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In an answer to another question, @FredWilson made the following comment:

At idle expect to see the fans off much longer than they are on in a normal system. The ratio of off to on depends on outside air temperature and humidity. At 75 degrees F and 50% humidity we expect to see 10 to 20 percent on time. On a hot, low humidity day fan on times of up to fifty percent are sometimes seen.

I was wondering if there might be somewhere a generalized table of fan on / off ratios by temperature and humidity which could be useful as an indicator of cooling system health.

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  • Fans will need to be on according to the power being developed by the engine, also a/c will, on some makes, put the fans on full time automatically. – Solar Mike May 18 '17 at 16:26
  • I have not seen any data on this from the manufacturers. I developed my own rule of thumb and they the just rough estimates. – Fred Wilson May 18 '17 at 16:51
  • For cooling system health, I would be more concerned with does it get up to temperature and does the fan kick on. You have several components of the cooling system - radiator, heater core, fan, pump, thermostat, hoses, passages, and coolant. If one of these fails you will either see leaks, and engine that doesn't get up to temp (thermostat stuck open), or engine that overheats. – rpmerf May 18 '17 at 20:40

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