A/C Compressor Cycles when in Defrost and Blend Mode

[LastCat]

This really isn't the season for A/C questions, but it is something that I am currently working on.

I have a 93 AWD I just bought, W engine, front and rear A/C and Heaters. I just did a tune up, plugs, cap and rotor. Runs good. Then I was cycling through the heater controls front and rear. A/C is working fine, and plenty of heat. But...

What I did notice is that the A/C Compressor comes on when I select 'Blend' and 'Defrost'. But if I select 'Heater' and 'Vent', then I can hear/see that the Compressor is off. Also, compressor comes on if A/C is selected. This is done with the engine running and the engine off. So I can't say its has to do with Vacuum, its seems to be electrical. Should the A/C compressor be cycling when not in A/C mode?

Is this normal?

I pulled the heater controls, and opened the switch (1 copper plates, 3 copper connectors, and 3 springs, very simple). All the connectors look clean. The housing looks great.

Anyone have ideas?

Thanks
Cheers
LastCat

[CaptSquid]

Perfectly normal. The defrost setting starts the AC to help dehumidify the air. Likewise the Blend setting. Don't worry about it.

[LastCat]

Thanks Capt.

I just found this online, ya learn something new everyday.

A/C Compressor Cycling or Not Engaging.

The clutch cycling orifice tube (CCOT) A/C systems such as the one in your Volvo are designed to cycle the compressor on and off to maintain a workable system pressure. While the thermal expansion valve systems in other cars vary other factors, the FOT (fixed orifice tube) needs to have the flow maintained at a steady pressure. That's why the pressostat is on the low side of your system. It senses when pressures are too low, and therefore too cold. It stops that to prevent icing. The reason for the frequent cycling of the compressor clutch is not dependent on only one factor. The clutch on the compressor will cycle when the refrigerant pressure drops to 25.

The pressostat causes the clutch to cycle. If the pressure is lower than about 20 on the low side (I prefer that to 25psi for better cooling with R134) then the evaporator will ice, as will the accumulator and its associated tubes. Then you've got no cooling at all, and the compressor will shut off. The compressor will cycle for the following reasons:

1. Ambient temperature low, refrigerant pressure low as a result of low ambient temp.
2. Ambient temp high, cabin temp low, evaporator exchanges little cooling because cabin is already cool.
3. Ambient temp warm to high, fan set on low, same reason as #2.
4. Ambient temperature high, cabin temperature high, system charge low.
5. Faulty pressostat switch

To put it all together, the system is cycled to maintain the most useful pressure for commonly encountered temperatures. With a FULL charge, the system will cycle very frequently in cooler temperatures, and sometimes will not have the pressure to EVER come on. That is how the CCOT systems work, and why they cycle the clutch a lot. The thermal expansion valve systems cycle the clutch less. If the charge is proper, in high temperatures, you might never see a time when the compressor clutch cycles off, until the cabin is cool, or the temperature drops outside. If the charge is too low, it'll cycle frequently to keep ice away.

If the charge is too high, you will blow the system up. That's why you need to know how much refrigerant is in the system. In very high ambient temperatures, it's occasionally advantageous to have a slightly low charge, only because the pressures of the system are maintained at a more optimum level despite the huge outside temperature differential. But that same system will be largely ineffective if the temperature is lower.

The whole picture is, you should make sure you have the proper charge as often as possible, especially if the system has a leak. That way, you'll be assured proper cooling, good compressor life, system integrity, and for you, sanity. Fortunately, A/C work is NOT very hard if you take the time to read about it a little bit and learn about what causes it to function as it does.

Lastcat

[Hotroder383]

All cars kick the AC on when you set the heater to defrost. I may be wrong in this but i have always unplugged the ac comp during the winter months. Most of the cars i have drove got bad gas milage with ac on(yes esp the Safari) I wanted to run it as little as pocciable.

[CaptSquid]

Bad move. Your AC helps dehumidify the air and reduces the fog on the windshield and side windows. Without the AC, it takes longer to defog the windows. You don't have to run the defroster all the time.