Why does your RPM go up and down when you turn on ac 1997 ford expedition?

The RPMs on your 1997 Ford Expedition increasing when you turn on the AC is perfectly normal. The AC compressor requires extra power from the engine to operate. This added load causes the engine's computer (PCM) to slightly increase the engine speed (RPM) to maintain the proper engine functions and prevent stalling. The increase in RPM is usually subtle, but noticeable.

If the RPM increase is *significantly* higher than usual, or if the engine struggles or hesitates when the AC is turned on, then there could be underlying problems, such as:

* Weak AC compressor: A failing compressor requires more power to operate, leading to a larger RPM increase.

* Low refrigerant: Low refrigerant forces the compressor to work harder.

* Idler pulley or belt issues: Problems with these components can increase the load on the engine.

* Dirty air filter: A clogged air filter restricts airflow to the engine, requiring more work to maintain RPM.

* Problems with the throttle position sensor or other engine sensors: Incorrect sensor readings can lead to improper engine management.

In short, a slight RPM increase is expected. A large or problematic increase warrants further investigation by a mechanic.