A/C not working on 1.4L Fiesta Mk7 (2010).

All fuses and relays checked and ok. Bypassing the A/C relay when engine off visibly engages the compressor clutch so that's ok. The problem must be associated with being unable to trigger the relay switch so the clutch never gets engaged.

I'm suspecting the a/c pressure switch is open because the pressure is too low and I need the system re-gasing (just bought car so don't know when it last worked or was re-gased). However before I spend money re-gasing I just want to check that the pressure switch does indeed measure LOW pressure rather than high. I have read that many a/c systems have two pressure switches, one for high and one for low. The circuit diagrams show my car has only one pressure switch so its not clear whether its high or low pressure its triggered by.

If its a high pressure switch and its been triggered by high pressure in the system then a re-gas won't be a solution. However, if a car only has one pressure switch I'd guess it would be the more useful low pressure one because running out of refrigerant gas is a more common problem.

Anyone know more about the a/c pressure switch?

 

ac pressure switch.pdf

It's a low pressure switch. When the gas is low it prevents the compressor running to stop it from destroying itself.

Thanks for confirming @unofix. I’ll get it regassed next then. 
 

Btw. In case anyone’s near Tamworth there’s currently a groupon deal for a 1/2 price regass. Just £25 for R134A refill. If you are new to groupon it’s only £20. 

Yeah, but where's the gas gone? See if you can get them to add a UV dye so that the leak can be detected with special glasses and UV light. 

On 1/8/2025 at 4:18 PM, Nic Williams said:

The circuit diagrams show my car has only one pressure switch so its not clear whether its high or low pressure its triggered by.

It's not a pressure switch but a pressure transducer i.e. it converts system pressure into an electrical signal and thus is able to measure the actual pressure and allow the PCM to make the call regarding whether it is too low, high or whatever.

All that said it is most likely to be low due to the risk of refrigerant loss as you say. Not that it matters either way - it needs someone with the right kit and expertise to diagnose and repair it so I would leave it to them. They (or rather their machine) will do a leak test before regassing anyway, and don't usually charge if a leak is found as they are unable to proceed further without it being fixed.

20 hours ago, MJNewton said:

It's not a pressure switch but a pressure transducer i.e. it converts system pressure into an electrical signal and thus is able to measure the actual pressure and allow the PCM to make the call regarding whether it is too low, high or whatever.

Thanks @mjnewton. That’s a useful clarification. So having just one pressure transducer will still mean over/under pressure protection in my car.
 

Thinking back at what I’ve read elsewhere makes sense now because others cars that have two pressure transducers have them on both the lower pressure pipe into the compressor and also the high pressure pipe beyond it. I’d wrongly assumed the one on the low pressure pipe monitored low pressure and the one on the high pressure pipe monitored high pressure. 

50 minutes ago, Nic Williams said:

Thinking back at what I’ve read elsewhere makes sense now because others cars that have two pressure transducers have them on both the lower pressure pipe into the compressor and also the high pressure pipe beyond it. I’d wrongly assumed the one on the low pressure pipe monitored low pressure and the one on the high pressure pipe monitored high pressure. 

It purely depends on the design of the system and is also influenced of course by the art of the possible in terms of functionality and cost. Basic limit switches are limit in functionality but are of course cheap, whereas dual/multi-function sensors can do more but at potentially higher cost. Historically aircon systems were purely solid state and so had no option but to utilise crude solid state switches, whereas with them them being ECU controlled they can utilise more advanced sensors in their control loop.