Unfortunately there is no accepted standard for the wiring of Volume or Expression controller pedals, different manufacturers wire their equipment in different ways so there is no assurance that a particular expression pedal will work with all equipment.
Some pedals use a mono plug and some use a stereo plug, so the required wiring will be different for each case.
Also the stereo plug can have the pins for the Tip(T), Ring(R) and Sleeve/Shield(S) can be wired differently, depending on the manufacturer. Below is an example of the Roland EV-5 wiring:
Stereo potentiometer wiring:
The wiring of the mono plug/socket is shown below:
The wiring of the stereo plug/socket is shown below: