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Fan reading problems and solutions |
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If you aren't getting the expected readings on your fans, |
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try the following: |
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- Do you get a fan reading in the BIOS or using a different OS? |
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If not, you may not have a fan with a tachometer output. |
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Look and see. Fans with tachometer outputs have at least 3 wires. |
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Fans with only 2 wires cannot, in general, report their speed. |
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- Try experimenting with the fan divisor settings: |
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Fan Divisor Settings |
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-------------------- |
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Fan Divisor Minimum RPM Maximum RPM |
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----------- ----------- ----------- |
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1 5314 1350000 |
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2 2657 675000 default on most chips |
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4 1328 337500 |
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8 664 168750 |
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16 332 84375 not supported on most chips |
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32 166 42187 "" |
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64 83 21093 "" |
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128 41 10546 "" |
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Pick a divisor so that the nominal RPM is about 50% |
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above the minimum. This is a good compromise between |
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margin and accuracy. Note that most chips only support |
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fan divisors of 1, 2, 4, and 8. |
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If you have a 0 RPM reading some or all of the time, |
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increase the divisor until you get good readings. |
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If you have a nominal reading less than 1.25 times the |
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minimum, increase the divisor to give you margin |
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so that you will not get spurious alarms. |
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If you have a nominal reading more than 3 times the |
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minimum, decrease the divisor to provide better |
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accuracy. |
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How to change fan divisors |
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Put an entry "set fanN_div X" in the appropriate section of |
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/etc/sensors.conf and run 'sensors -s' |
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(N is the number of the fan, and X is the divisor you want). |
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Further details |
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--------------- |
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Fan divisors are quite confusing. |
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Sensor chips count fan speed by using the fan signal |
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to gate an 8-bit counter driven by a 22.5 kHz clock. |
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So the _higher_ the counter value, the _slower_ the fan, |
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and vice versa. |
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The term 'fan divisor' is a misnomer because it doesn't divide |
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the fan signal, it divides the 22.5 kHz clock. |
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Thus you _increase_ the divisor if you have a slow fan. |
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The drivers account for the 'fan divisor' in their calculation |
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of RPM. So changing the fan divisor will NOT change the |
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nominal RPM reading, it will only affect the minimum and maximum |
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readings and the accuracy of the readings. |
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The actual formula is RPM = (60 * 22500) / (count * divisor) |
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The readings are most accurate when the fan speed is low |
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(i.e., close to the minimum possible RPM reading). |
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As fan speed gets closer to the maximum possible RPM reading, |
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the reading becomes quite inaccurate. Fortunately, most computer |
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fans do not approach 1,000,000 RPM ! |
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Fan readings 2X too high |
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This is rare, but can happen. This is typically caused by a fan which |
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outputs 4 pulses by revolution, instead of the standard 2. |
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As we learned above, you can _not_ fix this by changing |
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the fan divisor. You must add entries into the appropriate |
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section of /etc/sensors.conf: |
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compute fanN @/2, 2*@ |
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(N is the number of the fan) |
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------------------ |
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Copyright (c) 2000-2004 Mark D. Studebaker |
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