For a list of tested models, see the dellfand homepage -
http://dellfand.dinglisch.net/
UPDATE April 2011: for more thorough fan control, see the ik8utils package in the sixth post in this thread.
In theory Dell's bios should manage fan speed control via the Linux ACPI interface. So obviously you should not disable acpi with boot options. It's also worth checking the output of dmesg in the terminal. If you see something like "disabling acpi" then obviously this is why your fan is not turning on, and you should try to fix this by adding the boot option "acpi=force".
But if all else fails, go ahead and install this utility. The usage is -
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dellfand <mode> <sleep-seconds> <off> <low> <high>
0 - run in the foreground, print stats periodically
1 - run in the background as daemon, no output
sleep-seconds
dellfand will check the CPU temperature with this interval and adjust the fan speed according to the last 3 arguments
off
when the fan is on, turn it off when the temperature has dropped to this level
low
turn the fan to low speed when it reaches this temperature
high
turn the fan to high speed when it reaches this temperature
The default values, as defined in /etc/init.d/dellfand, are
off 30 low 38 high 40
but according to Google the temperature values for an Inspiron 3700 are
off 44 low 65 high 74
so it seems the default dellfand values are quite cool. It might be good to start with a compromise of 35/45/55.
So open an rxvt terminal and run this -
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dellfand 0 5 35 45 55
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dellfand 1 5 35 45 55