Service Manual

SECTION 6 - TROUBLESHOOTING
6-42 – JLG Lift – 3121231
Platform (Mast) Won’t Stay Elevated
Overview Of Procedure
The following procedure requests that the lift down, dump, and pump internal valves be checked to see if any are stuck
open, it also examines the lift down and dump valve circuits. Also suggests that the lift cylinder packing could be leaking
internally.
Check For These Obvious Conditions First:
• Manual descent valve is closed tight.
Platform (Mast) Descends Too Slowly
Overview Of Procedure
The following procedure examines the mast some hydraulic components for obstructions and defects.
Table 6-47. Platform (Mast) Won’t Stay Elevated.
STEP ACTION SPEC YES NO
1. Is the lift down valve stuck open? — Repair or Clean
Valve
Go to Step 2
2. Lift down valves could be open due to incorrect electrical signal. — Check Pump Valve
Electrical Circuit
Go to Step 3
3. Oil could be passing around the lift cylinder bore packing. — Replace or Rebuild
the Lift Cylinder
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Table 6-48. Platform (Mast) Descends Too Slowly.
STEP ACTION SPEC YES NO
1. Check mast slide pads shimmed to tight. — Reshim Mast Go to Step 2
2. Is there an obstruction in the mast? — Remove
Obstruction
Go to Step 3
3. The lift cylinder packing could be too tight in the bore of the cylinder barrel. — Rebuild or Replace
Cylinder
Go to Step 4
4. Check if the lift down valve is opening completely. — Clean or Replace
Valve
Go to Step 5
5. Is there a restricted hydraulic line (smashed)? — Replace Hydraulic
Line
Go to Step6
6. Check the flow valve in the cylinder valve block for a restriction, i.e. dirt. — Clean or Replace
Flow Valve
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