EBS system basics

This week's waffle is all about EBS-3 (Electronically controlled Brake System).

Brought in at euro 6 and After a few hiccups with the trailer control valve and EBS ECUs failing (check out this video here for that diagnosis) I'd say the system has been pretty solid. 

With our EBS system now having individual modules controlling brake operations connected via CAN to the EBS ECU, a Vehicle Stability Control module (I know it was an option on euro 5!) relaying lateral and vertical information and not to mention and brake equalization between tractor unit and trailer ... I'd say it's come along way in terms of functionality!

we will start at the business end and I will try not to Waffle on too much on how this all works as I may as well write a book on it (if only I was using this time to finish my manual on EAS-4!)

THE BRAKE SIGNAL ACTUATOR

Pressing the foot brake pedal does a few things, a micro switch is operated in the valve allowing the truck to identify a brake event is about to take place, all valves charge the brake Circuits with air known as in-rush or actuating pressure (someone correct me if I'm wrong on what that process is called please) 

This allows the pads to touch the brake discs and cuts down on application time.The further you push the pedal in the greater the the pressure, this is achieved by two brake position sensors which output a square wave pattern to the EBS ECU to tell the modulators what specific actuating pressures are required

So if we have a pedal doing all the measuring and modulators doing all the pressure why do we need a ECU now? 

Well the brake actuators might be smart but they can't calculate mass!

The EBS ECU 

All communication for the brake system comes through the ECU (engine and transmission on v-can 2) as trying to measure can resistance on the EBS system will give you some rather odd resistance readings due to the high speed network implemented so don't always think we're using 120 ohm resistors in this! 

All power and ground operation for EBS are controlled from the EBS ECU. So you have a total of 4 wires as an input to the modulators and VSC.

MODULATORS

I'm not going to go through all 3 modulators and the electrical operation as the principal is the same. Power, ground and CAN. THAT'S IT! 

Wheel speed is relayed through the modulators which passes the wheel slip data to the EBS ECU which calculates the required braking force and sends this back via CAN for the valve to implement the required pressure to enable deceleration for the duration of the brake pedal application. 

The valves contains 3 solenoids for operation, one for letting pressure into the valve, one for venting it and another for redundancy. The drive axle modulator even though it's one valve as a unit has this 3 solenoid set up for each side as unlike the front axle which has ABS VALVES the drive axle doesn't so implements individual control through these separately or together depending on wheel slip status. 

Trailer modulator

Electrically this valve is slightly more complex as we're dealing with trailer pressure and as such has an internal pressure sensor which measures the pressure to the trailer command line (yellow line) 

ABS VALVES 

Two valves Mounted in the front axle air pressure circuit. Nothing new here and have been around since I can remember. 

Used to implement individual braking of the o/s and n/s steering axle(s) via venting of the brake pressure that is applied from the front axle modulator to the brake chamber. 

Now all this electronics is all well and good when it works but what happens when it doesn't like in this week's video (EBS failure on a front axle modulator) well each vehicle is fitted with a pneumatic back up system or REDUNDANCY which provides a fail-safe in the even of EBS failure. This system is why we still have air pipes to and from the foot brake valve which provide a supply to the foot brake valve and signal feed from the foot brake valve to the front, additional, rear axle modulators and the trailer control valve. 

Well that's enough of that and I know it's ONLY basic operations and electrical functions I hope it helps you understand the system ABIT better and what your up against as technicians trying to diagnose these trucks and why not only understanding the system is important but coming up with a methodical testing plan is paramount to ensure a fixed right first time repair