I have a one question i have owner Tata Indica V2 . my question is when i start car ac rpm flucating up and down and similar pickup also same with rpm. what is the reasion and please give me solution

  • 1
    So, how much fluctuation : 500 to 1500 rpm? 800 to 850 ? the A/C load is not constant - depends on what load exists and it varies according to the A/C settings.
    – Solar Mike
    May 26, 2017 at 10:58
  • @SolarMike The AC load aspect is not so simple: a reasoning straight as yours might apply for an externally controlled variable displacement compressor, where the AC system is actually able to regulate the refrigerant flow, but internally controlled VDCs actually work on full flow until the evaporator reachs freezing point and then they start destroking, while fixed displacement compressors are always pumping the same amount of refrigerant around and therefore work in cycles. Compressor load is more likely to be affected by amb temp (condensing pressure and therefore discharge pressure).
    – Al_
    Jul 19, 2018 at 23:23
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    @Al_ what was the rpm fluctuation - until the OP clarifies then we are all p*ssing in the wind which was why I only made a comment to elicit more info - which never happened...
    – Solar Mike
    Jul 20, 2018 at 4:13
  • @SolarMike Oops, my fault. Misread the date.
    – Al_
    Jul 20, 2018 at 21:25

1 Answer 1


I suspect you are referring to the apparent dip in idle speed when switching the A/C on and subsequent correction. What actually happens is that the A/C compressor clutch kicks in so that the drivebelt attached to the A/C compressor pulley wheel is then presented with the additional load required to rotate the compressor internals that circulate the refrigerant.

This additional load placed on the engine causes the idle speed to slow down. What then happens is that the engine management software on the ECU detects that the idle speed has dropped and adjusts the idle control valve / throttle flap position accordingly. This then causes the RPM to jump back up again and then settle to a comfortable idle speed.

You see the same thing to a lesser extent when turning on other systems on the car such as the heated rear screen. It's most evident with the A/C compressor as this has is one of the biggest drains on the system in terms of additional equipment.

  • I agree with Steve. One additional thought is that cars with smaller engines are more likely to exhibit this behavior.
    – Tim Nevins
    Oct 24, 2017 at 15:32

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