Doh, it looks like this is not specific to me, and must be a HW issue. I just searched through the whole thread, and found this:
So I swapped out the 3 fans today.
....
New Fans
Sunon 40x40x20mm 3 pin fan #KDE1204PKVX
....
(FASTPATH Routing) #show environment
Temp (Celsius)................................. 0
It probably wasn't as obvious on FastPath, but it's clearly also showing 0 Celsius for that guy when 3-pin fans are used.
So I still don't understand why/how I got temp monitoring working on several boots when using three-pin fans.
I also don't understand how the lack of a connected 4th pin can mess up temp sensing. Maybe some low level issue? It's expecting a connection on that pin and that causes a HW bug?
Could I connect that pin to.. something? I don't really know how fans work. I know it's a PWM signal pin, so I suppose it sends a fluctuating voltage down it that controls the motor? I could put a high value resistor between it and ground? I don't want to risk damaging anything..
UPDATE: Ha! Yes that fourth pin sure is relevant. I got out my multimeter and, with the switch running and (Noctua 3-pin) fans operational, I (carefully!) probed the pins on one of the fan connectors:
- Between pins 1 & 2 (Black and red / Ground & 12V): +12V, of course
- Between pins 1 & 3 (Ground / Tach): Around 3 volts, slightly fluctuating
- Between pins 1 & 4 (Ground / PWM): Around 0.5 volts, slightly fluctuating
Then I went back to
show chass, planning to change the fan speed from 3 to down to 1, to see the effect on the PWM voltage. First thing I see:
Code:
Exhaust Side Temperature Readings:
Current temperature : 128.0 deg-C
Warning level.......: 80.0 deg-C
Shutdown level......: 90.0 deg-C
Intake Side Temperature Readings:
Current temperature : 128.0 deg-C
Hmm! Well that sure did something
What the hell is it using this fourth pin for, and why is it even connected to temp monitoring? Are they somehow monitoring temp
from the fourth pin?
Also notable is that 128 is well above the shutdown level, but of course it did not shutdown. So maybe shutdown is controlled using a separate temp sensor in the chip. Or more likely the code that controls shutdown knows the real temp, and it's just the temp I see that can be completely screwed by fourth-pin-manipulation. (Note that during all these tests I have a separate, external fan pointing right at the CPU to keep it cool regardless of how many fans are connected.)
This is really weird!
I tried connecting a few different resistors between ground and PWM with a 3-pin fan connected - 120k, 10k, 1k and 390Ω, all of which behaved the same: If connected on boot, I got the same 0.0 temps as usual. However if one of these resistors was connected between ground and PWM after booting (or disconnected and re-connected if already there from boot), show chass then showed the weird 128.0 temp I also saw when probing with my multimeter.
So.. I'm able to get
some result?
Not a very helpful one. I have no idea if I'm even on a useful path. Maybe it's actively monitoring this 4th PWM pin, despite it (I thought) being output only, and expects some specific and/or constantly varying resistance on it.
But can it even monitor the resistance of a pin it's also sending a voltage down? I suppose if it alternates between sending voltage and reading resistance. Maybe I should probe it with my oscilloscope. Not that there's much point; if it's doing anything complicated I'm not likely to be able to re-create it anyway.
My knowledge of electronics is sadly very limited. I'd really be grateful if anyone knows the first thing about fans/PWM etc and has any ideas. Maybe this concept of not working without a 4th pin is not uncommon?
I could just buy new fans, but now this feels like a challenge that needs to be solved
Anyway they're out of stock, so I can't buy PWM fans for a few days regardless.
Or I could just run with 3-pin fans and unable to see temps and detect dead fans. I have some confidence that even though
I can't see the temp, the shutdown routines probably can. Anyway if I just run the Noctuas at full speed, I don't expect to get anywhere near shutdown temps. It bothers me though, not being able to monitor