Your modified sine wave Xantrex 758 likely caused the hair dryer malfunction due to waveform incompatibility with sensitive electronics. While some devices work fine with modified sine wave inverters, modern residential refrigerators, computers, and electronics require pure sine wave power to prevent damage and ensure proper operation.
Surge Guard 40250-RVC and 40350-RVC are interchangeable. Buzzing usually means loose connections - tighten all terminals. If buzzing persists with tight connections, the relay may be failing and unit needs replacement.
Power remaining after key removal indicates failed ignition switch or stuck relay. Test for voltage on run circuit with key off. Check for hot relays in fuse box. Ignition switch replacement is straightforward if confirmed as cause.
A loud humming noise with a non-starting compressor typically indicates a failed start capacitor, bad compressor, or electrical issue. The start capacitor is the most common culprit and can often be replaced by DIYers with basic electrical skills.