Pressure Dropping Phase

If the braked wheel still tends to lock in spite of constant hydraulic pressure, e. g. because of road surface changing from dry to slippery, pressure in wheel brake cylinder must be reduced. Current higher than approx. 5 amperes on the electric solenoid valve causes a greater piston stroke, which opens the outlet.

Hydraulic fluid flows to pump reservoir and return delivery pump. Return delivery pump returns the brake fluid into the brake master cylinder against existing pressure. A damper is provided in each brake circuit to suppress the delivery noise. The pump always runs during regulation, even if return delivery is not required.

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1 - Brake booster with tandem master cylinder

3 - Electronic control unit a — Front wheel brake b - Rear wheel brake c — Electric solenoid valve, front, right d — Electric solenoid valve, front, left e — Electric solenoid valve,rear f — Check valve g — Pump reservoir h — Return delivery pump i — Pump inlet valve k — Pump outlet valve t —Noise damper

Speed Sensors

The rotary motion of wheels is picked up by inductive sensors and sent to the electronic control unit in form of electric signals.

Speed sensors consist basically of a magnetic core and a coil. The pole piece is surrounded by a magnetic field. As the road wheel turns the teeth of pulse gears move through this magnetic field. This changes the magnetic flux and induces alternating voltage in the coil.

This alternating voltage changes its frequency according to the wheel speed and consequently can be used to measure the road wheel speed.

The Porsche 928 S has one speed sensor for each road wheel.

Front Axle Speed Sensors

1 he front axle employs so called cross pole sensors with axial tapping, they are installed in steering knuckles and secured with one socket head screw each.

Rear Axle Speed Sensors

Speed sensors of rear axle, so-called flat pole sensors with radial tapping, are inserted in the wheel carrier

Pulse gear wheel is cut direct in propeller shaft and also has 90 teeth.

Speed sensors on front and rear axles must be inserted against "stop". This wilt automatically give a clearance of approx. 0.35 rum. Adjusting Hie clearance is not necessary.

Electronic Control Unit

Left-hand drive vehicles

In left-hand drive vehicles the electronic control unit is located on the driver's side, above the hood release on the side panel of the footwell.

Right hand drive vehicles

In right-hand drive vehicles the electronic control unit is located above the central fuse relay board.

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It is a small, multi-channel electronic computer, in which acceleration, deceleration and slip values are derived from eleclric signals of the speed sensors. By way of logic interconnection of these values control commands are produced for electric solenoid valves in the hydraulic unit.

The entire signal process can be divided functionally into:

• signal processing section,

• logic section and

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