Inside braking systems 04 March 2014

Braking systems have undergone considerable development in recent years, but what does it mean for technicians? In the first of a new series of features for CV technicians, Brian Tinham talks to Peter Jones, technical training manager at MAN

Brakes aren't what they used to be... Or are they? Yes, modern electrically-controlled pneumatic braking systems are functionally quite different form their forbears. And, yes, they offer faster reaction times than the old direct-applied pneumatics and longer pad service life – not least because many are designed to monitor pad wear patterns and compensate for any disparity by adjusting braking air pressures side to side, or even wheel to wheel. But beyond the fancy stuff, you'll recognise mechanical arrangements of callipers, discs and pads that have changed little in years.

Let's remind ourselves of the basics. All the OEMs use main brake manufacturers, such as Haldex, Knorr-Bremse and Wabco. And most of their systems work in similar ways. Essentially, electrical impulses – generated by the ECU in response to pressure applied to the brake pedal – are sent via CANbus to each wheel module. These are converted to electrical power and drive a solenoid, which controls the admission of pressurised air to each local brake chamber. The air, in turn, acts on each calliper piston to press the pads onto the disc. Air is then exhausted. Meanwhile, conventional pneumatics runs in the background, providing for an automatic backup, in the event of the electrics failing.

All brakes – including those on the lift axles, whether lifted or not – are applied every time the brake pedal is depressed. On tractors, the rear two axles on a 6x2 are powered and controlled as one, left and right. Also, for ESC (electronic stability control) and AEBS (automatic emergency braking systems), four wheel speed sensors are deployed – two on the front steer axle and two on the drive axle, whether pusher or tag. More on this later.

Wear adjustment
One other point: most automatic systems are designed to sense if one side is wearing more than the other. They then adjust the air pressure during braking to trend towards rebalancing pad wear. That said, the automated system requires no particular attention: it's a matter of being aware and making sure it's working. If it's not, that may be a sign of the calliper not working properly.

So, for technicians, not a lot has changed. Health and safety first – and as a minimum you should ensure that equipment available includes a crane to lift the discs, the callipers and drums, where fitted.

Then start by checking the foundation brake pad clearances, whether as part of the PMI (preventive maintenance inspection), or if a driver has complained of pulling or some other brake-related defect. Use a feeler gauge, nominally set at 0.7mm – although a gap of 1.2mm may be acceptable, depending on piston movement. As always, don't check the clearance between pad and disc: if they're wearing unevenly, you're not going to get a reliable reading. Instead, check, between the pad and the piston.

Pad wear
What about pad wear? Most modern systems are equipped with pad wear monitoring, so you can check the dashboard display, if fitted. Simply call up the dash diagnostic menu and ramp through the axles, looking at left and right wear values. Not all trucks have this system and it's simple enough to check wear at the wheel by looking at the central groove wear indicator in the brake pad, assuming the vehicle owner bought OE replacements.

As for when to change the pads, there are different schools of thought, but most suggest that it depends on several factors. If your task is a scheduled service or inspection, then the question to ask yourself is: 'Will this vehicle stop effectively before the next intervention?'. So, if you know you'll see it again for the next PMI in six weeks, that's one thing. If you don't know you'll ever see it again, that's another.

Similarly, if it's a tractor unit trunking up and down the motorway network, brake demand is likely to be low, so you can take a long view. If, on the other hand, this is a tipper subject to a lot of off-road working, with all that means in terms of grit and muck forming a grinding paste for the pads, then you might want to be more cautious. Unless, of course, your tipper works locally and you'll see it again for the next PMI. It's a judgement call but remember, brakes are safety critical systems. As a rule of thumb, a figure of 25% pad thickness remaining should certainly be a warning level.

Pad changing
Either way, anyone changing brake pads should check that the calliper is sliding freely from side to side by hand, without sticking, when they open it up. They should also look out for wear on the calliper slider bushes and check that there are no splits in the rubber boots. You should also check the discs for cracking, crazing and corrosion.

