AC cycling - advice please.
 

Last year we replaced the evaporator in our 1995 850 sedan.
Big job but no problems.
Had it evacuated and gassed up by a professional - no problems and it pushes out good cold air.

What it always seem to do though is the compressor cycles on/off every 5 seconds no matter what the outside temperature is. Even on hot days it still cycles in these 5 second short bursts.

I've had the gas pressures tested again and all is okay.

I've often suspected the pressure switch on the pipe about centre of the firewall is faulty. If I jumper it out, the AC stays on and doesn't cycle (at idle).

My question is, does the AC depend on this pressure switch to detect high or low pressure to control the cabin temperature, or does the AC unit inside the car have a thermister or some type of temp control?

In other words, I don't wish to jumper the pressure switch and go for a test drive to see if the compressor cycles at longer intervals if this switch is the cycling control and the AC explodes by being forced on continuously.
Or can I prove if this pressure switch is just a failsafe and behaving faulty in causing the constant cycling?

At idle, if I jumper this pressure switch, the AC stays on and the evaporator pipe coming through the firewall ices up which I haven't seen before.
But I don't want to go for a drive in case I'm wrong about the switch and i cause some serious damage.

The switch at the bulkhead is the low pressure switch, if you bridge it the compressor it will stay on even if the pressure dropped below safe levels.
Problem with that is it could seize the compressor if there wasn't enough oil being carried around with the gas if a leak developed.
There's also a high pressure switch behind the right headlight area which controls the high pressure side of things it's controlled directly with the engine ecu.
E.g. when the pressure in the system rises it puts the cooling fan on either slow or fast speed depending on the pressure or disengages the compressor for too much pressure.
Also things like a 5-10 second delay before engaging the compressor after start up to allow stabilising of the idle speed.
The A/C system on these cars is of the fixed orifice type rather than having an expansion valve in line to the evaporator which means that the low pressure switch controls the off/on cycling the compressor rather than an ice thermistor on the evaporator with the other type of system.

Hey thanks for the explanation. Terrific and appreciated.
So going by your last sentence as quoted above, this switch could be faulty and causing the AC compressor on/off every 5 seconds?
It does this day or night, hot or cold. On/off/on/off/on/off every 4 - 5 seconds and i know thats not correct.
The air is cold from the vents but it does take a while to cool the cabin if its been locked up a day in moderate sunshine because of this constant on/off cycling. I'm sure if it cycled on for longer periods like any other car AC I've experienced it would be even better.
Its a drag feeling the engine pull back every five seconds under load up long hills and such as well.

Do these switches come in different pressure (perhaps for different countries ambient temperatures) or are they all the same?
ie: if I find one in america for a Volvo 850/C70 will it be correct for me here in Australia?

Also - I saw mention of the AC pressure valve having a schroder valve under it so it can be swapped with minimum gas loss. Is this for the Low Pressure valve?

The pressure sensors are all the same. When the system is fast cycling this is a sign that the system is low on gas.
Both pressure switch and pressure sensor have a shradder valve beneath so replacing them results in virtually no loss of gas.

The only difference between the A/C system for the hot markets and "normal" is the quantity of gas which goes into the system.

One of the faults on my AC when I bought the car was cycling like you describe.

It was caused by the magnetic clutch on the compressor having worn and the gap being too big.

There is an explanation somewhere on the forum how to reduce the gap without removing the compressor by using cable ties on the clutch assembly.

That is the method I used and it has been working fine for years now.

The alternative is to remove the compressor and to re-shim the clutch.

Colin

If the clutch gap is too large, the clutch won't engage and cycle at all when warm but is ok cold.

My AC was cycling - I fitted the cable ties - now it doesn't.

Got the info from this very forum.

Colin

AC cycling isn't necessarily a fault as it depends on a number of factors..... Actual cabin temperature, cabin temperature setting, ambient temperature, evaporator temperature etc.
Regardless, cycling is not caused by too large a clutch gap.