- My model of MX-5 is: 2016 ND 2.0 Sport Nav
- I’m based near: Liverpool
- I’m looking for technical help or recommendations on: Engine Idle
My engine is idling at a slightly higher rpm when the air conditioning is turned on. It idles normally for around 12 seconds and then a click sound follows and the engine idles slightly higher for about 8 seconds before returning to normal idle speed.
It sounds like some element of the air conditioning unit is running when when idling higher and then shutting off when idling lower. This only happens when the AC is turned on. When driving with the AC on, it sounds like an intermittent hissing sound inside the car coming from the AC at the same time intervals as above. The air conditioning seems to be working fine aside from this.
Is this normal? Or is there something going wrong or about to go wrong?
I’ve attached a link to a video of the engine changing idling speed.
All normal.
When you put the AC switch to on it activates the electronic magnetic clutch on the AC Compressor which activates running it.
(Just an automotive process like a fridge/freezer which cycles on and off).
“Generally” when the AC is on and the compressor is running the engine revs increase to accommodate the running, (in particular when idling) as you have said.
The pump will switch to various modes during use depending on temperature ranges etc both inside and out, as obviously will have to work harder in very hot weather.
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That’s what I was thinking… I’m just concerned with the fact that it is ‘cycling on and off’ so often like every 10 seconds… I’ve never had a car do that before. Do all ND’s do it that much?
Can’t vouch for an ND as I have an NC.
Our Mazda 2 Skyactive was like that, whereas our Volvo is rather more subtle.
Some still do it, but “Perhaps” more quietly.
Model to model and manufacturer likewise.
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That is very common. Some cars are quieter on the clutch clunk, (usually because of better cabin sound proofing), but they all do something similar.
The proportion of “On time” vs the “Off time” in a given cycle varies depending on how much chill is needed, since the basic chill production is “all or nothing”.
The cycle rate of chopping up the chill time (more On = more chill) much more quickly than the slow crawl at which the cabin changes its temperature averages out to the end result as seen by the cabin sensor.
The technical term is Pulse Width Modulation, as usually found in regulated power-supplies, light dimmers, Class D amplifiers , some transmitters; it is such a handy trick the list of applications is almost endless…
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They all do it. The AC, much like the heater, is way overpowered for the cabin size of the car so the AC has to kick on and off as you are describing. This happens way less in the summer when you are really needing the cool. The AC will happily keep your legs and feet cool even with the top off.
The evaporator in the AC is not supposed to go below 4 degrees [a wild guess on my part there]. As the compressor kicks in, the cooling process starts. With it being reasonably cool now, 4 degrees is easily reached and so the compressor kicks off. The air blows through the evaporator, the evaporator gets warmer and so the compressor kicks in again to bring it back down to 4 degrees. And so the cycle continues. When the weather is warmer, it takes longer to get down to 4 degrees so the off gaps are shorter until the compressor is running full pelt on really hot days.
The compressor cycle is certainly not subtle in my Mk3. I get the same clunk you describe along with the serpentine belt having that bit more load upon it so it makes a hissy noise.
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Not “All” A/C systems have magnetic clutch type compressors. Some have variable displacement ones so don’t switch on and off. All MX-5s have magnetic clutch type ones though.
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We have 2 suzuki swifts and when the air con cycles on and off the compressors are quite noisy., to some degree every car I’ve had with ac has done. Perfectly normal for it to do what you describe.
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