Should a/c compressor cycle on/off while driving?

[abscate]

My 1999 V70 sticker says 825g R134a and 750g in "hot markets"

I think TN and NC are hot markets....whatever....but I think that gives an idea of the 'leeway' of how you can adjust the R134A load.

[cn90]

I just hooked my pressure gauge to the low-pressure port and made an interesting observation:

1. With AC button ON and Fan Speed at Low (@1/4 of the slide button): the AC Clutch cycles, and the pressure fluctuates between 22-35 psi. When the pressure is below 25 psi, the AC Clutch cuts off, and the cycles repeats itself.

2. With AC button ON and Fan Speed at High (@3/4 ---> Max of the slide button): the pressure hold steady at 32 psi, which is above the cut-out point and the AC compressor runs continuously.

My theory is when the Fan Speed is on 3/4 ---> Max, there is more heat removed the Evaporator, therefore more evaporation from liquid to gas states (simple Physics 101). Therefore more gas reaches the low-pressure side ---> keeping the pressure steady on the low-pressure side.

So if you don't want the AC to cycle, turn the Fan Speed up a bit.

[wheelsup]

Thanks JDS. No worries, I was able to at least suck out a good amount of air using my air powered unit. I did bleed the air from the line as well, so that certainly helped keep the integrity of the system in place.

I wonder why you'd have less charge in a warmer market? Would seem counter intuitive. Maybe because the pressure is higher the hotter the temp so no need for as much of a charge?

[wheelsup]

I just hooked my pressure gauge to the low-pressure port and made an interesting observation:

1. With AC button ON and Fan Speed at Low (@1/4 of the slide button): the AC Clutch cycles, and the pressure fluctuates between 22-35 psi. When the pressure is below 25 psi, the AC Clutch cuts off, and the cycles repeats itself.

2. With AC button ON and Fan Speed at High (@3/4 ---> Max of the slide button): the pressure hold steady at 32 psi, which is above the cut-out point and the AC compressor runs continuously.

My theory is when the Fan Speed is on 3/4 ---> Max, there is more heat removed the Evaporator, therefore more evaporation from liquid to gas states (simple Physics 101). Therefore more gas reaches the low-pressure side ---> keeping the pressure steady on the low-pressure side.

So if you don't want the AC to cycle, turn the Fan Speed up a bit.

What you are seeing makes sense logically. I notice once I get going for a bit the system will cycle a lot more than when I'm not moving. I believe the system is fully charged so am just going to roll with it for now.

[abscate]

Ill add a comment to this great thread.

I certainly went into this thinking if I turn my AC controls (ECC) fully to COLD (Blue dot) then the compressor should be engaged full time.

Thats wrong, per Jimmys note above. The system will cycle to protect the compressor from freezing in this mode as part of normal operation, using the system pressure as feedback.

[JDS60R]

Less charge means less head pressure. TN and NC are not officially hot markets although I would disagree on certain days .
They are thinking the equator is a hot market, I think. I tried to get the hot market products for the R to enhance cooling and had to special order them.

[abscate]

Well, heard back from Europarts and the condensor member to evaporator line does NOT ship with the tube...

DOH!!!!

So it looks like this explains my 5 minutes of AC I'm getting...I get cooling until the pressure gets equalized in the system and then nada. The orfices in the receiver/drier probably give enough impedance to explain this initial cooling.

[wheelsup]

Well, heard back from Europarts and the condensor member to evaporator line does NOT ship with the tube...

DOH!!!!

So it looks like this explains my 5 minutes of AC I'm getting...I get cooling until the pressure gets equalized in the system and then nada. The orfices in the receiver/drier probably give enough impedance to explain this initial cooling.

Haha that is funny. Glad you got it figured out. I bought mine on RockAuto.com for under $3 including shipping. I think they are a standard size, it's "white" orifice IIRC.

[abscate]

Usually my problems arise from orfices, as opposed to lack of them.

The good thing about this is it will be fun to see how fast I can do a job like this second time, with all the tools known and where to put your hands all recently done...I'm figuring 1/3 the time, minimum...

[wheelsup]

It will be easy. Get the 134a out of the system. 6mm hex to undo the HP line. Use the "quick-ish" disconnect tool to undo the evap inlet line. Take the ECU box out and unclip the expansion tank and place it on the top of the engine. Undo the large tubing from the ECU to the firewall (not sure what that is). This took the bulk of my time, maybe 3-4 mins. Get the line out and plug up the openings so moisture doesn't get in, although to be honest it probably isn't that important, you're only going to be open to the atmosphere about 2 mins.

Undo the line and put the new orifice in there. Button it up you're done.

I'm intimately aware of how much it sucks to do something twice. Last fall I put my RMS in incorrectly and it leaked like crazy. Second time around was only 12 vs. 18 hours. Yay.