Monitoring the internal temp of a Latitude C640

You buy an expensive multi meter with temp probe and snake the sensor into the unit via the fan intake vents.

 

Speedfan is available at download.com, it works very well

8kfanGUI is a graphical Windows application to show the internal temperatures
and to control the fan operation on the Dell notebooks.
It's running under all current Windows versions (98/ME/NT4/2000/XP/Server2003).

copy from above link:

The fan control and temperature detection is based on the fan and thermistor tests
of the Dell diagnostics software. I got the technical details by disassembling portions
of the Dell diagnostics software, because Dell was not willing to publish information
on how to get control over the fans to prevent overheating.
So this program uses undocumented Dell SMBIOS functions to detect the temperature sensors
and to control the fans. ...

Compatibility list:...

Fan control - Read CPU temp - Read fan speed - Read fan status - Temp control

Latitude C640 - Yes - Yes - Yes - Yes - Yes ..."

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Operate the above utility in its "Observer Mode" to see your internal temperatures
and fan operation.

I could never get "Speed Fan" to work well on my D800.