JLG Scissor Lift 321 Fault Code: Line Contactor Miswired On or Welded
Line Contactor Miswired On or Welded · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Code 321 on JLG M3369 and M4069 scissor lifts means battery voltage was already present at the power module B+ terminal at power up. The controller sees this as the line contactor being stuck closed or wired wrong, and it locks out drive, steer, and lift up until it's resolved.
High severity. The machine disables drive, steer, and lift up functions when this code sets, so it cannot be safely operated until the contactor circuit is checked and fixed. There is also a risk of a welded contactor meaning the machine is electrically live even when it should be off.
What does JLG Scissor Lift error code 321 mean?
Fault 321 is set when the power module detects battery voltage at its B+ terminal right at power up, before the line contactor should have closed. Normally the line contactor is the switch that connects battery power to the drive system only when the controller commands it. If voltage is already there before that command, the contactor is either welded shut (stuck closed internally) or its coil circuit is wired incorrectly so it is being energized when it shouldn't be.
Because this is a safety-critical power path, JLG's control logic responds by blocking drive, steer, and lift up functions entirely rather than letting the machine run with an uncontrolled power connection.
This code affects the M3369 and M4069 scissor lift models specifically.
What triggers a JLG Scissor Lift 321 code?
Battery voltage was present at the power module B+ terminal at power up.
Common causes of 321
- Line contactor contacts are welded closed internally, so they never open even when the coil is de-energized.
- The contactor coil circuit is wired incorrectly, causing it to be energized or closed when it should be open.
- Wiring faults between the contactor, the battery positive terminal, and the power module B+ terminal.
- A failed power module that misreads the B+ terminal voltage.
How to troubleshoot JLG Scissor Lift 321: first checks
- Check the wiring of the contactor for correct routing and secure connections.
- With the contactor disconnected from the machine, check resistance between its studs to see if the contacts are welded closed.
- Check the contactor main contact wiring back to the battery positive terminal and to the power module B+ terminal for shorts or miswiring.
- Check continuity between contactor connector pin 1 and ground board socket J1-19.
- Check continuity between contactor connector pin 2 and power module 12 position connector terminal 8.
- Measure voltage between the power module B+ and B- terminals. If 24V is present, replace the line contactor.
- If the contactor checks out, replace the power module.
How the code clears
No separate clearing step is listed. The fix is physical: correct the contactor wiring or replace the welded contactor, and confirm no voltage is present at power module B+ at power up before the machine will run normally again.
Affected models and serial ranges
321 appears in our records across 2 JLG Scissor Lift models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| M3369 | Serial range not listed in source records |
| M4069 | Serial range not listed in source records |
Frequently asked questions
What does JLG fault code 321 mean?
It means battery voltage was already present at the power module B+ terminal when the machine was powered up, which points to the line contactor being welded closed or its coil wired improperly.
Can I still drive or lift the machine with code 321 active?
No. JLG's control logic blocks drive, steer, and lift up functions whenever this code is active because the contactor issue affects the main power path to those systems.
How do I know if the contactor is welded shut?
Disconnect the contactor from the machine and check resistance between its studs. If there is continuity when the coil is not energized, the contacts are welded closed and the contactor needs to be replaced.
What voltage reading confirms the contactor needs replacing?
Measure voltage between the power module B+ and B- terminals. If 24V is present there, the line contactor should be replaced.
Which JLG models can get fault code 321?
This code applies to the JLG M3369 and M4069 scissor lift models.
If the contactor is fine, what else could cause code 321?
Check the wiring between the contactor and both the battery positive terminal and the power module B+ terminal, and verify continuity on contactor connector pin 1 to ground board socket J1-19 and pin 2 to the power module 12 position connector terminal 8. If everything checks out electrically, the power module itself may need replacing.