What causes This !!

When the Air Con' is switched ON and the compressor kicks in (whether at idle or on the move)....what sensor/switch or thingymebob causes the engine Rev's to rise?..

..is it the ECU that sets the rev's ?...i guess the TPS must receive a signal of some sort but from where!!....what actually takes place to cause the RPM to increase the instance the compressor starts to put load on the engine?


ps. there is a good reason for this question due to an odd erratic idle issue i have !....which appears to only happen once the A/C has been used :confused:

Thanks, in advance, Phil.

To add to the above, Is there an A/C Idle Control Module ?...if so, where is it?

Sorry for all the questions but i think i may be closing in on the answer to my erratic idle...hopefully ;)

Thanks chaps

Have you checked the belt? Is the compressor clutch damaged? Can you live without air conditioning?

Thanks for the reply...yes , the belt is fine, the compressor clutch is working, the Air con blows ice cold!
I can live without it but over here it is also useful for clearing frost off windscreens as well as in the warm summers!..

It appears my idle and running remain fine but when i use the air con..' it plays havoc! ...even if i then switch the A/C OFF it still idles at 1200 and surges when you least expect it

This may be getting you to do too much stuff, but I would get a voltmeter and check your battery voltage before and after you turn on your AC. Write these values down and see if there is any fluctuation.

I once had a totally dead battery, and there was no idle.. As soon as I jumped it, it would die unless I held the gas pedal down. It must have been sending some really dirty power through to the ECU.

I think what your asking is are there two idle air control motors. There is only one. When the ECU sees the AC on it sends out a signal to the idle air control to raise the idle to help with the increased load on the engine when the compressor is running. If you have an erratic idle during the time when the AC is on it maybe due to the IAC needing cleaning. Remove it from the throttle body and clean the carbon off cone end with carburetor cleaner. Also clean the throttle body were the motor goes in to. See if that helps. It doesn’t cost much and can't hurt

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Originally Posted by twostep1n2

I think what your asking is are there two idle air control motors. There is only one. When the ECU sees the AC on it sends out a signal to the idle air control to raise the idle to help with the increased load on the engine when the compressor is running. If you have an erratic idle during the time when the AC is on it maybe due to the IAC needing cleaning. Remove it from the throttle body and clean the carbon off cone end with carburetor cleaner. Also clean the throttle body were the motor goes in to. See if that helps. It doesn’t cost much and can't hurt

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Well done buddy, that is the answer i was looking for :thumbup:

So my thinking was kind of right!..it is the ECU which detects the extra load on the engine and raises the rev's (via the IAC) to a higher level.

A further two points.. I have already given the IAC a very good clean (the cone pintal and the whole throttle body) bare in mind, my motor has only done 37K miles and it all looked very clean anyway!!

Secondly, when stood at idle, the Revs increase to 1700 when the A/C is on...this seems way to high for me?..
..Furthermore, when driving along, if i am slowing to a stop and the A/C compressor kicks in, this causes the revs to rise far to much and the car momentarily surges forward ....this can be a bit unnerving if you are only feet from the car infront :eekgray:

Questions: Why do the rev's increase so much when the A/C compressor kicks in? And if it is the ECU which determines this, how can i stop it?

An erratic idle can be caused by a number of things. Only one of witch is the idle air motor. It can also be caused by a vacuum leak. There are several threads on the subject try a searching on "Idle".

Well, i have looked at and cleaned just everything i can lay my hands on under the hood!...even had a can of BG44K through the fuel tank.

I must say, when the Air con' is not 'ON' it runs really sweet (idles around 750 rpm in neutral)....When the A/C is ON it does now seem to drive better while on the move but you can still feel a slight surge in revs when the compressor cuts in but it does not seem so pronounced as it was....nevertheless, it is still there!

I guess i will just have to live with it.......does anybody know if there is a particular vacuum pipe which is related to the Air con'.....in the sense that it would have an effect on the revs of the engine when the A/C is ON (if it is solely the ECU that governs the idle speed when the compressor is working, then there will be nothing i can do about it? !)

Sounds like i could do with an Automotive Air Conditioning specialist :yesgray:

Cheers guys and thank you very much ;)

Just this past weekend, Wifey & Boy were with me in VX when stopped at a light & surge made me creep forward a little bit. That got me to remembering that there was a bit of discussion on this topic several years ago. Basically though:

1. that surge is normal (well, maybe not normal but not indicative of a problem).

