A battery?electric wheel loader was derating intermittently. The operator would be driving across the lot at full speed when, without warning, the ECU would cut power to the drive motor and drop the machine to a crawl. No active fault codes, just a historical "motor position signal lost" event.
We hooked up the laptop and watched the live data from the drive motor resolver/encoder. At low speed, the position signal looked fine. As speed increased, the angle reading started to lag and then suddenly jump. The ECU's software interpreted this as a loss of synchronism and derated the motor to protect the inverter.
We pulled the motor cover and found the encoder disc was loose on its shaft. The roll pin had sheared, and the disc was spinning slightly relative to the shaft. At low speed, the slip was small enough that the ECU could still track the rotor. At high speed, the slip accumulated until the reported angle was so far off that the controller declared the signal invalid and shut down. A new encoder disc with a proper roll pin and Loctite fixed the phantom derate. On electric drive axles, any intermittent "position signal" or "synchronization" fault should send you straight to the encoder mounting before you start replacing expensive inverters.
