Load Moment Limit Exceeded, Boom Functions Cut Out

Common Symptoms

• Cab display shows LOAD MOMENT EXCEEDED or a load moment warning icon with an audible alarm
• Boom raise and boom extend functions stop responding mid-cycle
• Yellow or red warning lamp on the instrument cluster illuminates and stays on
• Machine still allows boom retract and boom lower to recover the load
• Load indicator gauge on the cluster is pegged at or past the red zone
• Travel and steering may still function but boom controls remain locked out
• Alarm continues until boom is retracted or lowered to bring moment back within the chart limit

Probable Causes (Ranked by Likelihood)

• Load on the forks exceeds the rated capacity at the current boom extension and height combination per the load chart Very Likely
• Boom position sensor (angle or extension) sending an out-of-range or drifted signal, causing the controller to calculate an incorrect moment value Likely
• Load cell or pressure transducer on the lift cylinder circuit reading high due to contamination, damage, or calibration drift Likely
• Active work mode set incorrectly, for example machine in Rough Terrain or Outrigger-Down mode when outriggers are retracted, shifting the load chart Possible
• Attachment weight not configured in the controller, causing the net fork capacity to be overstated and actual total moment to be underreported until a heavy load is added Possible
• Hydraulic system pressure spike from a worn or stuck relief valve causing a false high reading at the lift cylinder transducer Less Likely
• CAN wiring fault or intermittent connector between the boom sensors and the load moment controller causing erratic moment calculations Less Likely

Step-by-Step Diagnostic Procedure

1. Step 1 -- Before anything else, retract and lower the boom to a safe carry position with the load close to the mast. Confirm the alarm clears. If it clears, the machine calculated a real overload. Cross-reference the load on the forks against the load chart decal in the cab for the actual boom angle and extension you were using.

2. Step 2 -- Check the active work mode on the cab display. Verify the mode shown (Standard, High Capacity, Rough Terrain, Outrigger, etc.) matches your actual setup. Switching to the wrong mode shifts the load chart limits and can trigger false lockouts. Correct the mode if needed and retest.

3. Step 3 -- With the boom fully retracted and lowered and no load on the forks, cycle the boom raise and extend. If the fault returns immediately with no load attached, you have a sensor or calibration issue, not a true overload.

4. Step 4 -- Inspect the boom angle sensor and boom extension sensor at the boom pivot and along the boom tube. Look for damaged wiring, bent sensor brackets, or connectors with corrosion or spread pins. Wiggle the harness while watching the load indicator on the cluster. A reading that jumps with harness movement confirms a wiring fault.

5. Step 5 -- Using CAT ET (Electronic Technician) software via the J1939 service port, read active and logged DTCs. Pull the SPN and FMI codes associated with the load moment system. Check live data for boom angle sensor output in degrees and extension sensor output in millimeters. Compare to expected values at a known boom position. Values that are stuck, erratic, or out of specification confirm a faulty sensor.

6. Step 6 -- With CAT ET connected, verify the attachment configuration stored in the controller matches the actual attachment on the machine. If a heavy work tool is mounted and not registered, re-enter the correct attachment weight per the CAT setup procedure. Recalibrate the load moment system using the on-screen calibration wizard in CAT ET if sensors and wiring check out.

7. Step 7 -- If sensors and wiring are good and the calibration is correct, check lift cylinder hydraulic pressure with a calibrated gauge at the test port on the lift circuit. Compare to the specification in the machine's service manual for the installed configuration. A relief valve stuck open will cause low system pressure and a lift capacity shortfall that can mirror a moment fault.

8. Step 8 -- If the fault persists after all of the above and CAT ET shows no sensor DTCs, escalate to a CAT dealer. The load moment controller itself may need reprogramming or replacement. Do not bypass or disable the load moment system. It is a safety-critical function under ISO 11228 and ANSI/ITSDF B56.6 requirements.

Frequently Asked Questions