2. careful at stoplights - make sure your foot is firmly on brake & if you are going to do something that will be distracting you whilst sitting at stoplight, maybe slip tranny into neutral.

OK Tom, thanks for that :thumbup:

I could well be normal for the VX !!!...lets see if other members show the same symptoms !!!

I must say, i have had a number of Automatic vehicles (don't forget i live in the land of the stickshift !) even though i am aware of when the A/C kicks-in...we can all sense/hear/feel that slight change in engine revs when stood stationary....But not to the extent that it wants to move the car forward even when the foot is on the brake pedal ;eekb;

I may just have to take your advice and press harder on the metal or as you say, knock it into neutral.

ps, just one little theory!!!....I have noticed that 1st gear is very highly geared, in the sense that just the slightest of throttle inputs make it jump of the line in a rapid way......I have read that Isuzu did make the first few gears this way to enhance its 0 to 60 times!
Not sure of this but maybe this is why the increase in revs caused by the compressor might be inclined to want to move the car forward!!!!......Could be wide of the mark on this though :confusedw

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Originally Posted by fildigger

OK Tom, thanks for that :thumbup:

I could well be normal for the VX !!!...lets see if other members show the same symptoms !!!

I must say, i have had a number of Automatic vehicles (don't forget i live in the land of the stickshift !) even though i am aware of when the A/C kicks-in...we can all sense/hear/feel that slight change in engine revs when stood stationary....But not to the extent that it wants to move the car forward even when the foot is on the brake pedal ;eekb;

I may just have to take your advice and press harder on the metal or as you say, knock it into neutral.

ps, just one little theory!!!....I have noticed that 1st gear is very highly geared, in the sense that just the slightest of throttle inputs make it jump of the line in a rabid way......I have read that Isuzu did make the first few gears this way to enhance its 0 to 60 times!
Not sure of this but maybe this is why the increase in revs caused by the compressor might be inclined to want to move the car forward!!!!......Could be wide of the mark on this though :confusedw

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Sorry, couldn't resist the slight edit in your post. :)

x2 on your theory ... plus, I believe we also have a lower stall speed torque converter for the same reason & with the same results.

Fildigger, mine raises the idle 100 - 150 rpm when the compressor kicks in. As you know, the PCM varies idle speed depending on engine temp, load from power steering pump, alternator, etc. but I think under normal circumstances "baseline" tickover is 750 to 800rpm - so if you're seeing 1700rpm from a warmed-up engine then something's wrong. The only time I've seen more than 1100rpm was when I had a dirty IAC. Cleaned it up in five minutes and things were back to normal. I know you said you cleaned yours but you might want to try again and make sure to do what twostep1n2 said and clean the seat that the cone seals against. On mine, however, it was actually the shaft that had gummed up. Carb cleaner and careful scrubbing with a small brass brush fixed it right up. Oh and I sprayed some contact cleaner on the connectors. Hopefully that will do it for you but if not, your next step would be to break out the propane and look for vacuum leaks. Hopefully it's a hose or faulty PCV valve - and not the infamous intake manifold gaskets...

It definitely needs to be fixed before you ram someone! Let us know what you find.

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Originally Posted by SlowPro48

Fildigger, mine raises the idle 100 - 150 rpm when the compressor kicks in. As you know, the PCM varies idle speed depending on engine temp, load from power steering pump, alternator, etc. but I think under normal circumstances "baseline" tickover is 750 to 800rpm - so if you're seeing 1700rpm from a warmed-up engine then something's wrong. The only time I've seen more than 1100rpm was when I had a dirty IAC. Cleaned it up in five minutes and things were back to normal. I know you said you cleaned yours but you might want to try again and make sure to do what twostep1n2 said and clean the seat that the cone seals against. On mine, however, it was actually the shaft that had gummed up. Carb cleaner and careful scrubbing with a small brass brush fixed it right up. Oh and I sprayed some contact cleaner on the connectors. Hopefully that will do it for you but if not, your next step would be to break out the propane and look for vacuum leaks. Hopefully it's a hose or faulty PCV valve - and not the infamous intake manifold gaskets...





It definitely needs to be fixed before you ram someone! Let us know what you find.

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Indeed !.....I will do that (clean everything up again)...I have sprayed Carb' cleaner and lots of it! around the intakes and surrounding area but never noticed any change in tick-over, so hopefully it is the IAC .

Thanks very much Slowpro ;)