A/C compressor operation question

[ArbitrageMan]

It warmed up this weekend and it got me thinking about my air conditioner.

I dont know anything about how this works, so go easy on me. Does this A/C compressor have a clutch?

And is the load on the engine from the compressor varried in this car? I guess what Im saying is, is the compressor on/off, or is the work it does varried?

In my last three cars, when I'd turn the A/C on, I'd feel a CLUNK as the compressor engaged. And in those cars I could definitely tell that the A/C was taking power away from the engine.

[ken in az]

Yes, ours is a clutch type that engages with a clunk. It will turn off at full throttle giving back your parasitic losses but the ac condenser will cause higher coolant temps which will inherently rob you of performance by the ecu pulling timing - per FSM

[sdkhalsa]

I was thinking of this same exact point. An old Q45 that I had did not cycle on and off with cabin temperature setting; it was on all of the time when the air conditioner was on. Are you sure that compressor turns off when the set temperature is reached? The fact that there is a clutch does not really tell the story.

[SteveTheTech]

The M uses a ZEXEL VALEO CLIMATE CONTROL variable displacement style compressor. When the AC request relay is energized the electromagnetic clutch is energized is stays energized although at some points it may be providing little to no pressure in the system, based on humidity and temperature demand.

There is some parasitic load on the engine and in high temperature situations heat soak on the engine is indeed increased. However; the fan control adjusts according to temperature, AC signal, and relative temperature both inside and around the engine. Once you have reached a cruising speed the load on the engine is decreased as the heat transfer efficiency increases.

Fuel economy and performance are not really effected by the AC being engaged on these cars, base parameters are all preprogramed to adjust to AC request and decrease in longterm fuel economy is nominal.

[ArbitrageMan]

The M uses a ZEXEL VALEO CLIMATE CONTROL variable displacement style compressor. When the AC request relay is energized the electromagnetic clutch is energized is stays energized although at some points it may be providing little to no pressure in the system, based on humidity and temperature demand.

Thats exactly what I was looking for! Thanks Steve and all for the info.

[NightWatch]

Fuel economy and performance are not really effected by the AC being engaged on these cars, base parameters are all preprogramed to adjust to AC request and decrease in longterm fuel economy is nominal.

The compressors on these cars are actually very efficient. The drag caused by rolling down the windows actually has a greater effect on fuel economy than running the A/C.

[ken in az]

The compressors on these cars are actually very efficient. The drag caused by rolling down the windows actually has a greater effect on fuel economy than running the A/C.

No doubt, I only notice a 1mpg difference when running the ac on the freeway.

And Steve great explanation!

[SteveTheTech]

The compressors on these cars are actually very efficient. The drag caused by rolling down the windows actually has a greater effect on fuel economy than running the A/C.

That's very true, the AC signal and power steering pressure signal are monitored by the ECM. There is also a load signal that monitors lights and other heavy loads (wipers and blower also) to adjust base fuel trim and timing. The window switch can actually cause the idle to dip as it is not monitored. These are by no means like the older cars that used cycling clutches and the pressure variance is minimal.