TRUCK SERVICE MANUAL
the top of the normal compression stroke releasing the
compressed cylinder charge to exhaust.
The blow down of compressed air to atmospheric
pressure prevents the return of energy to the engine
piston on the power stroke, the effect being a net energy
loss since the work down in compressing the cylinder
charge is not returned during the expansion process.
Exhaust blow down occurs as follows:
1. Energizing the solenoid valve permits engine
lubricating oil to flow under pressure through slave piston
control valve to both the master piston and slave piston.
2. Oil pressure causes the master piston to move
down, coming to rest on injector rocker arm adjusting
3. The injector rocker arm adjusting screw begins
upward travel (as in normal injection cycle) forcing the
master piston upward and creating a high pressure oil
flow to the slave piston. The ball check valve in the
control valve imprisons high pressure oil in the master
slave piston system.
4. The slave piston under the influence of high
pressure oil flow moves down, momentarily opening
To Switch Terminal
exhaust valve, while the engine piston is near its top
dead center position, releasing compressed cylinder air
to the exhaust manifold.
To Key Switch
10 Diode Position For
5. Compressed air escapes to atmosphere
Clutch Switch No. 1495 Negative Ground
completing a compression braking cycle.
Fuel Pump Switch
12 Brake Units
Operating A Vehicle With Jacobs Brake
Fig. 20-5. (N12032 J). Wiring diagram
In order to retard a vehicle on a downgrade using the
Jacobs Compression Brake, and if the engine speed
exceeds maximum rated R.P.M. for a desired road
speed, a lower gear should be selected. The selection of
a lower gear will generally allow complete control of the
vehicle by the Jacobs Compression Brake, leaving the
vehicle service brakes in reserve to be used for
Energizing the Jacobs Brake effectively converts a power
producing diesel engine into a power absorbing air
motion transfer through a master slave piston
arrangement which opens cylinder exhaust valves near