355 higher idle with AC compressor on

Normally drops due to extra load .

Can you post some numbers of your idles .

 

Correct, but at the same time the Idle Control Valve should get a signal from the ECU to allow more bypass air to combat this drop - and sometimes this results in a higher idle than without the compressor engaged. But it really should only be ~100 rpm higher than normal. If it's really high something may be off.

 

On the 348, the idle speed does not drop when the AC compressor is engaged. In fact, the speed drops a bit momentarily (for a second or so) and then goes back to normal, i.e. to what it was before the compressor engagement. On compressor disengagement, the idle speed goes slightly up momentarily and then settles down to normal. This is controlled by the engine ECUs via the Idle Control Valves (ICV). I recorded the PWM signal on the ICV before and after the compressor coming "on". It can be seen that the PWM duty cycle is increased at the time of the compressor engagement to compensate for the added load on the engine thus keeping the idle speed constant.

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The engine ECU will also compensate in a similar manner for the effect on the idle speed by added load coming from the alternator which is especially felt when the two cooling fans are switched "on".