Aircon - Is This Normal

[Slobbo]

Chaps,

Is it normal for the cars engine speed to fluctuate often(couple of hundred RPM) when the aircon is on. I know it is normal for the engine speed to rise when at idle (while at the lights for example)when the air con kicks in but it seems to cycle every 30 seconds or so.

If I turn the aircon off then the engine runs perfect.

It is more the frequency that it cycles that makes me wonder.

[9690phil]

Hi I had a ls400 forget the year of car think it was m reg. anyway when I put aircon on it really jumped up in revs. Took it to Lexus they had it for about a hour and told me it was a recall on them but it's no detriment to car. So who owned car at time of recall didn't have it done. And they told me it was about £800 to supply and fix the part. That was about 8years ago.

[9690phil]

Hi I had a ls400 forget the year of car think it was m reg. anyway when I put aircon on it really jumped up in revs. Took it to Lexus they had it for about a hour and told me it was a recall on them but it's no detriment to car. So who owned car at time of recall didn't have it done.

[scudney]

Yes the revs do go up very slightly when the aircon fan kicks in

[Cotswold Pete]

My previous LS used to do this weird cycling, but it had the Capacitor failure issue (stalling after long runs).

With current Mk4, the AC causes the engine to run at about 150ish RPM more when stationary. It does not cycle though, so I assume the capacitors are okay, though I notice my idle speed can drop to 250rpm at times, so I assume the ECU is either not quite 100%, though I have a slightly leaky exhaust (to be sorted in 3 weeks), so be interesting to see if this makes any difference.

With previous cars I have owned, found replacing the HT leads made a difference to idle rev cycling especially in extreme heat or extreme cold.

[Slobbo]

I know the ECU is fine as I have just had the caps done (because I thought this might be the issue). Could try the HT leads I guess but a new set of them is expensive to simply test.

Will research the recall.

It isn't a massive thing it just seems a bit excessive. Didn't seem to do it before the warm weather started.

[Slobbo]

Only recall on the LS seems to be a the following

PASSENGER VEHICLES. DUE TO IMPROPER ASSEMBLY OF THE TERMINAL FOR THE STARTER MOTOR MAGNETIC SWITCH, AN ELECTRICAL SHORT CIRCUIT CAN OCCUR IF ELECTRICALLY-CONDUCTIVE LIQUID, SUCH AS ROAD SPLASH WITH DEICING SALT, ENTERS THIS AREA.

So not sure what Lexus did for yours 9690phil

[9690phil]

It was Lexus Chester maybe not a redall but it be repaired foc but most definitely a fault that would have been done foc

[Slobbo]

Will see what I can find out. I have just bought an Autel diagnostics unit that is supposed to work with my LS400 (pre obd2 connector) but I've not managed to get it to work as yet. Maybe that will tell me something interesting.

If anyone needs some diagnosis info from your car just ping me. Should do all the Lexus models. Not just OBD, it is a proper workshop unit. Does live data, adaptions, key coding etc. I'm in Dunfermline Scotland so probably too far for most to go.

[ambermarine]

The early Ls400s had a clutch on the compressor that engaged and disengaged depending on the load the compressor was under .When stationary and revving at 650 which is normal it would frequently engage the clutch and the revs would go up to 850 or 900 after a minute the clutch would disengage and the revs dropped back to 650 .If the car was being driven the change was hardly noticable unless in cruise control because the higher revs kept the system pressured longer .The Mark4 was fitted with a new compressor type that maintains a constant pressure, so the clutch stays engaged all the time the A/C is switched on there is no change in the normal tickover revs and the clutch only disengages when the A/C is off.

[cruisermark]

The early Ls400s had a clutch on the compressor that engaged and disengaged depending on the load the compressor was under .If stationary and revving at 650 which is normal it would frequently engage the clutch and the revs would go up to 850 or 900 after a minute the clutch would disengage and the revs dropped back to 650 .If the car was being driven the change was hardly noticable unless in cruise control because the higher revs kept the system pressured longer .The Mark4 was fitted with a new compressor type that maintains a constant pressure, so the clutch stays engaged all the time the A/C is switched on there is no change in the normal tickover revs and the clutch only disengages when the A/C is off.

yes the above is correct - i noticed that when i went from a mark 3 to mark 4 - the ls430 in this respect goes a step further - where as in the mark 4 the revs can be heard and gears felt when the engine is cold - the 430 is smooth and seamless what ever the weather !!

[Titch LS]

Mark, FYI my 2000 mk4 is also smooth and seemless whatever the weather.

[Slobbo]

Yeah yeah rub it in. I will just have to put up with it then. I'm happy as long as it is normal.

Anyone know if the Mk4 compressor is interchangeable with a Mk2?

[ambermarine]

The answer is no ,as the ecu that controls the compressor is different , as are the electronics in the compressor.

[Jusme]

Not wanting to hijack your thread Rob, but as its about aircon. Started up yesterday in that heat, put the aircon on and waited outside of the car for a minute, got back in and the heat coming out of the vents was hideous, lasted about 5 minutes then the oh so cooling air came out.. Is this normal? Apologies again Rob. :winky:

[runsgrateasanut]

It is recommended that you cool the interior as much as possible pre turning on the Air con. Drive with windows down?

My strange experience is that up until yesterday I think my car seemed to start from cold at about 1100 and drop down to 650 when hot. I always notice the increase in revs when A/C switched on and so got into the habit of turning it off when approaching lights /crossings or even in a 30mph zone.

Replaced plugs and cables yesterday (all did not go smoothly) and today after a two mile trip started in the drive half an hour later was idling at 850 and when A/C turned on, it WENT DOWN TO 800. Bizarre

Perhaps the ECU is still calibrating changes, so will check after a good run. Seemed to be less exhaust smoke and cup on engine top only a very minor ripple BTW.