JLG Scissor Lift 77 Fault Code: Cyclic Program Test
Also called Indicates Motor Problems - Open Circuit Drive Motor Field Wiring, Problems with a Motor
Cyclic Program Test · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Code 77 on JLG scissor lifts covers a group of drive/pump motor and power wiring problems, including capacitor bank faults, low point A voltage, open circuit pump motor, and stalled pump motor conditions. It also appears as a memory/flash-test error that forces an emergency switch-off and blocks engine start. Drive and steer are prevented until the fault clears.
High severity. Code 77 stops drive and steer function and, in its memory-fault form, forces an emergency switch-off and blocks engine start. It should be treated as a stop-and-diagnose condition, not something to run through.
What does JLG Scissor Lift error code 77 mean?
Fault code 77 is tied to SPN/FMI 12 and groups together several related motor and power wiring problems reported by the controller. These include a capacitor bank that will not charge, a low reading at pump point A when the MOSFETs are off, point A collapsing when the MOSFETs pulse, and the MOSFET protection circuit activating due to heavy current drain.
In plain terms, the machine's power module is watching the wiring and switching devices that drive the pump motor. If it sees wiring errors, an open circuit in the motor, a stalled motor, or abnormal current draw, it locks out drive and steer to protect the electrical system and the motor itself.
A separate description ties code 77 to a memory or 'flash-test' fault. Here the controller continuously checks its own program memory, and if it detects an error it forces an emergency switch-off and prevents the engine from starting at all.
Common causes of 77
- Capacitor bank not charging, likely from a power wiring error causing illegal current drain, or a very low battery supply
- Pump point A reading low when the MOSFETs are off, likely from a power wiring error
- Pump point A collapsing when the pump MOSFETs are pulsed, likely from an open circuit pump motor or a power wiring error
- Pump motor stalled or drawing massive current, tripping the MOSFET protection circuit, from a stalled motor or a power wiring error
- Power module detecting an error in the power wiring at the F1 and F2 terminals for the drive motors
- Power module reporting a field open circuit fault code (16d), which is retained until power is recycled
- Program memory error detected during constant monitoring (flash-test), unrelated to wiring but tied to the same code
How to troubleshoot JLG Scissor Lift 77: first checks
- Inspect all pump and drive motor wiring for damage, loose connections, or corrosion, paying particular attention to the F1 and F2 terminal connections
- Check battery supply and overall power wiring for a low voltage condition that could prevent the capacitor bank from charging
- Look for signs of a stalled pump motor or physical binding that could cause the massive current drain the MOSFET protection circuit is reacting to
- Check the pump motor for an open circuit condition using appropriate motor testing procedures
- Cycle ignition off and on to see if the fault clears, since the field open circuit fault code is retained until power is recycled
How the code clears
For the power wiring and motor-related causes, check the power wiring and motor, then recycle power (switch off and back on) to clear the fault, since the field open circuit code is retained until power is cycled. For the memory/flash-test version of the fault, switch the ignition off and on again and check whether the fault clears. No other reset procedure is listed for this code.
Frequently asked questions
What does fault code 77 mean on a JLG scissor lift?
It points to a group of motor and power wiring problems, such as a capacitor bank not charging, low or collapsing voltage at pump point A, an open circuit pump motor, or a stalled pump motor tripping the MOSFET protection circuit. It can also appear as a memory (flash-test) error that forces an emergency switch-off.
Why won't my JLG scissor lift drive or steer with code 77 active?
The power module prevents drive and steer when it detects an error in the power wiring at the F1 and F2 terminals or a field open circuit fault, protecting the motor and wiring from further damage until the issue is checked.
Why won't the engine start after code 77 appears?
If the code is the memory/flash-test version, the controller forces an emergency switch-off and blocks engine start when it detects an error in program memory. Cycling the ignition off and on and rechecking is the listed step.
How do I clear code 77 on a JLG scissor lift?
Check the power wiring and motor for faults, then recycle power (turn the machine off and back on) to clear it, since the field open circuit fault is retained until power is cycled. For the memory-related version, switch the ignition off and on again and check again.
Can a low battery cause code 77?
Yes. A very low battery supply is listed as a possible cause of the capacitor bank failing to charge, which is one of the conditions behind code 77.
Is code 77 safe to ignore and keep working?
No. It prevents drive and steer, and in its memory-fault form it forces an emergency shutdown and stops the engine from starting. Treat it as a stop-and-diagnose fault before continuing operation.