According to the 71 FSM (which is where all my info is coming from), 30 MPH is about the minimum for the cruise to kick in. According to the section on the cruise, it takes all 3 - electrical, vacuum and speed - all working in combination to enable the cruise to work. If you don’t have a FSM that covers the later style Cruise Control (70 or so), MyMopars has later manuals with the appropriate info available to download. To troubleshoot cruise issues (both factory and dealer add-on), it’s a necessity!
