S3 E-Type

The PDWA, Pressure Differential Warning Actuator

The device is located next to the servo, its got a 2-pin electrical connector going to it's centre. Mine was "always on" and wouldnt reset. Opening it up is clear that there is gradual corrosion of the aluminium interfering with it's action.

Although there are 2 pins on the connector, they are shorted together, and the device works by a simple switch inside the plastic body shorting the pins to earth (and bringing the warning light on).

The thing that looks like a mangled spring is actually an earth strap between the black plunger in the plastic body and the aluminium casting.Beneath the plunger is a ball-bearing. This ball is what pushes the black plunger up in an imbalance situation.

Pic below shows the whole unit stripped out, there are pistons connected to each brake circuit and in mechanical contact with each other, so that the 2 pistons can slide side-2-side when there's a differential pressure. The ball bearing sits in the groove of the left piston. You can see how that will be forced up once that piston tries to move sidewards. In my case i just cleaned everything up & put it back together, it now works fine...