Why does your truck heat up when you turn on the AC at a stand still?
When your truck's AC is on at a standstill, the engine works harder to power the AC compressor. This increased workload generates more heat within the engine, which can cause the engine temperature to rise. The engine isn't moving to benefit from airflow for cooling, so this heat buildup is more noticeable. Essentially, the extra energy needed to cool the cabin is coming from the engine, and that energy conversion process isn't perfectly efficient; some of it ends up as waste heat.