I don't know if anyone knows, but I'd like to.

On the "classic" Dell systems that actually had parts inside that were
designed and maybe even assembled by Dell, there is clearly some type of fan
monitoring that is done by the BIOS. If you pull the plug on a fan, the
system will beep unhappily and tell you at POST that a fan is not
responding.

I've looked at a dead Dimension E521 board to see how they might do this, as
looking with software such as SpeedFan shows nothing. I found that this is
because the LPCIO simply doesn't have any fan speed reporting, temperature
reporting or fan control functionality. (Someone thought of using at least
temperature reporting for the E521--there are unpopulated spots on the board
clearly labeled as places for thermal sensors.) This is true of other Dell
systems, such as the Dim8400, 8300, 2400 and OptiPlex 170. (Systems like the
Dimension 2100 and its siblings (L5xx series) have temperature/fan
monitoring functions in their LPCIO ICs, but it isn't used or connected.)

I could not trace the board--it has too many layers. The mechanism for
actual fan speed control is obvious--a temperature sensor bulb that
protrudes into the airflow from the fan will speed it up if it gets hot.

I'd like to know how Dell does fan speed monitoring, and while I suspect
that this may require asking a Dell system board designer how it was done,
I'm hoping that someone on the group might know.

William