In large tractors (18 wheeler) they have a 2 position switch for high and low engine brake or if you like to think of it differently partial jake and full. Not sure if your system is set up the same or maybe pre-wired for 2 with an option to upgrade..?
Glen & Robyn (A.K.A. Puff & Crickit) Full Timers..
2006 F350 6.0 with 3.73 gears, 4X4, Extra-Cab Long Box, Oil/Tranny/P.S. Coolers. Coolant filter. SCT Livewire.
2011 Cedar Creek 36RE, Auto levelers + too much to list..!