1251 Introduction to electronicpneumatic brakes Fig 1240

The electronic-pneumatic brake (EPB) system controls the entire braking process; this includes ABS/ TCS braking when conditions demand, and the layout consists of a single electronic-pneumatic brake circuit with an additional dual pneumatic circuit. The electronic-pneumatic part of the braking system is controlled via various electronic sensors: (1) brake pedal travel; (2) brake air pressure; (3) individual wheel speed; and (4) individual lining/ pad wear. Electronic-pneumatic circuit braking does not rely on axle load sensing but relies entirely on the wheel speed and air pressure sensing.

The dual pneumatic brake system is split into three independent circuits known as the redundancy braking circuit, one for the front axle a second for the rear axle and a third circuit for trailer control. The dual circuit system is similar to that of a conventional dual line pneumatic braking system and takes over only if the electronic-pneumatic brake circuit should develop a fault. Hence the name redundancy circuit, since it is installed as a safety back-up system and may never be called upon to override the electronic-pneumatic circuit brakes. However, there will be no ABS/TCS function when the dual circuit redundancy back-up system takes over from the electronic-pneumatic circuit when braking.

The foot brake pedal movement corresponds to the driver's demand for braking and is monitored by the electronic control module (ECM) which then conveys this information to the various solenoid control valves and axle modules (AM); compressed air is subsequently delivered to each of the wheel brake actuators. Only a short application lag results from the instant reaction of the electronic-pneumatic circuit, and consequently it reduces the braking distance in comparison to a conventional pneumatic braking system.

RT front w


RT front w


RT Tauxiliary

RT Tauxiliary nS


A list of key components and abbreviations used in the description of the electronic-pneumatic brake system is as follows:

1 Electronic control module ECM 13

2 Air dryer AD 14

3 Compressor C 15

4 Unloader valve UV 16

5 Four circuit protection valve 4CPV 17

6 Reservoir tank (front/rear/trailer/auxiliary/parking) RT etc 18

7 Brake value sensor BVS 19

8 Proportional relay valve PRV 20

9 3/2-way valve for auxiliary braking effect 3/2-WV-AB 21

10 ABS solenoid control valve ABS-SCV 22

11 Single circuit diaphragm actuator SCDA 23

12 Redundancy valve RDV 24

Axle modulator AM

Spring brake actuator SBA

EPB trailer control valve EPB

Park hand control valve P-HC

Coupling head for supply CHS

Coupling head for brake CHB

Travel sensor TS

Speed sensor nS

Pneumatic control front PF

Pneumatic control rear PR

Electrical sensors & switches E

Air exit (exhaust) x

Fig. 12.40 Electronic-pneumatic brake component layout

The electronic-pneumatic part of the braking system broadly divides the braking into three operation conditions:

1 Small differences between wheel speeds under part braking conditions; here the brake lining-disc wear is optimized between the front and rear axles.

2 Medium differences between wheel speeds; here the difference in wheel speed is signalled to the controls, causing wheel slip to be maintained


similar on all axles. This form of brake control is known as adhesive adapted braking. 3 Large differences between wheel speeds and possibly a wheel locking tendency; here the magnitude of the spin-lock on each wheel is registered, triggering ABS/TCS intervention.

Note antilocking braking system (ABS) prevents the wheels from locking when the vehicle rapidly decelerates whereas a traction control system (TCS)

prevents the wheels from spinning by maintaining slip within acceptable limits during vehicle acceleration.

The single circuit electronic-pneumatic brake circuit consists of the following:

1 Compressed air supply, the engine driven reciprocating compressor supplies and stores compressed air via the four circuit protection valve and numerous reservoir tanks. The compressor regulator cut-in and cut-out pressures are of the order of 10.2 bar and 12.3 bar respectively. Service foot circuits operate approximately at 10 bar whereas the parking and auxiliary circuits operate at a lower pressure of around 8.5 bar.

2 Electronic control module (ECM). This unit determines the brake force distribution corresponding to the load distribution. It is designed to receive signal currents from the following sources: foot travel sensors (TS), front axle, rear axles and trailer control air pressure sensors (PS) in addition to the individual wheel travel and speed sensors (nS). These inputs are processed and calculated to simultaneously provide the output response currents needed to activate the various electronically controlled components to match the braking requirements, such control units being the proportional relay valve (PRV), redundancy valve (RDV), front axle ABS solenoid control valves (ABS-SCV), rear axle module (AM) and the EPB trailer control valve (EPB-TCV).

3 Brake value sensor (BVS) unit which incorporates the pedal travel sensors (TS) and brake switches (BS) in addition to the dual circuit foot brake valve.

4 Redundancy valve (RDV): this valve switches into operating the rear axle dual circuit lines if a fault occurs in the electronic-pneumatic brake circuit.

5 Rear axle electronic-pneumatic axle module (AM) incorporating inlet and outlet solenoid valves used to control the application and release of the rear axle brakes.

6 Electronic-pneumatic proportional relay valve (PRV). This unit incorporates a solenoid relay valve which controls the amount of braking proportional to the needs of the front axle brakes.

