1253 Rear axle braking Fig 1242ad

Rear axle — foot brake released (Fig. 12.42(a)) When the brake pedal is released the travel sensors within the brake value sensor (BVS) signal the electronic control module (ECM) which in turn informs the axle modulator to release the brake

Foot brake Proportional Proportional Pressure Single circuit Speed pedal (FBP) relay valve valve (PV) sensor (PS)/(diaphragm) sensor (nS)

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Compressor(C)

Four circuit protection valve (4CPV)

Foot brake Proportional Proportional Pressure Single circuit Speed pedal (FBP) relay valve valve (PV) sensor (PS)/(diaphragm) sensor (nS)

CfcH

Pressure limiting valve (PLV)

Brake value sensor (BVS)

Pressure limiting valve (PLV)

Brake value sensor (BVS)

3/2-way valve for auxiliary braking effect To rear (3/2-WV-AB) brakes

(b) Front axle - foot brake applied (normal brake operation)

(b) Front axle - foot brake applied (normal brake operation)

Fig. 12.41 (a and b) Electronic-pneumatic front brake system

(c) Front axle - foot brake applied under ABS/TCS conditions p, _xa

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(c) Front axle - foot brake applied under ABS/TCS conditions

(d) Front axle - foot brake applied with a fault in the electronic-pneumatics
Dual circuit
Fig. 12.42 (a and b) Electronic-pneumatic rear brake system

(c) Rear axle - foot brake applied under ABS/TCS conditions

r i "tt i i i ■

s ¡s

To front brakes •

Brake value sensor (BVS)

Coupling head for I supply (CHS)

Brake value sensor (BVS)

Coupling head for I supply (CHS)

I Coupling head for brake (CHB)

Relay | valve I Reaction (RV) I Piston

To front brakes

To rear brakes

I Coupling head for brake (CHB)

Relay | valve I Reaction (RV) I Piston

To front brakes

To rear brakes

(b) Trailer axles - foot brake applied (normal brake operation)

rear

RT front

rear

RT front

To front brakes

Fig. 12.43(a and b) Electronic-pneumatic tractor unit brakes coupled to towed trailer

To rear brakes

To front brakes

Fig. 12.43(a and b) Electronic-pneumatic tractor unit brakes coupled to towed trailer

To rear brakes

(c) Trailer axles - parking brake applied

(c) Trailer axles - parking brake applied

(d) Trailer axles - foot brake applied with a fault in the electronic-pneumatics

To front brakes

To rear brakes a

To rear brakes pressure in the wheel spring brake actuator brake lines. Consequently the axle modulator (AM) de-energizes the inlet and exhaust solenoid valves, causing the inlet valve to close and the exhaust valve to open, hence brake pressure will be prevented from reaching the spring brake actuator and any existing air pressure in the spring brake actuator (SBA) will be expelled via the exhaust solenoid valve. Air pressure in the pipe lines between the brake value sensor, the redundancy valve and axle modulator unit will also be exhausted by way of the foot control valve exit and the axle modulator (AM) exhaust solenoid valve exit.

Rear axle — foot brake applied (Fig. 12.42(b)) The rear axle reservoir tank delivers maximum supply pressure to the brake value sensor (BVS), the redundancy valve (RDV) and to the rear axle modulator (AM).

When the foot brake pedal is applied the travel sensor within the brake value sensor unit monitors the pedal movement and relays this information to the electronic control module. The brake switches will also close thereby informing the electronic control module (ECM) to operate the stop lights. The electronic control module (ECM) responds by signalling the axle modulators (AMs) to energize their corresponding inlet/exhaust solenoid valves. The exhaust valve will therefore close, whereas the inlet valves now open to permit rear reservoir tank supply pressure to flow via the 2-way valve to the dual circuit spring brake actuators (SBA) thereby operating the brakes. In addition, brake pressure is conveyed to the redundancy valve (RDV) where it flows though the 2/2 solenoid valve and then actuates the 3/2-way valve. This closes the 3/2-way valve, preventing redundancy circuit (pneumatic control pressure) control pressure reaching the axle modulator solenoid valves and at the same time exhausts the air holding down the relay valve's piston, hence it causes the relay valve to block the rear axle redundancy valve reservoir tank supply pressure entering the redundancy brake circuit.

Control of rear axle braking is achieved via the speed sensors giving feed-back on each wheel retardation or acceleration to the axle modulator and with the calculated brake pressure needs derived by the electronic control module (ECM) delivers the appropriate brake pressure to the wheel spring brake actuators.

Rear axles — foot brake applied under ABS/TCS conditions (Fig 12.42(c)) If the brakes are applied and the feed-back from one of the rear axle speed sensors registers excessive wheel lock/spin, then the electronic control module (ECM) will actuate the axle modulator (AM). Instantly the unstable breakaway wheel inlet/exhaust solenoid valves are energized, that is, the inlet solenoid valve closes and the corresponding exit valve opens, therefore blocking the air pressure passage leading to the wheel brake actuator and causing the air pressure in the actuator diaphragm chamber to collapse. The energizing and de-energizing (opening and closing) of each pair of solenoid valves is repeated continuously to adjust the magnitude of the wheel braking as demanded by the driver without the individual wheel locking when braking or for one of the wheels to spin due to poor road grip when the vehicle is accelerating.

