Copyright Goodheart-Willcox Co., Inc.
448 Hydraulic Systems for Mobile Equipment
A John Deere 4630 sprayer uses a jammer solenoid that pressurizes the
hydraulic system to a constant 3000 to 3100 psi (207 to 214 bar). In this applica-
tion, the jammer solenoid is constantly energized. The sprayer takes advan-
tage of the instantaneous response of the hydraulic system. One benefit of fi
this system over a true PC system is that the jammer solenoid can be disabled
during engine startup.
Jammer solenoids have the same effect whether a hydraulic system is open
center or an LSPC variable-displacement system. Any time the jammer sole-
noid is energized, the hydraulic system exhibits the characteristics of a PC
system: high system pressure, high noise, and responsive hydraulic controls.
The second reason for using a jammer solenoid is to eliminate the need of
a signal network with numerous shuttle valves (refer back to Figures 18-6).
A simple closed-center DCV can easily be incorporated into a variable-
displacement LSPC system with the use of a jammer solenoid. This system
design eliminates the use of a complex system of primary and secondary
shuttle valves. When manufacturers use jammer solenoids, they sometimes
use orifi in conjunction with the DCVs to control the cylinder speed. fices
John Deere 4940 and 4730 sprayers use a jammer solenoid, but not for all of
the system functions. The 4730 sprayer uses the jammer for everything except
adjusting the tread and raising and lowering the boom. The 4940 sprayer uses
the jammer solenoid for everything except adjusting the tread and tilt.
Signal Network Problems
A leak in a load-sensing signal network can cause other problems within a
hydraulic system.
Technicians were called to service an LSPC variable-displacement tractor
with an overheating hydraulic system. The technicians were having trouble deter-
mining why the machine was overheating. The OEM territory representative vis-
ited the technicians, examined the tractor, and requested that they follow the
specified service information. After carefully following the specified diagnostic
procedures outlined in the literature, the technicians were surprised at the source
of the problem. The service brakes had developed a leak in the load-sensing
circuit, which caused the pump to constantly run at higher flows and higher pres-
sures. This was the underlying cause of the overheating hydraulic system.
The technician found that the closed-center (service) brake control valve was
not fully blocking the pump’s flow when the foot pedal was released. The valve was
allowing fluid pressure to leak into the signal line leading back to the pump’s flow
control valve, causing the pump to upstroke. The brake valve was not serviceable. A
new service brake valve was ordered and installed, which remedied the overheating
hydraulic system.
Power Beyond in LSPC Systems
As mentioned in Chapter 9, power beyond in open-center systems simply 9
consists of a single coupler assembly. An LSPC power beyond option consists
of three coupler ports as shown in Figure 18-24. One port taps directly into