7 Two front axle ABS solenoid control valves (ABS-SCV) which control the release and application of the front axle brakes.

8 Electronic-pneumatic brake-trailer control valve (EPB-TCV). This valve operates the trailer brakes via the trailer's conventional relay emergency valve during normal braking.

9 Parking hand control valve (P-HCV) which controls the release and application of the rear axle's and trailer axle's conventional spring brake part of the wheel brake actuators. 10 Pressure limiting valve (PLV). This unit reduces the air pressure supply to the front axle of the towing vehicle when the semi-trailer is de-coupled in order to reduce the braking power and maintain vehicle stability of the now much lighter vehicle.

A description explaining the operation of the electronic-pneumatic braking system now follows:

Front axle foot brake released (Fig. 12.41(a)) When the brake pedal is released the foot travel sensors signal the electronic control module (ECM) to release the brake, accordingly the proportional relay valve is de-energized. As a result the proportional valve's (of the proportional relay valve unit) upper valve opens and its lower inlet valve and exit valves close and open respectively, whereas the relay valve's part of the proportional relay valve unit inlet closes and its exit opens. Hence air is released from the right hand wheel brake-diaphragm actuator via the right hand ABS solenoid control valve and the proportional relay valve exit, whereas with the left hand wheel brake-diaphragm actuator, compressed air is released via the left hand ABS solenoid control valve, 3/2-way valve and then out by the proportional relay valve exit.

Front axle foot brake applied (Fig. 12.41(b)) Air supply pressure from the front axle reservoir is directed to both the brake value sensor (BVS) and to the proportional relay valve (PRV).

When the driver pushes down the front brake pedal, the travel sensors incorporated within the brake value sensor (BVS) simultaneously measure the pedal movement and relay this information to the electronic control module (ECM). At the same time the brake switches close, thereby directing the electronic control module (ECM) to switch on the stop lights. Instantly the electronic control module (ECM) responds by sending a variable control current to the proportional valve situated in the proportional relay valve (PRV) unit. The energized solenoid allows the top valve to close whereas the lower control valve partially opens. Electronic-pneumatic control pressure now enters the relay valve's upper piston chamber, causing its piston to close the air exit and partially open the control valve, thereby permitting pre-calculated controlled brake pressure to be delivered to the wheel-diaphragm actuators via the ABS solenoid control valves for the right hand wheel and via the 3/2-way valve for auxiliary braking effect and the ABS solenoid control valve for the left hand wheel. For effective controlled braking the individual wheel speed sensors provide the electronic control module (ECM) with instant feed-back on wheel retardation and slip; this with the brake pedal movement sensors and pressure sensors enable accurate brake pressure control to be achieved at all times. Note the electronic-pneumatic brake (EPB) circuit has priority over the pneumatic modulated front pressure regulated by the brake value sensor (BVS) unit.

Front axle foot brake applied under ABS/TCS conditions (Fig. 12.41(c)) If the brakes are applied and the feed-back from the front axle speed sensors indicates excessive lock/slip the electronic control module will put the relevant ABS solenoid control valve into ABS mode. Immediately the ABS solenoid control valve attached to the wheel axle experiencing unstable braking energizes the solenoid valve, causing its inlet and exit valves to close and open respectively. Accordingly the wheel brake-diaphragm actuator will be depressurized thus avoiding wheel lock. The continuous monitoring of the wheel acceleration and deceleration by the electronic control module calculates current signal response to the ABS solenoid control valve to open and close respectively the inlet and exit valves, thus it controls the increase and decrease in braking pressure reaching the relevant wheel brake-diaphragm actuator; consequently the tendency of wheel skid is avoided.

Front axle foot brake applied with a fault in the electronic-pneumatics (Fig. 12.41(d)) If a fault develops in the electronic-pneumatic system the proportional relay valve shuts down, that is the solenoid proportional valve is de-energized causing its inlet valve to close and for its exit valve to open. Consequently when the brakes are applied the proportional relay valve's relay piston chamber is depressurized, making the relay valve's inlet and exit to close and open respectively. As a result, with the right hand ABS solenoid control valves de-energized air will exhaust from the right hand wheel brake-diaphragm actuator via the ABS sole noid control valve and the proportional relay valve. However, the collapse of the electro-pneumatic control pressure in the proportional relay valve causes the closure of the 3/2-way valve passage connecting the proportional relay valve to the left hand wheel brake actuator and opens the passages joining the auxiliary relay valve to the left hand wheel brake actuator via the left hand ABS solenoid control valve. Thus if the supply pressure from the front axle brake circuit is interrupted, the redundancy (pneumatic) rear axle brake pressure regulated by the brake valve sensor's foot control valve shifts over the 3/2-way valve into auxiliary braking effect position, that is, the 3/2-way valve blocks the passage between the proportional relay valve and the ABS solenoid control valve and then supplies modulated brake pressure from the 3/2-way valve to the left hand wheel brake-diaphragm actuator. Therefore the left hand front axle brake only, is designed to support the rear axle braking when the electronic-pneumatic brake circuit fails.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook

Post a comment