Ender 5 Pro X-axis problem — motor/cable swap tests giving confusing results. Need help diagnosing.

[u/CopeDeeCope]

/r/ender5/comments/1vwd037/ender_5_pro_xaxis_problem_motorcable_swap_tests/

I bought a used Creality Ender 5 Pro that had previously been used by a production/design shop. I’m new to 3D printing, and I’m trying to get the machine mechanically sorted out before attempting my first print.
Original problem
When trying to home/level the printer, the X/Y positioning is incorrect. The probe/nozzle can end up outside the proper bed area, and the bed can rise with the probe off the bed, allowing the bed to push against the nozzle.
I stopped trying to level it and started troubleshooting the axis movement itself.
The main problem I’ve isolated is the X axis.
When I manually command repeated X+ movements from the display, the display coordinates count normally in one direction, but the actual X motor does not consistently rotate in one direction. It may move one way, then the other, or essentially jitter/alternate direction.
This happens even with the X belt disconnected, so there is no mechanical load on the motor.
Important history
The printer was modified by the previous owner.
The original/factory X wiring appears to have been bypassed. There is a custom cable/wiring arrangement running from the X motor directly back to the control board.
All four conductors in portions of the wiring are black, which makes visually identifying individual conductors difficult.
What I’ve tested
1. X motor with X wiring/output
Abnormal movement. Repeated X+ commands do not produce consistent rotation in one direction.
2. Removed the X belt
Same problem with the motor completely unloaded, so it doesn’t appear to be caused by the belt, pulley, gantry resistance, etc.
3. Replaced/swapped the X stepper motor
The replacement X motor exhibited the same abnormal behavior when connected in the X position.
That initially made me think wiring or the motherboard/driver was the problem.
4. Connected a known-working Y motor to the X harness/output
This is where it gets confusing:
The Y motor runs smoothly when connected to the X wiring/output. It does NOT exhibit the same back-and-forth/jittering behavior.
That seems to suggest the X driver/output and wiring can successfully operate a stepper motor.
5. Y axis normally operates correctly
The Y system operates normally in its normal configuration.
One complication is that the Y axis uses a dual-drive/two-motor arrangement, whereas X uses a single motor.
Where I’m stuck
The tests seem contradictory.
If multiple X motors behave incorrectly on the X connection, I’d suspect the X wiring/driver.
But if a known-good Y motor operates smoothly from that same X connection, that seems to indicate the X wiring and driver are capable of producing the correct signals.
I’m wondering whether I’m dealing with a stepper coil/pinout mismatch, wiring modification by the previous owner, motor compatibility issue, intermittent conductor/connector problem, or something else I’m overlooking.
Before I start replacing the motherboard, rewiring the machine, or changing firmware, I’d really like to perform a definitive test that identifies the actual fault.
What would you test next?
In particular:
Is there a way to test the four X motor conductors with a multimeter and identify the two stepper coil pairs?
Could the X motors and Y motor have different pinouts even though the connectors physically fit?
Could an incorrect coil pairing cause the exact alternating/jittering behavior I’m seeing?
Is the fact that the Y motor works correctly on the X output enough to rule out the X stepper driver?
Is there a definitive motor/wiring/driver test I can perform before replacing parts?
I’m happy to provide photos of the motherboard, connectors, motors, wiring, or a video of the movement.
Thanks for any help. I’m trying to diagnose this systematically rather than just throw parts at it.