Pressure-Actuated Switching

The pressure switch responds to a rise or fall in system pressure by either breaking or completing an electrical circuit.

A Fluid System Control

A pressure switch may be used as a fluid system control in conjunction with a trigger gun. When the trigger gun valve closes, the increase in system pressure throws the pressure switch and shuts off power to the system.

The pressure switch may also be used with a pressure unloader to activate a timer in a system with complete automatic timed shutdown. When so employed, the switches installed normally open in a bypass port. When the system goes into bypass, pressure in bypass increases and the switch closes, activating a timer, which will shut off power to the unit after a preset interval. When the trigger is pulled, pressure decreases again, the switch opens and power is returned to the system. This installation will not work with a flow unloader where pressure is not retained in the system while it is in bypass mode.

Boiler Control
The pressure switch may be used as a boiler control as well. In this case the switch is activated to complete a circuit to supply power to the burner assembly. In other applications the switch may be used as either normally open or normally closed.

Switching Action

When the switch is used as a boiler control, it is generally installed so as to respond to an increase in system pressure. When system pressure rises, a plunger or piston rod in the system moves in it’s housing to activate or push a microswitch, which completes the electrical circuit to the burner. When system pressure falls, the plunger is returned by a spring to its normal position.

The Plunger And Mircroswitch
The pressure switch has two basic components: in the pressure switch, a hydraulic plunger which responds to pressure within the cleaner’s hydraulic or water handling system and an electrical component or microswitch which is activated by physical action of the hydraulic component.

The Plunger Assembly
Removing the threaded pressure inlet fitting from the housing will disassemble the pressure switch’s hydraulic component or plunger assembly. When the pressure fitting is removed, care should be taken that the spring or plunger does not fall out.

Inspect The Plunger Seal
The plunger seal and o-ring should be inspected for wear and replaced if necessary. Any debris or scale in the pressure fitting should be removed. The plunger and spring should then be replaced in the body and the pressure fitting screwed in until it is snug against the body. Some pressure switch plungers will have backup rings as well.

The Switch Assembly
The switch portion, which has a plastic housing, snaps onto the hydraulic component or housing, which is mounted in a port in the cleaner’s fluid system. O-rings seal the electrical portion from the hydraulic portion. A pair of machine screws at the clamp portion of the switch housing can be tightened to clamp the switch housing tightly onto the hydraulic housing.

Pressure Switch Wiring

The electrical portion of the switch is wired with three leads, only two of which will be connected, depending on whether the switch is intended for normally on or normally off operation.

In many instances the common wire will be striped. There is no overall standard for lead wire colors. The Giant 24015 pressure switch has a blue common wire, a brown wire for normally closed operation and a black wire for normally open operation. Either the brown or black lead will not be used. The unused lead should be cut short to prevent accidental contact with any other portion of the switch and capped.

When used as a fluid system control the pressure switch will generally be installed in the normally closed configuration. When used as a boiler control the switch may be installed in the normally open configuration.

Circuit Layout Must Take Pressure Switch Capabilities Into Account
When the pressure switch is used in most 115 V or larger electrical systems, a switching relay or magnetic contractor must be used. The switch, installed in a 24 volt or even low amp circuit, will complete the low voltage circuit, activating the relay or electromagnetic coil, which will cause completion of the high voltage circuit.

Most pressure switch types may be used without a relay if only the fuel solenoid and not the motor is being operated by the circuit the switch is on. The fuel solenoid has a very low amp draw, which is compatible with the microswitch’s rating and both may be included in the same low amp circuit.

Finding The Problem – Pressure Switch Troubleshooting
Diagnosis of pressure switch failure is relatively simple. Remove the inspection cover of the switch portion. On the most common type of switch this requires removing the four small screws, which hold the cover in place. There is normally an o-ring behind the side cover to help prevent moisture from entering the microswitch.

Check Plunger Action And Switch Activation
With the machine operating, check to see if the plunger is activating the microswitch inside the switch housing. If the plunger is not moving to trip this small switch, try tripping the switch manually with a small screwdriver or other device with an insulated handle. Simply depress the small plastic post on the microswitch to manually override the switch.

Check To See If Piston Is Stuck
If manually activating the switch activates the burner, the plunger is probably frozen in the threaded housing. Turn off the machine, making sure no pressure is present in the system and remove the piston housing from the machine. Manually work the piston to see that it is moving freely and clear any obstructing debris. Clean, reassemble and replace the switch on the machine. Lubricate the o-ring before reinstalling. If manual activation of the microswitch resulted in burner operation the burner should now function.

If Switch Can Be Activated Manually
If activating the switch manually did not result in burner operation, the switch is either defective or the problem is elsewhere in the system. Using the multimeter, check to see if there is continuity when the switch is closed. If there is no continuity the switch should be replaced. If there is continuity, the problem is elsewhere in the system.

Service Hint:
When checking pressure switch action, system pulsation should be at a minimum. Excessive pulsation can cause the plunger to rapidly or sporadically activate the switch. If such rapid or sporadic action is the problem, the pulsation problem should be tackled first. With it solved, it is likely that the switch will operate properly unless the pummeling it received from the piston damaged it and needs replacement.

Pressure Switch Troubleshooting
First check to see if the switch activates when the proper pressure is reached. The microswitch makes a click when activated or the inspection plate may be removed to observe switch action.

If the switch activates, use the multimeter to make sure current passes through the switch when the switch is activated. If current does not flow when the switch is activated, check to make sure the switch is wired in properly as normally on or normally off. If it is improperly wired, rewire it properly. If the switch is wired properly but current does not pass through it when appropriate (normally on or normally off), replace the switch. If the switch does not activate when proper pressure is applied, check to see if the plunger in the switch housing moves.

Ask, "Does the plunger move?" If the plunger does not more, it is probably fouled and stuck. Remove the plunger, clean it and reassemble it. Even if the plunger does move, it may not move far enough to activate the microswitch.

Ask, "Does the plunger travel far enough to activate the switch?" Check plunger travel by carefully observing switch operation to see if the plunger moves far enough to activate the switch. Check to see the switch can be activated manually.

If the plunger moves enough to activate the switch and the switch will activate, check to see that current is flowing properly through the switch. If the plunger will not move far enough to depress the switch, disassemble and clean the plunger assembly. If the switch still will not activate, see if the position of the microswitch can be adjusted, bringing the switch closer to the plunger. If the microswitch position cannot be adjusted, replace the entire pressure switch assembly.

Ask, "Is there proper current flow through the switch?" Use the multimeter to test the leads from the switch for the proper current flow. If current is flowing through the switch properly, check the other controls in the control circuit. If current does not flow through the switch, replace the switch.