JLG Boom Lift 55 Fault Code: Problems with Vehicle Speed or the Encoder
Problems with Vehicle Speed or the Encoder · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 55 on the JLG M600 boom lift covers a group of drive system faults tied to vehicle speed measurement and the speed encoders. It can mean no movement was detected at full traction motor power, one or both speed encoders reading out of range, invalid encoder pulse counts, a tilt/positrac module communication failure, or a vehicle runaway condition. Because several of these sub-faults involve loss of speed feedback or uncontrolled movement on grade, treat code 55 as a drive safety issue until diagnosed.
High severity. Some sub-conditions under code 55 are informational (drive continues at cutback on one encoder), but others (runaway, no movement detected, both encoders faulty, tilt module not communicating) directly affect vehicle control and grade safety. Treat any occurrence as a stop-and-check situation before resuming drive operation.
What does JLG Boom Lift error code 55 mean?
Fault code 55 is a family of related messages on the JLG M600, all connected to how the controller measures and manages vehicle travel speed using the left and right speed encoders and, in one case, the tilt/positrac module.
Encoders tell the controller how fast the wheels are actually turning. If the controller commands full traction motor power but measures no speed, or measures speed in the wrong direction, or gets encoder voltages that are out of range or pulse counts that are impossible, it cannot trust the drive system and will limit or stop vehicle movement.
Some of the sub-faults are less severe: if only the left or right encoder is out of range, the machine will keep driving at a reduced (cutback) speed using the good encoder. Others, like both encoders failing, a runaway condition, or the tilt module not communicating, are more serious and affect the machine's ability to control speed and grade roll.
Common causes of 55
- Traction motor fault
- Power wiring error to the traction motor or encoder circuits
- Speed encoder fault (either left, right, or both)
- Brakes not releasing even though brake pressure reads normal
- Vehicle overloaded so the motor cannot turn the wheels
- Speed encoder input voltages out of range (one or both encoders)
- Speed encoder indicating an impossible number of pulses (invalid speed reading)
- Speed encoder fitted incorrectly, causing a reversed/runaway speed reading
- Positrac/tilt module not communicating with the controller
How to troubleshoot JLG Boom Lift 55: first checks
- Stop driving and note exactly which of the code 55 messages is displayed, since the fix path differs for each (no movement, left/right encoder, both encoders, tilt module, runaway, invalid speed).
- Inspect the wiring and connectors to both speed encoders and the traction motor for corrosion, looseness, or damage.
- Confirm the brakes are actually releasing, not just that brake pressure reads normal, since brake pressure can read normal while the brakes stay applied.
- Check that the encoder(s) are mounted and oriented correctly; an incorrectly fitted encoder can cause a false runaway/reverse-speed reading.
- Verify the machine is not overloaded beyond what the traction motor can move.
- Check for communication faults or wiring issues at the positrac/tilt module if the display indicates a tilt module failure.
- If only one side's encoder is faulted, confirm the machine is indeed driving at cutback speed on the remaining good encoder before continuing to a full diagnosis.
How the code clears
No separate clearing step is listed for fault code 55. Diagnosis should focus on identifying which of the listed sub-conditions triggered the code (encoder wiring, motor, brake release, overload, tilt module, or incorrect encoder fitment) and correcting that specific issue. Once the underlying wiring, encoder, motor, brake, or module problem is fixed, normal operation should resume; retest vehicle movement and speed feedback before returning the machine to service.
Affected models and serial ranges
55 appears in our records across 1 JLG Boom Lift models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| M600 | Serial range not listed in source records |
Frequently asked questions
What does JLG fault code 55 mean on an M600 boom lift?
It is a group of drive-related faults tied to vehicle speed and the speed encoders. It can range from a single faulty encoder causing cutback speed, to both encoders failing, an unreleased brake, an overloaded machine, a tilt module communication failure, or a runaway/reverse-speed condition.
Can I keep operating the machine if only one speed encoder is faulty?
The machine will continue to drive at cutback speed using the remaining good encoder, but this should be treated as a warning to get the faulty encoder diagnosed and repaired promptly, not as normal operation.
Why does the machine show no movement even though the traction motor is at full power?
This can be caused by a traction motor fault, a power wiring error, a speed encoder fault, brakes that are not actually releasing despite normal brake pressure readings, or the vehicle being overloaded so the motor cannot turn the wheels.
What causes a vehicle runaway warning under code 55?
It usually means the controller measured speed in the wrong direction while the traction motor was at full power. This is most often due to a speed encoder being fitted incorrectly, but it can also result from a speed encoder fault or the same conditions listed for no movement detected, especially if the machine is on a grade.
What does the tilt module failure message mean under code 55?
It indicates the positrac/tilt module is not communicating with the controller. When this happens, the controller defaults to assuming a massively tilted condition and does not attempt to prevent vehicle roll on grade, which is a significant safety concern.
Is fault code 55 safe to ignore if the machine still drives?
No. Even when the machine continues driving at cutback speed on one working encoder, code 55 points to a drive control or safety system issue that should be inspected and repaired before continued use, especially given the runaway and tilt module scenarios it covers.
Does brake pressure reading normal rule out a brake problem as the cause?
No. The diagnostic description specifically notes that brakes may fail to release even when brake pressure reads okay, so a normal pressure reading does not confirm the brakes are actually releasing.