Idle Up Mechanism ?

[marwankw]

Hi, I have a 1998 LS400; my problem is that when I start my AC after a while the RPM needle starts to fluctuate between 600 and 800 continuously every 3 sec.

Is there any idle up mechanism that starts working when AC is on only? because I have no fluctuation when I switch off the AC.

[wwest]

Yes, that's the result of the A/C compressor cycling on and off. A good sign.

[marwankw]

I don't mean the normal fluctuation, its every 3 sec. the RPM needle is going up and down, i think that something is wrong with AC idle up mechanism.

[SRK]

The engine RPM should not vary with the compressor load. Neither of my cars do that. Some cars produce a higher RPM when the AC is on, but even then the RPM remains steady. Perhaps the compressor load is excessive.

[Jacob]

Yes, that's the result of the A/C compressor cycling on and off. A good sign.

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I think my compressor is always on; is this something to worry about?

Thanks,

Jacob

[LexusLooker]

Most cars built since the mid '80s raise the RPM slightly just as the compressor kicks in (I guess the A/C send the engine computer a signal that the compressor is about to engage). With the additional load of the compressor, the net result is no change in RPM. You don't feel that "bog" that you did years ago.

I think I read somewhere here that the LS raises the RPM when the A/C is on anyway. On mine, idling in D, the RPMs are about 650 with the A/C on (whether or not the compressor is running). With the A/C off, idle drops to 400.