10-22-2016, 08:44 PM
Good catch. That switch activates the low air pressure warning light on the dash if the pressure drops below 60 psi. www.bendixvrc.com/itemDisplay.asp?documentID=4935
"The electrical contacts provided in both the LP-2™ and LP-3™ indicators remain closed by spring force until the air brake system pressure below the diaphragm is above the setting (force) of the low pressure indicator spring. The setting of the indicator and piece number is marked on a label on the valve body. If a label is not present, then the vehicle manual should be consulted for the proper setting. The nominal setting of the indicator is 60 psi; however, pressure settings may vary depending upon the vehicle.
OPERATION
To describe the operation, we shall assume that the low pressure indicator is set for 60 psi. When air pressure at the supply port and under the diaphragm is above 60 psi, the
electrical contacts remain open because the force exerted by air pressure underneath the diaphragm overcomes the force exerted by the spring above the diaphragm. When air pressure below the diaphragm drops below 60 psi, the spring exerts a force which is greater than the force exerted by the air pressure below the diaphragm. This causes the diaphragm (and the piston in the LP-3™ indicator) to move and allow the electrical contacts to close. This completes or closes the electrical circuit to the warning device, warning the driver of low air pressure in the system."
"The electrical contacts provided in both the LP-2™ and LP-3™ indicators remain closed by spring force until the air brake system pressure below the diaphragm is above the setting (force) of the low pressure indicator spring. The setting of the indicator and piece number is marked on a label on the valve body. If a label is not present, then the vehicle manual should be consulted for the proper setting. The nominal setting of the indicator is 60 psi; however, pressure settings may vary depending upon the vehicle.
OPERATION
To describe the operation, we shall assume that the low pressure indicator is set for 60 psi. When air pressure at the supply port and under the diaphragm is above 60 psi, the
electrical contacts remain open because the force exerted by air pressure underneath the diaphragm overcomes the force exerted by the spring above the diaphragm. When air pressure below the diaphragm drops below 60 psi, the spring exerts a force which is greater than the force exerted by the air pressure below the diaphragm. This causes the diaphragm (and the piston in the LP-3™ indicator) to move and allow the electrical contacts to close. This completes or closes the electrical circuit to the warning device, warning the driver of low air pressure in the system."