Refer to the workshop manual for cracking depth tolerance and don't be guided solely by what you think you see. Incidentally, the same goes for the clearances: there will be variations between vehicles, both in terms of gvw and marque. Only when you're satisfied that all the above is satisfactory should you install the new pads, remembering to replace all of them on both sides.

Air pressure
Beyond the brake mechanisms themselves, you may need to consider braking air pressure, particularly if a problem has been reported by the driver. For routine checks, first read the pressures using the dashboard gauges, the truck's onboard diagnostic menu or the workshop diagnostics.

You should also look and listen for air leaks. If they're present, you'll notice leaks long before they're visible on the gauges. Likely causes might include an air pipe that's rubbed or a connector that's corroded. Take care to check all systems after depressurisation.

Similarly, examine the wiring condition from the harness to the brake components. Again, check that there are no signs of rubbing or exposed connections.

Note that, while load sensing valves are a thing of the past, many truck manufacturers were still using this older technology until around 2005, so you may still come across them. However, today load sensing is achieved through the EDC (electronic diesel control) unit. That module derives the vehicle weight from the power, and hence fuel, required to move the load. It requires no technician attention.

Note also that while the EDC cannot function properly, if a vehicle stops with ignition off over the brow of a hill – since fuel consumption moving off downhill will be low so the system will assume low load – in fact, the preset start condition is 125 tonnes of braking, so full braking power is available.

Automatic emergency braking
Moving on to AEBS, which uses the same the same front-mounted radar sensor as used in ACC (adaptive cruise control), where fitted, there are several key points to note. The sensor must be forward facing; its plastic case must be fitted, to avoid chipping of the glass lens; and the case must not be painted. Failures with any of the above will render the system useless.

The remaining task is to ensure proper set-up. First, check the rear axle wheel alignment, using standard equipment, to ensure that the vehicle is running square so the radar is facing forward. Then use a purpose-designed tracking gauge two metres in front of the truck to set the radar angle. Adjust the laser so that it hits the small mirror to the side of the sensor glass and reflects back to the cross hairs on the instrument. Don't align it to the middle. Your diagnostics will enable you to read the required settings from the ECU. Then adjust the truck-mounted radar, using the two alignment screws.

Electronic stability control
Finally, on ESC, while in the event of a fault, the controller is a bolt-on, bolt off unit, there are two points to remember. The system uses a pair of sensors, one mounted under the steering column, the other nominally in the middle of the chassis.

So, first, to avoid damage to the former, if the steering shaft has to be disconnected, the steering wheel must be maintained in the central position. The sensor itself is a coil so excessive turning of the steering wheel risks breaking the sensor.

Then, second, the chassis sensor must not be moved. The device is mounted centrally and facing forwards because it's designed to detect vehicle roll and yaw. If it is placed elsewhere to overcome some equipment packaging issues, the roll parameters it sees may not match the mapping program and the device could malfunction. Worse, if its orientation is also changes, it will see pitch instead of roll, and will again malfunction.


Before you do anything
Delphi warns that before you do anything, consider other equipment on the vehicle that may, if faulty, mimic faulty brake systems.

First, inspect the wheels: look for damage to the rim that might be causing buckling.
Next, check the tyre inflation pressures and wear patterns.
Third, check the adjustment of the wheel bearings.
Fourth, examine the suspension system, checking components for signs of wear or damage.
Wheel alignment problems may result in an incorrect diagnosis of braking system faults. For example, if the camber or caster has an out-of-tolerance problem, this can cause a vehicle to pull to one side, in much the same way as a brakes problem.
Once other relevant vehicle systems have been ruled out, it's time to focus on the brake system.


< strong>Checklist

1. Consider health and safety, and use appropriate equipment
2. Check foundation brake clearances
3. Check pad wear
4. Check calliper function
5. Check for calliper slider bush wear and boot condition
6. Check discs for cracking, crazing and corrosion
7. Check braking air pressure
8. Look and listen for leaks
9. Examine wiring condition from the harness to the brakes
10. Where fitted, ensure AEBS sensor is properly aligned.

Author
Brian Tinham

Related Downloads
59854\Inside_braking_systems.pdf

Related Companies
Delphi Diesel Systems
MAN Truck & Bus UK Ltd

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