Well today I got my Air Con re-gassed; and it's blowing cold air again, thankfully - considering we've had a few 40 deg C plus days here in Sydney of late!

Only problem is it's blowing REALLY cold air. The guy who came around to re-gas my 850SE said that the air temperature was reaching -1 deg C. He didn't seem to think there were any leaks...

It would seem that the compressor is running constantly when the Air-Con is switched on; when I thought that the compressor would cycle to try to keep the air temperature at around 5 deg C!

I'm wondering if the air-con system has a thermostatic switch somewhere that would help maintain a slightly warmer temperature - I'm scared to leave it running in case everything freezes up and something seizes.

My 1995 850SE has manual air-con, not auto.

Any ideas? Google's not proving to be too helpful in this area for me this evening!

 

There is a pressure sensor and temp sensor for the AC. I bet one of the two sensors are going out.

 

Thank you, sir! I think you're probably right.

Any idea where those two sensors are located - or their exact names so that I can search for replacement parts?

(Man, I really need to get myself a workshop manual!)

 

AFAIK a bad pressure sensor would make the compressor fail to start, not to stop.

Are you guys sure that a manual A/C has a cabin temperature sensor? Isn't that only for the Electronic Climate Control?

 

You're right, man - I don't think my manual system has any cabin temp sensors. I've been reading through the service records that came with the car, and I noticed that in a previous service, the mechanic made a note that there may be "a possible leak in evaporator". It must be a pretty slow leak, however, as the Air Con Re-Gassing Guy didn't mention any leaks when he de-pressurised the system.

Another weird thing I noticed, that may or may not be related:

When the Air Con is switched OFF, the temperature gauge works as normal, eventually reaching the "3 o'clock" position. If the Air Con is switched ON, the temperature gauge fails to reach "3 o'clock", and won't go any higher than about "5 o'clock".

Even with the engine hot, and the gauge reading "3 o'clock"; if I turn on the Air Con, the temperature gauge will slowly drop to "5 o'clock".

 

There's no cabin temp sensor, but there's still an evaporator temp sensor. (I think) It's possible that a pressure sensor could fail in the closed position, though that's unusual... maybe someone bypassed the low pressure (cycling) sensor... I've seen that before, too. Take it back to the shop and tell them it's doing this- too low of a pressure could damage the compressor.

It's also entirely possible that there's a slow leak in the evaporator... they fail all the time on these cars.

 

From my memory

There are two pressure switches one is the low pressure line at the firewall, it is a normally open contact that closes when the low pressure side has high enough pressure for the system to operate, if it didn't close the clutch wouldn't pull in and no AC.

The high pressure switch in the high pressure line just behind the right headlight, is a normlly closed contact that opens when the high side pressure reaches the high pressure limit, if it doen't open the clutch won't drop out and you have an over pressurized system and could see what you're seeing. I'd take a look at it.

 

Thanks for the replies, guys.

I'm at work at the moment, and just slipped out to quickly check around the evaporator, and evaporator drain hose, and can find no signs of water or dampness - so IF the evaporator core IS leaking, then it must be very slow, and draining through the hose - and not into the cabin; I'll have to keep an eye on this, I know this is a VERY painful thing to fix.

I'll have someone check the High Pressure Switch, as this diagnoses seems to explain my issue.