Old Dog

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Cummins/Holset put different waste gates on based on the customers specs for full rated hp/torque. Waste gates were frequently used to de-rate engines to protect transmissions. Dodge did it for their auto transmissions on the 5.9L. You can go higher but will most likely damage something else. First step is to id what it should be, chances are it's 18 for a reason if everything is functioning correctly.
 

The problem is caused by dirt or contamination getting between the needle and seat of the soleniod vavle allowing a slow leak by. In addition to running the pump in both directions, first drop as largest rare earth magnet that will fit down into the hydraulic tank. It picks up the praticulant before it gets in the needle valves. Don't run the pump more then 15 seconds in a two minute period to prevent over heating the motor.
In case you have replace a soleniod vavle, they go for about $400 and you just unplug it and thread it off. Hard part is relieving the pressure before removal.
On my type of slide if it creeps in the it's extend soleniod for the slide. Slides are numbered g for 1 - Driverside front, counterclockwise to 2, 3, 4. And generator
HWH has instructions for identifying soleniods. Slideouts and bleeding procedure.
 
CLEAN EVERYTHING BEFORE WORKING ON THE HYDRAULIC SYSTEM or all the old dirt and oil will get in there. Engine scrubber and a toilet brush work good.
Hope the helps

As previous stated, white is negative in automotive applications (if black and white. If red and black, black is negative) if you wire it the way you described it will probably reverse all the functions since DC Motors are polarity specific. You will get 12 vdc on the white wire since you are testing battery voltage at the negative terminal of the battery. Green is NOT ground on 12dc automotive applications. You said 3 wires from the coach, white, green and orange, and from the chair you have Red/white and Red/black as well as 2 black and 1 orange. a lot of possible combinations.
You should consult a schematic for the system to sort it correctly. Good luck

The 2 Group 31 batteries are hardy up to the task if you have the bigger 525hp Cummins. I installed a 3rd one (really tight fit) and that helped. Are you familiar with the Battery Boost switch function? It ties together both house and starting battery banks, there is a good description in the owners manual. It will eliminate the problem assuming your Big Boy Relay is working correctly. Hope this helps