The solenoid does help the engine. The help is needed due to the strain of the compressor. Here is how it works--
The compressor pulley always spins. It free spins when the compressor clutch is not engaged. When the AC is turned on, the compressor clutch engages. The pulley now does real work to operate the compressor. This pulls some power from the engine. At idle, this strain pulls the idle speed down...enough to sometimes stall the engine. The solenoid gives the engine a bit more gas to compensate.
The same thing happens when the thermostat causes the compressor to cycle. If it calls for AC, the compressor clutch is activated and the load is added to the engine. The solenoid compensates so that the engine automatically idles the way it should when the compressor clutch engages and disengages.
The solenoid only comes into play at idle. Your foot on the gas compensates when driving.
The compressor load (about 5 HP from what I have read) is always on the engine when its clutch is engaged whether idling or driving. At idle, it is very noticeable. When driving, it is less noticeable but you can tell that performance is reduced.
Remember back in the old days when people would talk about losing MPG due to air conditioning? It is because of the extra load put on the engine.
Some vehicles have a vacuum operated cut-out that automatically shuts down the compressor clutch during hard acceleration so that all power goes to the drivetrain. I did not install one in my application.
Matt
