High-Voltage Accessories: HVAC, Charging, and Auxiliary Systems
HV accessories
Electric A/C compressors are driven by HV three-phase motors with an internal inverter, use POE-type oil compatible with the electrical insulation, and communicate on a data bus. The HV PTC heater or heat-pump system provides cabin heat without engine coolant, so heating draws directly from the pack and reduces range. Both accessories set faults when isolation resistance drops; a leaky compressor or heater element can disable charging.
Charging systems
Understand the path: charge port, onboard charger (AC to DC), HV junction box, contactors, pack. AC Level 1 and Level 2 use the J1772-style connector, where the EVSE sends a 1 kHz control-pilot square wave whose duty cycle encodes the available current; the vehicle closes the proximity circuit through a resistor so the EVSE knows it is plugged in. DC fast charging bypasses the onboard charger and communicates digitally with the vehicle to negotiate voltage and current. Charging requires the HV interlock to be intact, isolation to be acceptable, and the 12 V system to be healthy.
Diagnosis traps
A vehicle that will not charge is often an EVSE or ground-fault problem, not a vehicle problem - test with a known-good EVSE and inspect the outlet. Insulation faults commonly block charging rather than stop driving. Charging also maintains the 12 V battery on many vehicles, so a no-charge condition can flatten it. After HV accessory replacement, clear codes and verify with a charge cycle and A/C performance test.
Rule of thumb
Before condemning an onboard charger, confirm pilot signal presence, 12 V health, and insulation resistance - the three cheapest checks come first.