During ABS/TCS braking conditions the redundancy (pneumatic circuit) brake system must not become active; to achieve this, the 2/2 solenoid valve is energized and closes so that air pressure is maintained underneath the 3/2-way valve piston. Accordingly the space above the relay valve remains open to the atmosphere and the 3/2-way valve inlet remains shut, hence locking out the redundancy brake circuit. Braking will revert to normal electronic-pneumatic control when the difference in wheel speeds is relatively small.

Rear axle — foot brake applied with a fault in the electronic-pneumatics (Fig. 12.42(d)) Should the electronic-pneumatic brake circuit fail, the axle modulator (AM) solenoid valve de-energizes so that the solenoid inlet valves close, whereas the exit valves open. Consequently compressed air under the 3/2-way valve piston escapes from the left hand solenoid exit valve thereby permitting the 3/2-way valve inlet valve to open. Foot control valve modulated brake pressure now enters the relay valve's upper piston chamber where it controls the delivery of redundancy circuit pneumatic pressure to both wheel brake spring actuators via the 2-way valves which are now positioned to block compressed air reaching the axle modulator solenoid valves. Note with the redundancy circuit operating there will be no active ABS at the front and rear axles.

Trailer axle — foot brake released (Fig. 12.43(a)) When the brake pedal is released the foot travel sensor signals the electronic control module (ECM) to release the brakes by de-energizing the proportional valve (PV). As a result the proportional valve (PV) inlet closes and its exit opens so that air pressure in the throttle valve and relay valve piston chamber is exhausted. Accordingly the relay valve inlet closes and its exit opens, thus permitting the brake pressure leading to the coupling head to collapse and for the brakes to be released.

Trailer axles — foot brake applied (Fig. 12.43(b)) Maximum supply pressure from the rear axle and trailer reservoirs is routed to both the brake value sensor (BVS) and to the proportional relay valve (PRV) respectively.

When the driver operates the foot brake pedal, the travel sensors located inside the brake value sensor (BVS) unit measure the pedal downward movement and feed this information to the central electronic control module (ECM). At the same time the brake switch closes thereby instructing the electronic control module to switch on the stop light.

The electronic control module (ECM) responds by directing a calculated variable control current to the proportional valve (PV) which forms part of the EPB-trailer control valve unit. Energizing the proportional valve's solenoid, closes the exit valve and opens the inlet valve in proportion to the amount of braking requested. Controlled air pressure now enters the relay valve piston upper chamber; this closes the exit valve and opens the inlet valve in proportion to the degree of braking demanded. Modulated brake pressure will now pass to the coupling head brake circuit where it is then relayed to the trailer wheel brake actuators via the trailer-mounted relay emergency valve.

Trailer axle — parking brake applied (Fig. 12.43(c)) With the 'park' hand control valve in the 'off' position the hand control valve central plunger closes the exit and pushes open the inlet valve. Compressed air from the parking reservoir tank is therefore able to flow to the relay valve part of the EPB trailer control valve via the open inlet valve inside the hand control valve.

When the 'park' control valve lever moves towards 'park' position the central plunger rises, causing the exit to open and the inlet valve to close. Air pressure therefore exhausts from above the lower control piston within the relay valve. Supply pressure acting beneath the reaction piston will now be able to lift the reaction piston and inner valve assembly until the upper control piston plunger closes the exit. Further upward movement then opens the inlet valve, thus permitting supply air pressure to flow through the partially open inlet valve to the 'coupling head for brake', and hence to the trailer attached relay emergency valve where it modulates the supply pressure reaching the wheel brake actuators.

Trailer axle — foot brake applied with a fault in the electronic-pneumatics (Fig. 12.43(d)) If there is a fault in the electronic-pneumatic system the proportional valve (PV) is de-energized, causing its inlet valve to close and its exit to open; pressurized air is therefore able to exhaust from the relay valve's upper control piston chamber via the proportional valve exit. Note the relay valve consists of an upper control piston and a lower assembly with upper and lower piston regions and which incorporates an internal double seat inlet and exit valve. As a result the upper control piston moves to its uppermost position, the inlet valve initially closes and the trapped supply pressure and the redundancy brake pressure (foot control valve pneumatic pressure) acting underneath the inner assembly's upper and lower piston region, pushes up the assembly against the hand control valve park pressure sufficiently to close the exit valve and to open the inlet valve. Brake pressure will hence be delivered to the trailer wheel brake actuators via the 'coupling head for brakes'.

Brake line to trailer defective (Fig. 12.43(b)) If the brake line to the trailer fractures the output pressure from the relay valve drops, causing the pressure above the throttle valve piston to collapse; this forces the throttle valve piston to rise and partially close the throttle valve, thereby causing a rapid reduction in the supply pressure flowing to the coupling head supply line (diagram not shown with throttle valve in defective pipe line position). As a result the relay emergency valve mounted on the trailer switches into braking mode and hence overrides the electronic-pneumatic circuit brake control to bring the vehicle to rest.

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