I took a Jacobs exhaust brake from a parts truck I bought and put it on my 96 one ton based RV. It was the unit supplied by Dodge/Cummins dealers for the 12V trucks. It works great until I hit just over 2000 rpm and then it cuts out. I can hear a click from the firewall where the pneumatic and electronic components are and then it opens the brake valve. 2000+ rpm is the optimum range for an exhaust brake to work and I cannot find an adjustment mechanism. I know others have this feature and was hoping someone familiar with this unit may know. Could be a bad pressure switch I guess, vacum pressure is normal. Any ideas appreciated!