My Journey:
iMac 27" 2020 performance slowed to a crawl after 20-30mins of use, main fan failed, CPU over heats.
Confirmed via iMac diagnostics, something wrong with main fan.
Opened iMac and confirm not spinning. Voltmeter reported 12v|-12V|2V|1V
Looks like onboard PWM has failed. Other issue was that plugging in replacements fans, this also killed them.
Based on this thread, I ordered "Noctua NA-FC1" fan controller $AUS40.00. Finding a new iMac fan was a bigger issue, but got there...
As I was lucky, having both the 12V supply + the -12V return on the original fan mainboard socket, I cut the old fan plug off and soldiered it to the Noctua controller supply. Only those are required to power this controller.
Thanks to the above thread confirmed the 4 x fan wiring from the controller. Nervously powered up the iMac - worked a charm.
Fan speed setting: This controller has manual fan speed setting and no way to adjust once the screen is taped up. Setting the fan speed to high results in excellent cooling, however the fan noise is high. I originally set the fan to midway speed setting, I found that once I temporarily attached the screen, it is was pretty quite, however the issue I had was that the microphone picked up fan noise. So I further slowed the fan to around 1/4 speed. The noise reduced, giving minimal microphone interference, but more pleasantly, the same heats tests I preformed, resulted in almost identical cooling. I did slightly open the ram door at the top a couple of mm's to allow for better airflow.
Taped up the screen - works a charm. Plus the fan turns off when the iMac is off or sleeping.
