JLG 600AJ Aux Power Pump and Emergency Descent Failure

Symptoms

• Aux power switch clicked down but zero motor sound is heard from engine bay
• Emergency down descent operates extremely slowly or boom does not drop physically
• Heavy cable heat or burning plastic odor when the aux switch is held
• System alerts show high current load warning on diagnostic screen

Possible Causes

• Burned out or failed high-current 12V DC auxiliary motor starter relay
• Seized or heavily worn internal mechanical design on the 12V auxiliary gear-driven hydraulic pump
• Main power feed high-amperage terminal fuse blown in structural engine harness box
• Worn high-discharge carbon motor brushes or wet winding inside the aux electric motor

Diagnostic Procedure

1. 1
   Locate and Verify High-Current DC Aux Relay

   Locate the direct chassis-mounted starter contactor relay wired to the aux pump motor. Direct measure input power post (must be stable battery positive +12.6V). Command auxiliary power from the ground box and measure output post voltage.
2. 2
   Perform Voltage Drop Test across Aux Circuit Cables

   Connect multimeter. Measure voltage drop along the ground return cable from the aux motor frame back to the negative post of battery. Check for drops exceeding 0.5V, which indicates loose joints.
3. 3
   Test Hydraulic Solenoid Block During Aux Travel

   Ensure that when aux switch is activated, the auxiliary hydraulic bypass block valves are also powered to allow pilot-to-open system pressure back to tank. Proving 12V is hitting auxiliary bypass coils is vital.
4. 4
   Manually Rotate Pump Shaft to Check for Seizing

   Disconnect battery. Remove external protective shroud. Try to spin the coupling plate connecting electric motor to hydraulic pump by hand. If the assembly is stuck and cannot be rotated, the dynamic internal gears of the pump are seized.

Common Repairs

• Replacing the high-amp 12V DC auxiliary starter solenoid switch helper
• Replacing the complete dynamic auxiliary hydraulic gear pump
• Rebuilding or replacing high-discharge auxiliary backup batteries

Safety Precautions

• Never run auxiliary 12V DC pump motors consecutively for more than 2-3 minutes; these starters run extremely hot and can weld internal contact points.
• Ensure boom paths are physically clear of obstacles before forcing manual descent override circuits.

Required Tools

• High-Current Amperage Probe (0-200A DC)
• Digital Multimeter
• Battery Cable Terminal Cleaners
• Socket Sets

Related Fault Codes

People Also Ask

• How do you test the auxiliary power pump motor on a JLG 600AJ?
• What causes JLG fault code 4-3 aux power circuit overload?
• Where is the emergency descent power relay located on a JLG 600AJ?
• Why is my JLG boom lift emergency down function operating extremely slowly?
• How to check for a seized 12-volt DC emergency pump on a JLG lift?
• What is the amp draw limit for JLG auxiliary power unit motors?

Related Guides

Suggested Follow-Up Guides

• Step-by-Step Replacement Guide for JLG 600AJ Aux Power Relays
• Rebuilding JLG 12V DC Emergency Pump Motors
• Diagnostic Guide: JLG Ground Control Box Control System Fault Code 1-1
• JLG 600 Series Hydraulic Schematic and Emergency Descent Valve Cleaning
• Testing and Calibrating Pressure Relief Valves on JLG Auxiliary Pumps

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