5-29
5
BOOM
TROUBLESHOOTING
When the boom control lever is pulled slowly, the boom drops at first
1
Check the load check
valve of the control valve
Faulty
Repair
or replace the housing
block
Normal
2
Check
for leakage inside
the boom cylinder
Faulty
Repair
or replace the boom
cylinder
1. Check the load check valve of the control valve
a. Disconnect the hose connecting the control valve
and the boom cylinder from the control valve.
b. Connect the hose just disconnected to the boom or
bucket block.
c.
Try to operate the boom cylinder.
If operation is normal, the load check valve is faulty.
Before performing steps “a” through “c” above, set
the machine as follows.
• Fully extend the bucket cylinder, completely re-
tract
the arm cylinder and lower the bucket to the
ground.
When the spool first starts to pull, the oil’s pressure
and the flow rate are small, so if the load check valve
is faulty, it causes a load to act on the boom cyl-
inder
, which causes the oil to flow backward for a
time. This is what causes the boom to drop at first.
2. Check for leak inside the boom cylinder
If the oil is leaking from the head side (A) to the rod
side (B) due to a faulty arm cylinder piston or tube, the
lift arm first drops because of the low oil pressure and
small flow rate when the spool begins to pull. The driv-
ing
force of the cylinder is also insufficient. In this case,
the spontaneous drop of the boom becomes large.
• Check the cylinder
“5-27 and 28”
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