are the Fuse and Line Filter, Magnetron TCO, Primary Interlock Switch, and the Board Relay – all of which should allow current to flow when the microwave is on at full power. Test each of these to identify the source of the problem. Reconnect the wires to the transformer before testing.
Readings:
- Fuse: Transformer Common Terminal to Line side of the fuse = 0 VAC (with unit in Cook)
- Fuse: Transformer Common Terminal to Load side of the fuse = 0 VAC (with unit in Cook)
- Board Relay during Cook (pin 1 to pin 2): 0 VAC
- Mag Thermal Cutout: Resistance across terminals with wires disconnected
Test all 5 interlock switches the same way.
Readings:
Not pressing the actuator:
- Com to NC = 0 Ω
- Com to NO = Open
After pressing actuator:
- Com to NC = Open
- Com to NO= 0 Ω
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