littleball_s4

Some little things about ABS:

1) The logic in the -87 car is only wheel speed related (system doesn't know about long or lat g's) but it's smart enough to know when the wheel(s) is stopping faster than a car ever would, so it releases the brake pressure in that channel(s), even if all 4 wheels have the same speed.

2) The rear end has two sensors, but the pressure is same left and right. This is so because if the car brakes with both left wheels on ice and both right ones on sticky tarmac, the front right will brake a lot more than the front left. This creates a spin moment, so the car will pull to the right. However, the rear braking force will be the same left and right (very low because of the left slipping a lot) creating no spin moment. If it had the same split pressures in the rear, that spin moment would add to the one you already have in the front, making it so high that the car would actually spin if you are not very skilled with the steering wheel.

3) The valve has 3 positions each of them is on/off:

3.1) Put the pressure the driver wants ("disengaged").
3.2) keep the actual pressure, ignoring driver if he wants more (like a check valve)
3.3) Open the circuit momentary to the reservoir, reducing pressure.

You can feel the abs when it uses position 3.3. As the pressure goes down, the wheel start spining faster, the abs goes back to pos 3.2. But if the grip improves, it needs more pressure, so goes back to pos3.1, the pedal is connected to the caliper one more time and you have to pressurize that part of the circuit, so the pedal goes down a little bit. If the grip is changing often (you are braking in a bumpy area, for example) or the system is not exactly 100% perfect, the cycles will be fast and it will feel like a vibration on the pedal. This is normal, although the "ideal" operation will have only one cycle per braking instead of several hundreds, as it happens in real life.

AO

Sorry, I re-thought this, and you are correct. My bad.