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The car is a Vauxhall Agila A and the compressor is a Delphi CVC internally controlled compressor, so it's basically supposed to never cycle while the AC is on. The transmission is obviously manual (very low end European car). The car has a Bosch Motronic ME ECU. The car hasn't got a rpm indicator.

Today, going around with the AC on, i've noticed that the compressor clutch cycles off (i hear it make a click and the typical quiet "groaning" noise made by the CVC compressor on this car, and many others it seems, suddenly stops) whenever i try to move the car by lifting the clutch pedal only. It won't do that when i rev it from stationary (so that i think i can rule out AC system pressure/charge issues), it never cycles while idling or driving, and it doesn't seem to cycle whenever i use both the throttle pedal and the clutch one at the same time. It then immediately re-engages as soon as the RPMs start increasing again.

It looks as if the ECU is programmed to disengage the clutch automatically whenever the engine RPMs lower too much, but i never found anything like this in any documentation about automotive ac systems i read so far. On the other hand, i've read that the clutch usually gets disengaged temporarily whenever a strong acceleration is desired.

Are ECUs supposed to behave that way (maybe a way of preventing the engine from stalling when the AC is on), so that what i experience is perfectly normal AC compressor clutch behaviour, or have i got a problem (bad AC compressor relay, clutch coil on the way out, alternator/battery problems even if the battery warning light never comes on)?

I know that too much compressor clutch cycling can ruin the friction surfaces in the clutch assembly, that's why i want it to be limited to the strict necessary.

Thanks for reading and for any help.

  • Most, if not all, A/C cluches are electromagnetic so it’s magnetic clamping that provides the drive - not like a clutch centre plate... – Solar Mike Mar 24 '18 at 16:49
  • Yes, i know that. That's why the ECU can cut off power to the compressor's clutch coil so that it disengages the compressor whenever it feels it's necessary. I'm strictly speaking of the compressor's magnetic clutch here. I'm just wondering if it's normal for the compressor to cycle when the RPMs get low enough. Maybe i've got an idle/crankshaft sensor problem and the AC compressor is trying to tell me that. – Al_ Mar 24 '18 at 17:11
  • I did not read the novel, just the last paragraph ... – Solar Mike Mar 24 '18 at 17:27
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So, I'm coming at this question from a bit of a different direction. My experience is with United States spec'd vehicles, but I gotta believe the principles hold.

Yes, manufacturers will use the engine control computer as an input to the Air Conditioning (A/C) system and A/C clutch engagement. Why do that?

  • You can delay A/C clutch engagement until after a slight RPM bump first. In some conditions A/C load is considerable on an engine. When done correctly, the rpm bump up will minimize A/C startup load. The customer won't notice the additional load. This bump up process takes just seconds.

  • You can shut off the A/C system when there are other problems, to protect the engine. When you go to a near engine overheat condition, the system should shut the A/C system off. You don't need to burn your engine up. Replacing an engine is a very expensive repair. Instead, add coolant, ensure the cooling fan is working correctly, unblock the radiator for good airflow.

  • It wouldn't surprise me if they shut off the A/C system upon other demands to the engine (e.g. during a period of high acceleration demand from the customer...) Wanna go fast? Now?

  • I don't really know the details on the Delphi variable output compressor. The fact that the A/C clutch disengages at ultra low engine RPM doesn't really surprise me. That's probably a good thing. It helps get you from point A to point B, with best $$$ economy possible.

  • You mention not having a tachometer. You may not have a gauge in the instrument cluster, but I guarantee the engine computer knows the RPM at any given moment in time.

And as for your worry about the A/C clutch plates getting worn out. Not to worry. I've spent decades working on Automotive A/C systems (as a US based automotive engineer ) and I've never seen worn out plates thru normal engagement.

  • Note: what is possible... if the air gap between clutch plates is too large, then you won't get proper engagement (slip) and that will wear out plates. Check the air gap with engine off. Check your service manual for the correct gap for your vehicle. The air gap is set by spacer washers under the outer clutch plate mount.

  • What I have seen is a broken compressor with seized shaft and an engaged clutch. Yes, that will totally burn up the clutch plates. No surprise.

Again, I read that mere frequent A/C clutch cycling as very low chance of wearing out the clutch plates. You need not be worried. Sounds like your car is working just fine.

Good luck with it! Be safe, have fun...

  • Yes, i know that even if i have no tachometer the ECU still knows the engine rpms and much more. In fact, the engine has a crankshaft position sensor, so the ECU can see the rpms. Told so just to explain that i can't precisely see the rpms at which i hear the compressor cycling off and then back on (haven't got a live data capable OBD reader with me). Just wondered if this information was used to control whether to keep the compressor engaged or cycle it off. Anyway, thanks for answering with so much detail! – Al_ Mar 25 '18 at 19:31

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