Low Insulation Resistance at the Traction Motor

Something else - the pattern says contamination, not a fault.

• All three phases are low and they are low by the same amount (2.4/2.6/2.5 MΩ). A winding-to-case failure is a localized breakdown: it shows up on one phase, and it typically reads near zero, not a tidy 2.5 MΩ.
• Phase-to-phase resistance is balanced and correct (0.020–0.021 Ω), which argues against a shorted turn or an open winding.
• The residue at the terminal cover is the lead. Coolant is mildly conductive - it is not an insulator, and glycol residue left to dry leaves a conductive film that tracks across surfaces. A surface leakage path from all three terminals to the case is exactly what produces a balanced, moderate drop in insulation resistance.

The readings are also low enough to matter: at 500 V, 2.5 MΩ is a real leakage path, and it explains a derate that appears after 10 minutes - the leakage current grows as the assembly heats and the contamination spreads.

Clean and dry the terminal area per the OEM procedure, then retest before condemning anything.

• Remove the contamination with the specified cleaner, dry thoroughly, and re-run the 500 V insulation test at the same temperature. Insulation resistance roughly doubles per 10 °C rise, so retest at a comparable temperature or you will misread the result.
• Inspect for tracking, carbon paths, and a distorted or hardened seal. Clean up a leakage path and the leak is still there - find and fix the entry point: seal, O-ring, terminal-cover gasket, or a cracked housing. Pressure-test the cooling circuit if the OEM procedure allows it.
• Change the diagnosis if: one phase stays low after cleaning, the reading drops as the motor warms, or the reading is unstable while flexing the terminal block. Any of those points to a genuine winding or internal insulation fault and the motor is condemned.
• Also confirm there is no residual coolant inside the inverter side of the assembly before it goes back together - reassembling onto a wet terminal block reintroduces the problem and can take out the new inverter.

The tester is the reason. A 500 V insulation tester applies hundreds of volts DC to whatever is connected. Applied into an inverter's gate-drive and DC-link circuitry, that voltage can destroy semiconductors and capacitors that were fine. Insulation testing is done on the isolated load with the electronics disconnected - always, and the OEM procedure will state it.

The 10-minute derate is a heat signature. If the insulation were a fixed 2.5 MΩ, the vehicle would either fault immediately or not at all. Faulting after ten minutes at highway speed means the leakage current is temperature- and time-dependent - consistent with a damp, contaminated surface rather than a clean break. It also tells you the failure will get worse in service: the derate is the vehicle protecting itself from a shock and fire hazard, not a nuisance.

Three weeks of backorder is not a reason to skip the clean-and-retest step. If cleaning restores ≥ 20 MΩ with a sound seal, the motor stays; if it does not, the wait is the wait, and the vehicle does not go back on the road with a known leakage path to chassis.