I have a problem with fans on Mikrotik CRS354-48P-4S+2Q+.
I changed the preinstalled 3pin fans Chiefly CC4028B12M with 4pin fans Arctic S4028-15K. All 4 fan connectors in switch has 4pins.
Problem is that the original fans spins quietly at 4K after boot, but the new fans spins at 8K (not quietly) after boot.
Since all fan connectors in the switch are 4pins, I would expect that the mainboard accept both pwm and non-pwm fans. Also even PWM fans can be controlled by voltage, but it’s generally not the primary method for speed control. So, there shouldn’t be any problems.
I would expect the mainboard to read the speed of the fans and use PWM or at least set the appropriate voltage to slow the fans down.
Question is why the new fans spins double the speed and what can be done about it?
You can check 4th pin with a multimeter which is supposed to be PWM signal. What voltage are you getting ? From the datasheet of the new fans RPM range is from 1400-15000. What you can do to quiet it down is to pull PWM pin down to the ground and fan should run at the lowest speed.
What are the speed readings in system/health after you installed new fans ?
Also in system/health tab you have Settings where you can specify minimum speed of the fan.
According to datasheets, the old FAN has 7000 RPM max speed, and the new one has 15000 RPM max speed. Hence, the new FAN will rotate with double speed. Everything is logical.
Your expectations are wrong, because mainboard doesn’t control RPM as a value, it’s meaningless. It adjusts RPM based on temperature and some internal predefined PWM values, which were chosen for specific FANs installed on device.
If there are no any speed control settings in this device model, you may reduce a speed by installing some speed reduction adapter for PC FANs (it’s just a resistor in +12V power line).
The resistor values should be:
27 Ohms for around 30% speed reduction
37 Ohms for around 40% speed reduction
47 Ohms for around 50% reduction
There are pre-made cables with these values, example: https://www.aliexpress.com/item/1005004974264597.html