Dell inspiron monitor issue

ok, so I cracked my notebook monitor, and got a new one delivered. Installed the monitor, on first appearance everything looked greatr. However, I have come to learn the monitor dims when the power supply is plugged in, and it works at full brightness when running solely on the battery. This is the opposite scenario as to what normally happens. Does anyone have any ideas as to what my problem might be? Greatful for good advice.

Gappman<>