MAP sensor

[tom cloud]

>How does it work:  To be honest I don't understand the chemistry of how a 
>signal is generated proportional to the pressure that is exerted on the 
>sensor.  I do know that the MAP output is in Hz

Pressure sensors are commonly made using a diaphragm as one plate
of a capacitor -- when pressure is exerted on it, the capacitance
changes.  The easiest way to read capacitance is to make it part of
an L-C tank or an R-C network in an oscillator.  Then, changes in
capacitance cause a change in the frequency of oscillation, which
can be directly related to the pressure on the diaphragm.

Then, if one side of the diaphragm has a vacuum on it, the output
is considered 'absolute' (PSIA).  If it is exposed to the ambient
atmospheric pressure, it is called 'gauge' (PSIG).  Of course, these
terms apply to any type pressure sensor -- i.e. whether the reading is
absolute or gauge.