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

[wheelsup]

There is no need to draw the vacuum on both sides on our cars. See my above explanation. We have an accumulator and orifice tube. The orifice tube creates the restriction on the high side and lowers the pressure from high to low using a small tube behind an o-ring. Both sides will equalize in pressure.

When I sucked down the system for 2 mins and turned closed the service port, I saw pressure rise. I interpreted this as moisture however I don't believe it was the case. I believe it was the pressure coming thru the orifice tube equalizing the pressure.

http://youracauthority.com/acterms/Auto ... nter.shtml

Do you have a link for the adapter needed to attach the gauge set 1/4" NPT red hose to the high pressure side? I haven't been home to mess with it.

[wheelsup]

OK update.

The compressor was cutting out due to the low pressure of the system cutting it out.

I added roughly 5 oz of refrigerant, it will still cycle at higher RPMs off idle. At idle it holds 25 psi, which still seems low. Ambient temp was 85*F the output temp w/ recirc on was 38*F while at idle not moving. When the compressor runs it sounds nice and smooth, very quiet.

At highway speed it still seems to be cycling just as much as before. So I guess that is normal for my unit. Temps are nice and cold.

I'll hope for the best. I really hope there isn't a leak. When I sucked it down it seemed to hold the negative pressure (all ~14 psi of it) for 15 mins or so without moving, so I would assume any MAJOR leak would be shown there. I know pressurizing the system would have been better, but I have no way of doing that.

I do wonder if the running pressure @ idle should be higher. 25 psi sounds low. I don't want to overcharge the system however to bring the pressure up. Also wonder if there is a leak, as the running pressure before was much higher, although it was probably about 10-12*F hotter then. I can't imagine a leak big enough to leak down over 6 days would not have moved the pressure on the manifold set.

[abscate]

Well, I should be tightening mine up and pumping it down today

My "member" to condensor hose (9171318)looks really snarly and I'm on the fence about using it. Condensor too, for that matter. I would like 3-5 years out of the car, so maybe I should just replace both?

I usually don't discuss members on the Internet, truly, but this one I could only find online at fcpeuro





Matt.i sent you a PM

[jimmy57]

First, ALL A/C DIAGNOSIS IS DONE WITH ENGINE SPEED AT 1500 RPM +/- 200 RPM. (OK, if you have a car with a variable displacement compressor this is faintly less important. The cars in this forum do not have a VD compressor) I am not making this rule. It is in every factory service manual.
The pressure you get with engine idling is more or less meaningless as compressor is at a speed where its rated displacement is not being reached. This is why a perfectly working system will warm up when you stop and idle on warmer days.

On orifice tube systems "normal" pressure is widely variable. "it is running 35 psi and that seems about right" is far from a true statement. The refrigerant pressure varies with ambient temp, fan speed, temp difference between car and incoming air across evaporator and of course, engine speed if below 1200 RPM.
Since there is no throttling of refrigerant flow on orifice systems, the system works at full capacity all the time. When the heat energy goes down as car cools down and moreso if the heat level is lower due to cooler days, the compressor is turned off to prevent freezing. The pressure and temperature of refrigerant are directly related so the compressor is switched off at 22-25 psi and will be switched on at 42-45 psi. Cycling once the vent temps go down to 50 F range is totally normal and desireable and indicates proper charge. Cycling frequency is energy level dependent. Cycling rapidly with vent temps of 60 F or above is a sign of low refrigerant as an undercharged system goes to the low low side pressure because compressor sucks down the low side, not because the temp of the refrigerant is getting lower causing the pressure drop. Rapid cycling can occur with full charge and the orifice inlet side screen partially blocked with debris (usually from failing compressor).

Procedure for a/c evaluation:
1. run engine at 1500 RPM
2. windows closed
3. recirculated air setting
4. Fan at 75% speed
5. air discharging from dash face vents
6. allow system to run for 8-10 minutes
Now monitor vent temp and system pressures
If you turn down the fan speed and the compressor cycles at the pressures above with vent temps in the 44-52 F range then your system is working as designed. IF you turn up the fan speed and cycling stops with higher vent temps, again this is normal.
If it is a cooler day then the cycling may occur within 2-3 minutes of starting the evaluation. If it is a 100 F + day it may never cycle and vent temps may not get to much below 55F and not cycle.
Car parked in the sun will decrease chance of cycling as heat load into car is greater.

High side pressure is also variable with heat load due to ambient temp and inside car temp.


Note: provide a fan to help airflow across condenser if ambient temps are above 95 F

[wheelsup]

Well, I should be tightening mine up and pumping it down today

My "member" to condensor hose (9171318)looks really snarly and I'm on the fence about using it. Condensor too, for that matter. I would like 3-5 years out of the car, so maybe I should just replace both?

I usually don't discuss members on the Internet, truly, but this one I could only find online at fcpeuro

I got your PM. I don't need the gauges, I have a set of those. The pump doesn't really help detect leaks. At max differential pressure you're talking 14.7 psi. That is with a perfect vacuum. The proper way to do it would be to pump down the system, then pressurize with nitrogen or other inert gas to XXX psi, then see if the system pressure falls. If not you are good. I don't have a way of pressurizing the system to that level of pressure unless you use compressed air which is laden with moisture so probably not the best way to do it...

Thank you for the offer, it was very nice of you. Wish we were closer so we could've done it at the same time, and pooled our resources.

[wheelsup]

First, ALL A/C DIAGNOSIS IS DONE WITH ENGINE SPEED AT 1500 RPM +/- 200 RPM. (OK, if you have a car with a variable displacement compressor this is faintly less important. The cars in this forum do not have a VD compressor) I am not making this rule. It is in every factory service manual.
The pressure you get with engine idling is more or less meaningless as compressor is at a speed where its rated displacement is not being reached. This is why a perfectly working system will warm up when you stop and idle on warmer days.

