Arduino Mega 2650 Voltage Read

Please help I am working with the Arduino Mega 2650, and have the Seeed Studio Relay Shield v1.3 attached.

I am trying to read voltage to A15. A15 is getting a voltage from a pressure sensor 5v in / ground / signal
signal is going to A15

using the example given I am getting crazy readings which appears to be from the "float" Is there a better way to read voltage on this pin? my sensor is going to be giving voltages like .02v .4v .73v .96v 1.2v 1.4v up to 3.8v so this float causes a big issue.

I am a beginner so any help would be appreciated.

the code i was using was the example voltage code and i just changed the pin A0 to A15

A link to the pressure sensor please.

I am scanning the documents i have on this sensor. Right now i can tell you it is a ford 7.3L ICP pressure sensor

I am having troubles uploading files to this board so here are 3 links
i know the reference voltage to this sensor is 5V

http://dyceroprojects.com/icp1.jpg

http://dyceroprojects.com/icp2.jpg

http://dyceroprojects.com/icp3.jpg