expression
under adjustment; being adjusted; out of service
Used for equipment, systems, or schedules that are currently being adjusted or are temporarily unavailable. Often seen on signs or in notices.
このエレベーターは調整中です。
This elevator is out of service for adjustment.
システムは現在調整中です。
The system is currently being adjusted.
修理中 means 'under repair', implying something is broken and being fixed. 調整中 can be used even when nothing is broken, just being adjusted or calibrated.
Compound of 調整 (adjustment) + 中 (in the process of).