ABS/ASR Hydraulic Unit and Controls
ABS/ASR Hydraulic Unit:
The ABS/ASR hydraulic unit (A7/3) is located in the left front component compartment on all models. It is designed as a one-piece hydraulic unit, that is, it contains the hydraulic control components for the ABS and ASR hydraulic functions together in one housing. The ABS/ASR hydraulic unit (A7/3) has two plastic covers. The smooth cover snaps on in 4 places (arrows); a bolt holds the cover with ABS/ASR lettering.
Connection Identification Letters:
The identification letters on the connections refer to the following:
vl Left front brake
vr Right front brake
v Tandem master cylinder front wheel circuit
BA Reservoir connection (to ASR charging pump, M15)
Connection Identification Letters:
hl Left rear brake
hr Right rear brake
SP Bleeder valve
p Pump
E Pressure reservoir inlet
H Tandem master cylinder rear wheel circuit
Pressure Limiting Valve:
The pressure limiting valve (14) is a safety component and protects the hydraulic control circuit from excessive pressure.
The pressure limiting valve (16) protects the system against excessive pressure in the reservoir. If reservoir pressure exceeds 230 bar (3335 psi), the pressure limiting valve opens and releases excessive pressure back through line "BA" to the ASR charging pump (M15) and from there directly to the brake fluid reservoir.
The switchover solenoid valve (y5) switches the system from normal operation to ASR operation.
The switchover solenoid valve (y5) as well as the left and right rear wheel solenoid valves (y3 and y4) are in normal position "O" (non-energized), open for brake fluid passage during service brake applications. If the driver operates the brake pedal, brake fluid pressure from the tandem master cylinder acts directly on the brake calipers.
During an ASR control operation, the switchover solenoid valve (y5) shifts so reservoir pressure connects directly to the left and right rear wheel solenoid valves (y3 and y4).
The left and right rear wheel solenoid valves control the brake pressure in the right and left rear brake calipers during ASR or ABS operations, corresponding to the control signal from the ABS/ASR control unit in the three pressure phases:
^ Pressure increase
^ Pressure retention
^ Pressure release
During an ABS control operation the left and right rear wheel solenoid valves operate simultaneously.
The pressure switch (s1) monitors the pressure in the pressure reservoir (13). Below a pressure of approx. 160 bar (2320 psi), charging begins. Once a pressure of approx. 180 bar (2610psi) is reached, charging stops.
ASR RETURN/PRESSURE PUMP
In the ABS/ASR hydraulic unit, one motor drives both these pumps together. It drives the return pump directly and the pressure pump through a reduction gear. The ABS/ASR control unit controls the ASR return/pressure pump via the ASR return/pressure pump relay on the ABS/ASR hydraulic unit. The ASR return/pressure pump can only work together with the ASR charging pump, so the pumps all run simultaneously.