The Valves.....pressure differential switch
In this page we'll cover the pressure differential switch functionality - its purpose and how it works.
Please note that most of the systems I cover will be related to or specific to AMC - American Motors Corp. Although trained as an automotive technician covering all aspects and brands, I did have my favorites. In the case of the pressure differential switch, the same principals apply to all companies.
The pressure differential switch is exactly what the name implies - it's a switch. There is a "shuttle" or piston inside that moves to one side or another upon the loss of pressure on one side of the system. The photos below will attempt to show what this switch is and does and how it works, and moreover, what it doesn't or can't do.
The pressure differential switch does not affect fluid flow or pressure to either side. It's job is strictly to warn of a pressure loss or difference.
Typically, it takes 80 to 150 psi. difference for it to "trigger". It does not have the ability to block fluid flow, nor does it need to with a divided "safety" type master cylinder.
The brake warning system has been required standard equipment since 1970. It is connected between the master cylinder and the brake wheel cylinders or calipers. It monitors differences in pressure in the brake lines of the two hydraulic sub-systems, and alerts the driver with a light if an imbalance occurs. When you turn the key to the ignition position, the brake warning light on the dash comes on during a "self-test".
The brake hydraulic system is divided into two sub-systems to increase safety. A pressure differential switch, connected to the dash warning light, is positioned between or straddles the two systems. If a leak occurs, and/or pressure in one of the lines is sharply reduced for any reason, pressure from the other side forces the piston to move to one side activating the switch and triggering the warning light.
There are two different designs shown below. One type uses two wires (one being ground) and complete a circuit when the plunger is raised. The plunger spring acts as the spring pressure on the piston keeping it centered. The other style uses a single wire and the switch is grounded internally when the plunger drops. This style uses a spring on each end of the piston to center it.
If brake fluid is found around the switch or center part of the unit, it must be replaced or o-rings replaced as it indicates an internal leak.
Click for a larger view.
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The inner workings: Shown centered, normal operations. |
Shifted to one side, triggers light, lost pressure on one side. |
Shifted to the other side, lost pressure, triggers light. |
Position relative to housing. |
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Shuttle pushed to the far right by pressure loss on the right. Note the center red line indicates where the switch plunger would be. The red line on the right shows how fluid can and will continue to flow. |
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Shuttle moved to the extreme left, raised the switch plunger (as noted by middle red line). Line on the left indicates how fluid can still flow pasat the end of the shuttle. |
You may be able to see the way the bore is cut so that fluid can continue to "flow" or pressure get by the shuttle no matter which position it is in. |
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The image to the right shows the switch in normal operation, piston centered and switch contact raised and not grounded. The green indicates normal and equal brake fluid pressure. |
The image to the right shows the switch in "tripped" operation, piston off-center and switch contact dropped, grounding it. The light green on the left side indicates normal brake fluid pressure which pushes the piston to the right where the blue indicates lower pressure, but unrestricted "flow". |
