URAL Motorcycles 4320M, Operation Manual, Page: 130 / 321

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Compressed air is fed from air tank to outlet I. Outlet II is connected with the brake
valve, and outlet III- to air chambers of axle brakes.
If there is no pressure at outlet II, piston 2 is up. Inlet valve 3 is close by spring force 4,
and discharge valve 6 is open.
When compressed air is fed to outlet II from brake valve , air enters the over-piston area -
chamber 1. Piston 2 by compressed air force moves downwards. Discharge valve closes, and then
inlet valve opens. Chambers connected to outlet III are filled with compressed air from the air
tank through outlet I and open discharge valve. The truck is braked via axle brakes.
4.5.2.8 ABS modulator is shown in Figure 108. ABS modulator is aimed at quick (milli-
seconds) pressure grow, drop or maintaining in brake cylinders during braking process depending
on control signals of the electronic unit.
Operating principle:
a) Pressure grow
Both valve magnets I and II are not exited, discharge hole of “i” valve and discharge hole
of “h” valve are closed. Preliminary control chamber “a” of diaphragm “c” there is no pressure.
Compressed air at the outlet 1 enters from chamber A through open inlet hole “b” to chamber B,
and then to brake cylinders through outlet 2. Simultaneously, compressed air also goes through
hole “d” in the chamber of preliminary control “g” of diaphragm “f”, and the discharge hole “e” is
closed.
1-Outlet; 2-outlet to brake cylinders; 3-atmospheric outlet; A,B-chambers; I,II-magnets; a,g-chambers of
preliminary control; b-inlet hole; c,f-diaphragms; d-hole; e-discharge hole; h,i-valves
Figure 108 - ABS Modulator
b) Pressure Drop
If ABS electronic unit gives a signal to drop pressure, valve I magnet is excited, “i” valve
closes the connection with outlet 3, and preliminary control “a” chamber gate opens. Compressed
air being inside chamber A flows into preliminary control chamber “a”, and diaphragm “c” closes
inlet hole “b” of chamber B. Simultaneously, valve magnet II switches, valve “h” closes the gate
of “d” hole so that compressed air being inside the preliminary control chamber can be discharged
to the environment through outlet 3. Diaphragm “f” opens discharge hole “e”, and brake pressure
at outlet 2 is discharged to the environment through outlet 3.