Copyright Goodheart-Willcox Co., Inc.
446 Hydraulic Systems for Mobile Equipment
Jammer Solenoids and the Implications of Connecting the Pump
Outlet to a Signal Network
In Chapter 16, jammer solenoids were described in open-center hydraulic
systems. Jammer solenoids are also used in LSPC variable-displacement sys-
tems. Even though the solenoids provide the same effect—placing the system
on demand (maximum system pressure)—the jammer solenoid has an oppo-
site design when used in LSPC variable-displacement pump systems.
In an LSPC variable-displacement hydraulic system, anytime the pump
outlet pressure is directly connected to the signal pressure, the pump will sud-
denly enter the stall mode. Why is this? The fl ow compensator has pump outlet
oil pressure acting on the top of the spool while pump outlet oil pressure and
the fl ow control spring pressure are acting on the bottom of the spool. There-
fore, the fl ow control spool remains shifted upward, asking for more oil. As
the pump attempts to upstroke, however, the PC spool senses the high system
pressure and holds the pump destroked in the stall mode.
A jammer solenoid in an LSPC variable-displacement pump is normally
closed, which is the opposite of the design in an open-center system. See
Figure 18-23. When the jammer solenoid valve is energized, it opens to con-
nect pump outlet pressure to signal pressure. John Deere uses an LSPC jammer
solenoid on their sprayers. Case IH uses LSPC jammer solenoids on their com-
bines. AGCO uses LSPC jammer solenoids on their Massey Ferguson, Gleaner,
Challenger, and Fendt combines.
Case IH used the 12-volt diode assembly that was discussed in Chapter  16
on their 2100 and 2300 series combines. However, like most manufacturers,
late-model machines have the jammer solenoid directly controlled by the ECM,
allowing the manufacturer to eliminate an external diode module. The jammer
solenoid is sometimes called a signal valve solenoid because it creates a false
signal by connecting pump outlet pressure to signal pressure.
The Missing Piece
Understanding the theory behind jammer solenoids in an LSPC variable-
displacement system has helped an instructor diagnose an LSPC system built for
laboratory use. The system consisted of an LSPC variable-displacement pump, a
set of DCVs, a priority valve from a Case IH Steiger 4WD tractor, and a steering
DCV from a John Deere tractor.
The first time the hydraulic system was operated, it appeared to be operating
as a traditional closed-center PC hydraulic system. Time was spent reviewing the
fabrication of the hydraulic system, including the signal network, as well as having
conversations with the pump supplier. Eventually, the instructor focused on the
jammer solenoid concept, “anytime pump outlet pressure is connected directly
to signal pressure, the pump will enter the stall mode.” He used cross-sectional
drawings to look for a place where pump outlet pressure could be hydraulically
connected to signal pressure. The instructor determined that if the steering pri-
ority valve compensator spool was missing, the pump would immediately enter
the stall mode.