ZhiDou EV forum

Loose Motor Cable Connection at the Motor Controller: What Can Happen (and What to Look For)
Hi everyone,
I’m putting together this post because a loose connection on the motor controller (inverter) motor terminals can cause a lot of weird symptoms and, in the worst case, expensive damage. I’ll add photos to this post (terminal condition / melted parts / discoloration).
Important: HV systems are dangerous. Disconnect 12V, wait for discharge, and only work on this if you know what you’re doing.
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Why a “slightly loose” terminal is a big problem The motor phase cables (U/V/W) carry very high current. If the terminal is not clamped properly, you get:

• Higher resistance at the contact point
• Heat buildup under load (I²R losses)
• Arcing / sparking as the connection vibrates or shifts

This turns a small mechanical issue into an electrical + thermal failure.
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What can happen to the controller / terminals

• Overheating of the terminal (hot spot right at the lug / stud / busbar)
• Discoloration (darkened copper, “rainbow” heat tint, burned insulation)
• Melted plastic parts around the terminal block or cover
• Pitting and carbon tracking from arcing (black soot, cratered metal surface)
• Loose lug becomes worse over time (thermal cycling relaxes clamping force)
• Stud / thread damage (overtightening after it’s been hot can strip threads)
• Controller failure in severe cases: phase overcurrent events, repeated faults, or damaged power stage (IGBT/MOSFET) from unstable phase connection

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Symptoms you might notice while driving (listing common signs that can appear before total failure)

• Intermittent loss of power / “limp mode”
• Jerking or shudder under acceleration
• Fault codes related to motor phase / overcurrent / inverter protection
• Unusual humming, buzzing, or clicking (especially under load)
• Strong hot smell near controller area after driving
• One phase cable/terminal noticeably hotter than the others

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Why arcing is extra dangerous here If the phase connection opens momentarily under load, the inverter can see abnormal conditions:

• Voltage spikes and harsh switching transients
• Protection trips (overcurrent / desat / phase fault)
• Repeated stress on the power stage if it happens many times

Even if the car still “works,” it may be slowly cooking the terminal and stressing the controller.
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What to check (visual + basic mechanical) I’ll show examples in photos, but in general look for:

• Burn marks, soot, melted insulation near U/V/W terminals
• Oxidation or dull/blackened metal at the contact
• Cable lug not sitting flat on the terminal surface
• Evidence of movement (shiny rub marks, uneven contact)
• Cracked ring terminal / frayed strands / overheated crimp

Note: If you retighten, use correct tools and correct torque if available. Over-tightening can crack parts or strip studs.
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Conclusion A loose motor terminal can start as a small issue but can quickly become:

• Heat damage to the terminal block
• Arcing and carbon buildup
• Unreliable drive behavior and fault codes
• Potential inverter/controller failure

If you’ve seen similar damage on ZhiDou (or other small EVs), feel free to reply with your photos or torque specs / best practices.