I'm not sure if this question is for this section or for general issues, but still - I noticed that no matter on what temperature I set the air conditioner, when it's on AUTO, the A/C red light is on. Does this mean that the A/C compressor is always running when I set on Auto?! Even if the temperature outside is 10C and I set the inside temp on Auto at 30C?!??!

 

Yes, to dehumidify the air, the compressor runs even when heat is called for.

Cheers
DD

 

I believe it may run occasionally if the outside air temp is high enough and there is a need to dry the air. Otherwise, no.

 

After reading Mikey's reply I should clarify my answer.

I don't know if the compressor actually *operates* continuously in auto mode. It might cycle in-out. But, if the a/c button is lit, the compressor is at least on-line to operate even if hot air is called for.

Or, in other words, asking for cabin heat will not automatically or necessarily disengage the compressor.

I'll check today on my own car....although the X308 might be different than my X300.

On my older Jags....80s vintage....the compressor operated in all modes, period. Only evaporator freeze-up or loss of freon would disengage the clutch.

Cheers
DD

 

I was wondering this too. I have always turned the AC off as I found it weird coming on at -20.

 

Don't confuse the little light being on or the A/C belt and pulley turning as indications that the compressor is actually doing anything. The electric clutch cycles the compressor on/off as required without the driver having to do anything or really in spite of what they may want. No compressor runs 100% of the time except in extremely hot conditions that exceed the system's capability to keep up.

Over riding the system to turn it off in cold conditions does little or nothing. Leave it in auto mode and let it do it's job.

 

I agree... and in fact If you read the owners manual, I am pretty sure it advises to leave the A/C on ( as in auto mode) for lubrication purposes even in winter months. I have not read it for some time, but if you have yours, take a look. During warm weather, from outside the car with A/C on you can hear the compressor cycle on and off as demand requires.

 

Just checked.

FWIW.....

On my X300, with the system in "auto" and the a/c button lit, the compressor cycles in-out even when cabin heat is called for. I suspect the cycling is regulated by pressure and/or evap core temperature. I've never researched it for the X300....never had a problem with the system (knock wood)

When the a/c button is NOT lit, the compressor does not engage at all.

If "def" is selcted the compressor will engage, regardless of other settings....but that's typical operation and most cars have been set up that way for decades

Cheers
DD

 

If the A/C state light is on, the compressor IS engaged unless it is being inhibited by the engine management system, [high eng. temp, wide open throttle and low engine speed for V12] or the ACCM because of high refrigerant pressure. On the X300 V12 and 1997 XK8, there is also a compressor speed sensor, monitored so the clutch can be disengaged if it sees an impending compressor seize, that way it can save the serpentine belt to maintain water pump and steering pump.

Cheers,

 

I have an 03 XJ8, and the Driver's Manual DOES mention "turn on A/C" (occasionally)
for "lubrication and to keep any seals pliable."

 

The AC does run in "auto" when heating to dehumidify the air.

Mine is always in auto

 

So.....my compressor shouldn't be cycling in-out?


Cheers
DD

 

The light on the button means it's in automatic mode. Climate control takes care of running the compressor when needed. It won't run if not necessary, so it's quite safe to keep it in auto.

I guess the switch is there just so that you're able to turn it off completely if needed... dunno why it deactivates the climate control when you turn it off though. Kinda like the cruise control button overriding the sport mode. Or the fact that you need to depress a button to activate something. Or that windows open with a single press, but to close them you need to hold the button (while the sunroof opens and closes with a single press)

If you want to force the compressor to run (for lubrication or whatever), you need to set a low temp on the climate control, or use the DEF-button. And even then the compressor will not run unless the outside temperature is over 3 degrees celsius, to keep the evaporator from freezing.

 

I just leave mine on Auto and let it get on with it. I adjust the temp up or down, but Auto is on. The system is very clever and works well in all seasons at all temperatures, that is why it has been equipped with an AC clutch so AC is only used when the car senses it's needed.

A little off subject though:
I tire of all these "Mondeo Money" jag buyers who try and save an egg cup of petrol by turning the AC off and running round in winter with steamed up glass, and then moan when it blows out hot and doesn't function properly when they come to use the AC on that special occasion once a year! On a 4.0 engine the AC being off will make little or no difference to fuel consumption.

Rant Over.
S.

 

Well said, like hyper-miling a Hummer...

 

Well, when mine is properly charged the compressor very, very rarely cycles; the expansion valve takes care of the regulation. If the charge gets low, the low side pressure will fall below the 29 psi limit of one side of the triple pressure switch. That part of the switch is really there to kill the compressor if the refrigerant is lost; but it will cause excessive cycling if the charge is low enough. I 'spose that could happen also, if the expansion valve isn't quite throttling right.

Cheers,