Solved the fan problem;Found this 555-generator online (china)
I got the idea from this site:
The RPM and his calculations are for a different type of server.
I measured the frequency of the TACH wire on the original fans which is about 120 Hz at full speed.
So i set the frequency on the generator in that range
The indication I get now on UBUNTU: @HP-server:~$ sensors
Adapter: ISA adapter
Core 0: +46.0°C (high = +74.0°C, crit = +100.0°C)
Core 1: +46.0°C (high = +74.0°C, crit = +100.0°C)
Regarding the fan problem; I did some research and the only solution seems to be a new (quiet) fan with 3 wire plug (tach wire OK) or a pulse generator to emulate tach signal. A 3-wire fan 90mm goes around 50$ and up. That's 100$+ for an old machine.
The generator is a LM555 chip with a few resistors and a capacitor. Only a few $ at best.
Cannot find the component or temperature sensor on the MB driving the fans. Dismantled and cleaned the system thoroughly but still the same result.
My Linux OS distribution name and version: Ubuntu server on HP 5700
This is an older HP desktop I use as web and mail server. Lately the fans (CPU and Case fans) run at full speed for no reason. Temps on CPU and MB are below 40C. The fans (90mm 4-pin) are too noisy, so I want replaced with fans that run around 2000rpm quietly.
The problem now is that HP MB detects missing fans and does not boot until F1 on a keyboard is pressed. (there is no need for display or keyboard connected all the time since it is a server and access is via ssh) That means if power is interrupted (happens from time to time here) the server does not boot up because of the missing fans.
No i can not disable the fan detection in BIOS! So I'm thinking to connect pin 3 (tach) of the fan header to ground (-) to simulate fan presence.
Anybody has any experience with this?