[rescue] Dell custom fan pinout

[Mouse]

I'm writing this largely to get the info below into the list archive,
in the hope that it may save someone else having to guess another day.

I had occasion to replace a fan today.  A fan had seized up (wouldn't
turn under power, would barely turn by fingers and wouldn't keep
turning on its own even when turned manually).

I had four fans which were the right size.  The machine had the usual
four-pin fan connector; one of my four spares had the usual three-pin
connector.  I put that one in place and promptly discovered why that
fan had been taken out of whatever machine it came out of: its bearings
are worn to the point that it buzzes loudly and moves comparatively
little air.

The other three had a different connector.  Their manufacturer marking
said "JMC DATECH" and they were also marked "MODEL 9825-12HBTA".  My
minimal Web skillz, on a work machine, informed me that this was
probably a Dell fan, with (unsurprisingly for Dell) a custom
idiosyncratic connector.

So I did a little searching.  I didn't find any clear statement of its
pinout.  But I did find hints that led me to guess, and my guess
appears to be good enough for the fan to work.

Here's an ASCII graphic of the fan connector - the one on the end of
the wires from the fan, that is, not the connector on the board - held
with the wires extending away from you.  That is, you're looking down
into the holes in the connector.

Strictly speaking, I'm guessing about the tach line.  But connecting
12V (it's marked as taking 12V) to the other two lines makes the fan
spin nicely, and the wire colours make it seem reasonable.  (The fans I
have have black to pin 1, red to pin 2, and white to pin 3.)  The
boxology to the left of the pins represents the locking clip, which for
my purposes is useless.

 ----+---+
| |  | o |    <-- pin 1: Ground
,-|--|   |
|    | o |    <-- pin 2: +12V
`-|--|   |=   <-- mechanical keying tab
| |  | o |    <-- pin 3: Tach
 ----+---+

It turns out, conveniently for me, that this is compatible enough with
the usual three-pin peecee fan connector that I could plug it in
without needing to cut any plastic off either connector.  Pleasantly,
the locking clip does not get in the way of doing this.

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