Brand:LG Model Number:MHEC1737D Main Symptom:Cooling Fan for the Magnitron and other internal components seems to have slow down drastically. What happens & when: The microwave just turned two years old and whenever we have used microwave section of the oven you could always distinctly hear the cooling fan running at an audible speed. Then, one day while using the microwave, the cooling fan suddenly slowed down and to the point that you could barely hear it. I purchased a new cooling fan motor from Appliance Parts Pros, installed it, and you could distinctly hear the cooling fan running at an audible speed. However, that only lasted for approximately a week or so, and once again, now the new cooling fan seems to have slowed down to a point that you can barely hear it. I took the microwave back down from over the stove and removed the sheetmetal casing. I plugged it in to another outlet and ran a cup of water through a 30 sec warmup at 100% power and the cooling fan is running enough to feel the air blowing over the internal cavity, but very quietly. I checked the voltage to the cooling fan motor during the water warmup and had approximately 113V at the motor leads. I also purchased a new Control PCB from Appliance Parts Pros, installed it and had the same result. Have you ever heard of this kind of issue before with the cooling fan and should I just go ahead and continue to use it this way?
Error Code (if any): I have ran the ThinQ Diagnostic on my phone and it said everything is OK. No error codes have ever shown up.
Parts or tests already tried: I purchased a new cooling fan motor from Appliance Parts Pros and then a new Control PCB, but still have the same issue.
Photos / video link:
Thanks for the detailed explanation and for sharing the voltage reading. You’ve already done quite a bit of troubleshooting, and the fact that the replacement cooling fan worked normally for about a week before becoming quiet again is an important clue.
On your LG MHEC1737D, the cooling fan helps remove heat from the magnetron and other internal components. I would not recommend continuing to use the microwave until we confirm that the fan is moving enough air. Inadequate cooling can cause the magnetron to overheat and potentially damage other components.
The good news is that you’ve already replaced both the fan motor and the main control board, so I wouldn’t recommend replacing either one again just yet.
Here’s what I’d check next.
1. Check for restricted airflow.
Since the replacement fan initially worked normally, I’d first look for something that could be restricting airflow or putting additional strain on the motor.
With the microwave unplugged, check the following:
Make sure the fan blade turns freely without rubbing or binding.
Inspect the fan housing for anything interfering with the blade.
Check the air passages and exhaust openings for grease, dust, or other obstructions.
Make sure the fan blade is securely attached to the motor shaft.
Also, verify that the replacement fan motor is properly positioned and that its blade isn’t contacting the housing.
2. Consider the voltage reading.
You mentioned measuring approximately 113 VAC at the fan motor while the microwave was operating.
That’s useful information, but we need to know the motor’s rated voltage before deciding whether 113 VAC is normal.
If the motor is rated for 120 VAC, 113 VAC is slightly lower than nominal but wouldn’t automatically explain such a dramatic reduction in fan speed.
A loose connection, damaged wiring, or a voltage drop under load could also contribute to the problem.
3. Check the fan motor wiring and connections.
Since you’ve already replaced the control board, I’d pay particular attention to the wiring between the control board and cooling fan.
With the microwave unplugged, inspect any safely accessible connections for loose terminals, discoloration, or signs of overheating.
A poor electrical connection could potentially explain why the fan worked normally after the motor was replaced and then began acting up again.
Important safety warning: Microwaves contain a high-voltage capacitor that can retain a dangerous electrical charge even after the microwave is unplugged. I don’t recommend operating the microwave with its outer cover removed or performing additional internal live-voltage tests unless you’re trained in microwave high-voltage safety.
My recommendation
At this point, I wouldn’t purchase any additional parts until we determine whether the fan is actually turning too slowly or is simply operating more quietly.
The fact that you’re still feeling airflow is encouraging, but it doesn’t necessarily mean the magnetron is receiving adequate cooling. Also, the ThinQ diagnostic reporting no problems doesn’t rule out a cooling airflow issue.
One question that will help narrow this down:
When the replacement cooling fan first started running quietly after that first week, did you notice that the amount of air coming out of the microwave’s exhaust openings had also decreased significantly, or is the airflow still about as strong as it was when the fan was running loudly?
That distinction will help us determine whether we’re dealing with an actual fan-speed problem or a change in operating noise.
Jeffrey, thanks you for your quick response. I would have to say it has decreased somewhat, but not 100% sure. I just ran through a check of the noise filter board on the main power coming in and it all checked out good. The voltage drops on both the incoming and outgoing from 122V to 115V, so I think the 113V at the cooling fan motor is reasonable. What I can’t explain is how the louder fan noise and cfm output suddenly dropped on the old motor after 2 years of operation that way and then dropped on the new motor after only 2 weeks of operation. Very strange and makes me wonder if some other component could cause such a drop. I wish I could test the fan motor to see what rpm it is turning, as the oven schematic shows it as a 2650rpm motor. The schematic does say 35-55 ohms, so should I check that either not running or running?
Not running is about 38-39 ohms. Would it be advised to use an external 120V supply to try to power the fan motor to see if it spins faster doing that?
Powering the fan at 122V straight out of the wall socket produced the fan speed and noise that I remember it had always produced for the first 2 years and right after I installed the second motor for 2 weeks. It is definitely turning faster and more cfm.
I noticed in the schematic that the power to the cooling fan also looks to supply the stirrer motor. There is a brown power wire to the cooling motor on one terminal, a yellow wire coming out on another terminal to the stirrer motor, and black and white coming out of another terminal, all on the cooling fan motor.
The brown wire is the positive power wire from the control board to the cooling motor. The yellow wire is a positive power wire to the stirrer motor from the cooling motor, and the black and white wires look to be a negative wires from the stirrer motor (white) and cooling motor (black).
Could the stirrer motor be dragging the voltage down on the cooling fan motor? I have confirmed that the stirrer motor is working, but it could be going bad possibly.
On the stirrer motor or the cooling fan motor? I am a retired Mechanical Engineer and definitely not a wiz at this electrical stuff. If you could tell me what the resistance test would be precisely I would very much appreciate it.
OK and is that running or not? Also, if I unplug the stirrer motor, maybe I can then try using the 30 second cycle to heat some water and see if the cooling fan motor runs at the higher speed and cfm?
So online it says the resistance of the stirrer motor should be 2-3 ohms and it tests at 128 ohms. I also ran a 30 second cycle with the stirrer motor unpluged and the cooling motor ran perfect, high speed and much more audible noise. I plugged the stirrer motor back in and the cooling motor still is running correctly, but I’ll it would last for long until the stirrer motor drags it back down. Wow, so glad to hopefully have solved this mess and I guess I better get a stirrer motor ordered. Thanks for all your help and insight Jeffrey, it really made a difference.