99+ P80 EBD (Electronic Brake-force Distribution)

[aharres]

First off, I've got a 1999 V70 T5 (no TRACS or STC whatsoever) with porsche 993 turbo calipers up front (big reds) and P2R calipers in the rear. I was wondering if anyone has any insight as to how the EBD system in our cars work. I believe it was introduced in 1999 and it will modulate the rear brakes to give a more well balanced brake force application. But I've noticed that in certain situations on medium to heavy brake application, there will be a rhythmic pulsating that can be felt in the brake pedal and can be heard in the rear of the car. It sounds and feels to me a lot like a singular "abs pulse" going to one or both of the rear wheels at a rhythm that correlates with wheel speed. I went and removed the big 50A fuse for the abs and this behavior is completely gone and the rear wheels lock up fairly easily. I also took my dad's 2000 S70 GLT SE which has STC and I noticed the same behavior on a specific piece of road that almost always triggers this behavior. It's a right turn lane that has a dip near the beginning and I'm assuming the EBD or ABS is getting triggered because the rear of the car wants to lift up coming off the dip while braking. It's stange though because it seems to persist during the entire braking period even after everything is settled and the road is smooth.

 

From the owner's manual: EBD is an integrated part of the ABS system. EBD regulates the hydraulic pressure to the rear brakes to help provide optimal braking capacity.

 

Has anyone else experienced this before?

Does the EBD use the same mechanics as the ABS system would to prevent locking of the wheels?

[apeacock]

 System description 
 
· The brake system, with integrated EBD function, has undergone major changes compared with the previous models. The type designation is ABS Mark 20, manufactured by ITT (TEVES). 
The most important changes are: 
 
 Diagonally split circuit brakes (LF+RR, RF+LR). 
Each wheel is individually controlled. 
New master brake cylinder (equal amounts of brake fluid in both circuits). 
Servo cylinder with pedal position sensor (The sensor is connected to the engine control module (ECM)). 
Hydraulic unit (two valves have been added to certain variants). 
New wheel speed sensors (Active sensors). 
The control module, connected to the network.  
 
The system can be complemented with a stability and traction control, Traction control alternatively STC. Car/variant/market dependent, standard/option. 
Common components for different car projects. 
 
 Hydraulic system 
 
Also new is that the rear wheels are also individually controlled which has meant that 2 valves per wheel have been introduced, 8 in total. If the system is supplied with Traction control or STC, 2 Hydraulic valves, 2 solenoid valves and 2 by-pass valve are installed. The control module is screwed into the ABS hydraulic modulator. The control module receives the wheel speed sensor signals which are put onto the network. 
 
 Wheel speed sensor 
 
The sensors are active, which means that they have a built in electronic circuit. The control module transmits a voltage of Ubat , the sensor sends back a digital power signal of 7 or 14 mA, depending on position. This solution gives a more robust signal, compared to the voltage signal. The current flow over the connections is higher which minimizes the chance of corrosion. 
 
 Pulse wheel 
 
S/V 70 have 48 toothed magnetic pulse wheels on all four wheels. 
 
 Control module 
 
There are three versions of the control module available: 
 
 ABS 
ABS+TC 
ABS+STC 
(the control modules are not programmable).  

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 Anti-lock Braking System (ABS) 
 
Is activated by the likelihood of the wheels locking during braking and ensures that the maximum braking effect is retained without the wheels locking. This is to retain the cars driveability and steering capacity. 
The function comes into operation at speeds in excess of 7 km/h (4 mph). At lower speeds the wheels can lock, but this has no practical effect on the brake or steering capacity. 
The ABS system is a four channel system. This means that each wheel is controlled individually. 
There is an inlet valve (C1, C2, C3 and C4) and outlet valve (D1, D2, D3 and D4) for each channel in the ABS hydraulic modulator. Furthermore there are two receivers (A1 and A2), one for the primary (LF+RR) and one for the secondary (RF+LR) circuits. 
In start position all the valves are in the resting position, that is to say that the inlet valves are open and the outlet valves are shut and the receivers have no pressure. 
There is a return valves for each inlet valve. The valve is opened by the high brake pressure at the wheel when the pedal is released. This makes the brakes release quickly and avoids “ delayed release” . 
 
 Anti-lock braking system (ABS) control 
 
When the brake pedal is depressed brake fluid is pushed out into the system and the brakes are applied. 
If for example the right front wheel displays a tendency to lock the following occurs: 
 
 the pump is started and is then running while ABS control is in operation. 
inlet valve (C4) closes. This means that the hydraulic pressure on the brake cannot increase even if the pedal is depressed further. This phase of the control is called pressure retention. 
If the wheel locks anyway the outlet valve (D4) opens and the brake fluid is allowed back to the pump, via the receiver (A2). The hydraulic pressure is eased and the wheel accelerates. This phase of the control is called pressure reduction. 
When the control module senses that the wheel accelerates the outlet valve shuts (D4) and the inlet valve (C4) opens. The hydraulic pressure increases and the wheel is braked. This phase of the control is called pressure increase. 
The control process with the three phases is repeated until the risk of the wheels locking is over or braking stops.  
 
In the illustration the front and rear right wheels are in the pressure retention phase, the left front wheel is in the pressure increase phase and the left rear wheel in the pressure reduction phase. 
 
 
When the pump is operating the brake fluid is returned to the lines between the master cylinder and the inlet valves. If one of the outlet valves opens at the same time as the inlet valves are closed the brake fluid will travel only to the master cylinder. This means that the brake pedal is pushed upwards. If one or more of the intake valves opens part of the brake fluid goes to the wheel brakes and the pedal is pushed a little way upwards, or sinks slightly. When the pressure is controlled it causes a pedal movement that signals to the driver that the ABS is operating. 
 
 
In the master cylinder pistons there are central valves. The valves are normally open and connect the space in front of the piston with the brake fluid reservoir via a channel. When the pistons are affected at braking the valves shut so that pressure can build up in front of pistons. 
 
 
The receiver compensates for the volume/pressure on pump intake side. After finishing the control the pump can continue for approximately 1 second, this is to ensure that the receivers are empty. 
 
 Electronic Brakeforce Distribution (EBD) 
 
Is activated during normal braking (without ABS control) and divides the braking force on the rear wheels. 
Normally there is a variation in the front and rear wheels slippage dependent on how hard braking is and how heavily the car is loaded. The Electronic Brakeforce Distribution (EBD) function ensures that the rear wheel speed is 0-2% higher than the front wheel speed. The function is, in principle, load sensing. 
The function is controlled by the control module and controls only the valves to the rear wheels. The pump does not operate, the brake fluid is taken up by the receivers.  

[aharres]

Awesome information! Did you get this from VIDA?

[apeacock]

10 hours ago, aharres said:

Awesome information! Did you get this from VIDA?

Yeah right from there. Didn't see anything else on it.