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
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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
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
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
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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
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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
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