This is about a 2010 mazda2 but I’m putting query here as many many more people read fiesta forum than Mazda 2 and principles of aircon much the same I guess.  Switching aircon , light comes on but revs don’t dip and pulley does not engage.  No power to pulley solenoid.  I ran a wire from battery to solenoid and that makes it engage. But no cold air.  Fuses are fine.   Is it normally the case that there is a aircon gas pressure sensor and if that reads low pressure it stops the computer/gem/bcm from energising the pulley solenoid?

1 minute ago, isetta said:

Is it normally the case that there is a aircon gas pressure sensor and if that reads low pressure it stops the computer/gem/bcm from energising the pulley solenoid?

There is indeed a pressure sensor in the aircon system. If there is no gas or very little gas then it will prevent the compressor running to stop it running dry and causing damage.

Thanks. I see the logic in that. it’s my wife’s car. If it was mine I’d probably just put up with no aircon but she’s more fussy than me. Perhaps wait until after summer when we might get a special offer cheap price on regas.   But could be a waste of money as it might have a leak and there’s no way of establishing that without a degas. So could be money down the drain

I think Kwik Fit have some sort of deal that if they re-gas your car and it doesn't work they give you your money back. I'm sure that @StephenFord has mentioned it before.

Waiting until after summer sort of defeats the object...  The car might get written off in spring and you won't get the benefit of it!

There are 2 main ways to detect leaks.  First is holding vacuum in the system once it's been emptied.  This is what all garages should do for at least a few minutes.  Trouble is, it's not really long enough to detect minor leaks.   The other method is to pressurise the system with nitrogen.  Any AC specialist should have a cannister of nitrogen for this purpose.  

So you've really got two choices.  Use a chain garage who should be cheaper (check the ATS Groupon) and may offer a no-chill, no fee, deal.  Or use a proper AC specialist who will cost more but should be able to detect leaks first.

I take your point about if it’s written off in the meantime.   I always say that when people I know get a crack in their windscreen and replace it straightaway.   I always say if your mot is not due, wait until it is.  If you replace now and something happens before next mot (write off, theft, windscreen damage) then it was a waste of money (typically £75 xs)

And then there’s also people who replace them when the crack is not even a mot failure due to its location/size.  What a waste of money and a waste of the world’s resources. Some windscreen companies would have you believe your car will self destruct if there is a crack or even a chip in the windscreen.  Of course it’s in their interests to spread that worry.

You can usually find some offers on wowcher and the like for half price or some other deal, if they're gonna regas they usually do a system check first.

6 hours ago, isetta said:

And then there’s also people who replace them when the crack is not even a mot failure due to its location/size.  What a waste of money and a waste of the world’s resources. Some windscreen companies would have you believe your car will self destruct if there is a crack or even a chip in the windscreen.  Of course it’s in their interests to spread that worry.

When my armrest was torn I replaced it, when one of the boot's clips broke I replaced it, if I had a window chip I'd get it fixed, if I had a window crack I'd fix it through insurance. If it looks a little dirty I take it for a wash.

 

Some people just like keeping their car in good nick, rather than just seeing the money.  🤷‍♂️

That said, I've a scratch on the rear bumper (plastic, no metal shown) and scuffed up alloys.  Getting a bit steep for something purely cosmetic there. If it were a newer car I'd repair it. 

 

As for air con gas, every couple of years I swap it out like clockwork to keep it pressurised and water out of the system.  I use the AC all year around too to keep it maintained, it's one of those things that fall into disrepair by not being used. If you think filling it is expensive you should check the cost to get a new AC compressor fitted .