Johnson Controls P100AP-354 Pressure Control

How does this work? It has continuity at zero psi. My fan is running at start up and it doesn't get to 275 psi. I want it to cycle my fan for low ambient conditions.

Description
P100 Series switches are encapsulated,
nonadjustable, single-pole, single-throw
(SPST), direct-mount switches for use with
non-corrosive refrigerants. The switches are
available with automatic reset in both open
low and open high configurations and manual
reset with open high action.
Refer to the P100 Series Encapsulated
Pressure Controls Product Bulletin
(LIT-121485) for important product
application information.
Features
• compact size and light weight allow for
direct mounting
• trip-free manual reset (manual reset
models); contacts cannot be overridden by
continued depression of the reset button
• use with all non-corrosive refrigerants
• encapsulated switch, dust tight
• 1/4 in. Internal flare fitting with built-in
Schrader type depressor is standard
• electrical connections are made to 48 in.
leads
Applications
• low limit, high limit, and fan cycling control
in commercial refrigeration
• ice machines
• food service equipment
Repair Information
If the P100 Series Encapsulated Pressure
Switches fails to operate within its
specifications, replace the unit. For a
replacement switch, contact the nearest
Johnson Controls® representative.