Driving the aircon. system?

[barry]

Thanks for all the replies. I have a better idea now of what is taking place in the system.
Sounds like my system is quite normal.
Did wonder what that little horizontal wheel did in the centre. Will investigate further. Didn't appear to do anything last attempt.

Regards
Barry

[uk_dave&gill]

Just for clarification about the little wheel in the centre....

Is it white side for ambient temperature air through the face vents and blue for air-conditioned air?

[HarryM1BYT]

Originally Posted by SD1too

James,

This is not quite right. In 'auto' the compressor is running constantly to maintain the evaporator at about 4 degrees C. The air conditioning is therefore operating all the time. If the cabin temperature rises above the selected temperature, the air flaps are moved to change the blend of heated air and cooled air until the selected temperature is reached. The system mixes air, it does not turn the compressor on and off.

I've not actually checked the system on the 75, but standard car practise is to cycle the compressor on and off as needed, which makes James correct. You are also correct in that the fine control of cabin temperature is by adjustment of air mixing flaps.

[SD1too]

It's in the owner's handbook, but for those who haven't got one this is what mine says:

Air supply from the face level vents is further controlled by the heater by-pass control in the centre of the fascia panel.

The purpose of this feature is to enable cooler air to be directed towards the face at those times when the heater is required to provide hot air in order to keep the interior of the car warm - particularly useful during winter. The by-pass is controlled by turning the thumbwheel:

• towards the BLUE spot to open the by-pass
• towards the WHITE spot to close the by-pass.

If the air distribution control is set to foot, screen or window vents:

• Heater by-pass closed - all air flow is prohibited.
• Heater by-pass open - unheated (fresh) air supply only.

If the air distribution control is set to face, or foot and face vents:

• Heater by-pass closed - heated air supply (temperature as heater setting).
• Heater by-pass open - a blend of heated and unheated air (which has by-passed the heater) providing a cooler output.

A bit over complicated perhaps, but a great feature which no other manufacturer (as far as I can remember) offers.

Simon.

[SD1too]

Originally Posted by HarryM1BYT

... but standard car practise is to cycle the compressor on and off as needed, which makes James correct.

Harry,

There isn't a "standard car practice"; it depends upon the design of system. Some compressors do cycle, for example the CCOT system I have on my SD1, but it does not cycle in response to cabin temperature demands. If you think about it, such a system would be too slow to respond to changes. Cabin temperature control is achieved by other means. There are air conditioning text books available if you're interested in learning about it.

The 75/ZT system is not a cycling system; the compressor clutch stays engaged throughout. It's a variable displacement compressor.

I'm afraid that James is not correct Harry, as I have explained.

Regards,

Simon.

[uk_dave&gill]

Originally Posted by SD1too

A bit over complicated perhaps, but a great feature which no other manufacturer (as far as I can remember) offers.

A bit?!

Would it be fair to summarise then that, in summer, with air-con in use, then open is best (to get cooler air) and in winter, with heater in use, then closed is best (to get warmer air)

Or is this completely wrong, as it's only heater output that it changes, not air-con output?

Or is the best way to simply move it about on a regular basis and find it what feels best on each occasion?!

[HarryM1BYT]

Originally Posted by SD1too

The 75/ZT system is not a cycling system; the compressor clutch stays engaged throughout. It's a variable displacement compressor.

I'm afraid that James is not correct Harry, as I have explained.

Regards,

Simon.

OK - I got it wrong

[SD1too]

Originally Posted by uk_dave&gill

Would it be fair to summarise then that, in summer, with air-con in use, then open is best (to get cooler air) and in winter, with heater in use, then closed is best (to get warmer air)

It's personal preference. In winter I hate hot air blowing at my face so I use it a lot then, as the owner's handbook recommends.

Or is this completely wrong, as it's only heater output that it changes, not air-con output?

It works the same way whether the air con. is operating or not. The only difference is that with air con. the "cooler" air will be much, much cooler!

Or is the best way to simply move it about on a regular basis and find it what feels best on each occasion?!

Probably, yes!

Simon.

[uk_dave&gill]

Originally Posted by SD1too

It works the same way whether the air con. is operating or not. The only difference is that with air con. the "cooler" air will be much, much cooler!

Ah! This is the bit I am interested in...

Imagine the scenario: Been out for the day with OH and mother, to a town 30 miles away. Go to garage to fill up as running on fumes. Surprised to find a huge queue at all the pumps - surprised because I don't read the papers or watch the news and didn't know there was "going to be a strike"! Sat in queue for 10 minutes with air-con on full blow, but surprised to find the face level vents blowing quite warm

On journey back, the air temperature dropped a bit but still not as cool as hoped

After reading this post this morning, checked position of centre wheel and found it on full white (by-pass closed)

So..... Would it be fair to say that, now I have moved it to full blue, I should get nice icy air again?!

[MangoMan]

Sorry if this is a repost, but this link might help - A/C under 'Before You Drive'....

Oh well, must be - Sorry.

Cheers.