Under normal braking conditions, the master cylinder supplies fluid to the HCU and routes the pressure to the calipers. The solenoids are not energized allowing fluid to pass directly from the master cylinder to the caliper. The top solenoid is an isolation valve that isolates the HCU from the master cylinder during stability control interventions; more on that later.
The next valve goes by the many names including “inlet,” “apply” and “isolation.” When it is energized, this solenoid allows pressure form the pump to pressurize the circuit or allow other valves to dump the pressure and release the caliper.
Below the inlet valve is the release/outlet/dump valve. When energized, it dumps the pressure and releases the caliper.
The typical ABS intervention begins with the release and apply solenoid valves energized to get a locked wheel moving again. The main braking pressure is still applied by the master cylinder. The apply solenoid is cycled to allow the pump to maintain pedal height and proper braking pressure.
The outlet valve is closed and the inlet valve is opened. The pump applies pressure to the wheel.
If the wheel is still outside the wheel slip parameters, the cycle will start over. This happens very quickly. The operation of the solenoids and pump will cause a “kick back” or pulsation in the pedal.
ESC operations begin with the ESC isolation solenoid valve energized. This allows the isolation of the master cylinder from the HCU (yellow). The release and apply solenoid valves are also energized and the pump turned on so the correct action can be taken. This allows the charging of the accumulator. The apply solenoid is cycled to allow the pump to apply braking pressure to an individual caliper.
How Do Soleniods Work?
An ABS solenoid coil is energized by a power MOSFET (Metal Oxide Semiconductor Field-Effect Transistor). The circuit grounds the solenoid coil and the valve is opened or closed. A diode is used to dissipate the current in the collapsing solenoid coil. A solenoid coil should have between 10-15 Ω of resistance. But, on modern units, it is impossible to measure the values of the solenoids.