On orifice tube systems "normal" pressure is widely variable. "it is running 35 psi and that seems about right" is far from a true statement. The refrigerant pressure varies with ambient temp, fan speed, temp difference between car and incoming air across evaporator and of course, engine speed if below 1200 RPM.
Since there is no throttling of refrigerant flow on orifice systems, the system works at full capacity all the time. When the heat energy goes down as car cools down and moreso if the heat level is lower due to cooler days, the compressor is turned off to prevent freezing. The pressure and temperature of refrigerant are directly related so the compressor is switched off at 22-25 psi and will be switched on at 42-45 psi. Cycling once the vent temps go down to 50 F range is totally normal and desireable and indicates proper charge. Cycling frequency is energy level dependent. Cycling rapidly with vent temps of 60 F or above is a sign of low refrigerant as an undercharged system goes to the low low side pressure because compressor sucks down the low side, not because the temp of the refrigerant is getting lower causing the pressure drop. Rapid cycling can occur with full charge and the orifice inlet side screen partially blocked with debris (usually from failing compressor).

Procedure for a/c evaluation:
1. run engine at 1500 RPM
2. windows closed
3. recirculated air setting
4. Fan at 75% speed
5. air discharging from dash face vents
6. allow system to run for 8-10 minutes
Now monitor vent temp and system pressures
If you turn down the fan speed and the compressor cycles at the pressures above with vent temps in the 44-52 F range then your system is working as designed. IF you turn up the fan speed and cycling stops with higher vent temps, again this is normal.
If it is a cooler day then the cycling may occur within 2-3 minutes of starting the evaluation. If it is a 100 F + day it may never cycle and vent temps may not get to much below 55F and not cycle.
Car parked in the sun will decrease chance of cycling as heat load into car is greater.

High side pressure is also variable with heat load due to ambient temp and inside car temp.


Note: provide a fan to help airflow across condenser if ambient temps are above 95 F

Jimmy,

Thanks. I did end up seeing that procedure in the FSM that I downloaded off here. What confused me is a previous poster who stated his is solid at around 32 psi on the low side when fully charged. What I am seeing is as the RPM of the motor goes up, the pressure drops, compressor cuts off, and pressure rises. Then compressor clicks on again and process is repeated. It's of course less at first when the temp is warmer. Whats unnerving is how much it happens on the highway, every 6-10 secs it seems it's off, then on for 15-18 secs. I guess that is normal?

With output air of around 40*F I am happy and feel confident the system is working as needed. I have been without AC so long (properly charged at least) that I just thought a compressor cycling was bad. Only time will tell if the system is sealed up properly. I bought a cheaper evaporator off FCP and had to use a large round screwdriver to twist the input air into place, the quick connect fastener clicked in fine but there is just a feeling in my stomach that says I screwed something up. If I made the hole a little out of round the o-rings might not be sealing correctly. I did spray soapy water on the line and to date do not see any bubbles. Crossing my fingers.

For the time being, I'm incredibly happy that the air coming out of my vents, for once in a LONG TIME, actually has no smell to it. I installed the cabin air filter mod so hopefully it stays like that! I do plan to use some sort of spray cleaner every now and then to clean bacteria off the evaporator. Also I turn the a/c off and keep the fan going for the last few minutes of my drive to hopefully dry out the thing as best as I can before stopping.

[abscate]

My pleasure. My system held vacuum all morning, ran cold-cold-cold for 10 minutes then cycled off with no cold air, so Im still in TBLSHT mode.

Jimmy - thanks for that tutorial - that was really helpful. I will try to report data using that testing protocol from here. I guess I use a telescoping pole to hold revs at 1500 or else con one of the kids with a steady foot to sit in the car.

I agree that the vacuum pump sucks (!) for finding leaks. I do have dry Nitrogen from welding I could use to pressurise.

Here is a really NooB question - I replaced the condensor to Evaporator line on this job, assuming the orfice valve is contained inside this assembly. I didnt take it apart to check.

[wheelsup]

The orifice valve is in the line from the evap to the HP sensor area (held down by the hex bolt). Yep separate the two sides. In order to get the line out I had to take the ECU box out (super quick) and pull the large hose looking thing from the firewall out as well.

Pull the line out and clamp it in a vise. Pro tip (ie I found out the hard way): MARK THE TWO PIECES on each line (not the nuts) so you know where they go in relation to each other before undoing them. I would use something like white-out, as a black marker did not show up well for me. Then you just unscrew the two sides and pull the orifice out with needle nose pliers, lube the new one (o-ring) and put it back in. Then line up the two lines and screw it back together.

If you replaced the line, the orifice should be in there already. I don't think there is a way the system could give you cold temps without a restriction in it. To confirm, with the system on and operating, run your hand along that line. Up top by the evaporator should be cold, down near the HP switch it should be warm-ish. That is the HP liquid from the condenser.

The only thing I wish I had done differently is used a real vacuum pump to evacuate the system.

[JDS60R]

Matt,
I see the confusion. We see 32 psi ( lowest number on low side before cycle)on a stabilized system ( stabilized system = run for 8 to 10 minutes as described in FSM) I think this is where the confusion came from.

The pressures are always dropping and rising with the cycling of the compressor, air flow etc. We do fill a little more than Volvo suggests and have had great results as we get a little more cooling and the customer comes back less often as AC systems all leak. We do not see excessive cycling.

Everything Jimmy attached from Volvo is correct. We just prefer the results with a little more R134a in the system.

[JDS60R]

Matt,
Wish you were here. I would have let you borrow one of my vac pumps and a micron gauge.