[Doug]

While one turn clockwise on the low pressure valve may be sufficient to check pressure, 3.5 turns is full open and more efficient for filling.

[Will]

Dan, thanks, will do. I was not sure if further opening would impair the circulation of refrigerant to the compressor.

Gerald

[John]

To fill the OTR system on the Prevost you need to make sure that the Carrier compressor is running on all 6 cylinders. They all need to be working and then you can use the guide of liquid in the receiver tank lower sight glass. The level should be no higher than the middle of the sight glass (colored bead bouncing in the liquid refrigerant).

If all the cylinders are loaded then they should all be warm to hot to the touch. The center cylinders are on with the clutch activating. The outer cylinders are controlled by pressure or electrical unloaders. The unloaders are used to control the low side pressure so that the system can work in warm to cool temperatures without cycling the clutch on the low pressure switch.

If you make sure these are working then your recharge will be more acurate and you will not under-fill or over-fill the system.

[Archived]

Thank you very much. I’ll make sure to check the cylinder temps. At this point I’m waiting for delivery of an A/C valve service wrench for the king valves. Then we’ll give this a try.

Gerald

[Dorel]

Today I finally had all the bits and pieces together (gauges, freon, wrench, pressure tables etc) and went to work.

I connected the gauges and had 70 psi with engine off. Ambient temp was about 85 deg F. This confirmed that it needed charging. I cranked up the engine, put it on high idle and max A/C. It took a few pounds of freon fairly quickly but then the charge rate slowed down considerably. After an hour or so, it had taken 9 lbs of refrigerant and pressures were between 30-35psi on the low side and 150-200psi on the high side. Still low for the ambient temp. It should have been approx 45/225 according to the tables.

The bus interior was cooling down nicely (air at vent was 58 deg F) and the compressor started unloading. That pretty much stopped the charge process. The red ball in the accumulator was still not floating, so the system still needs a few pounds of refrigerant.

I’ll give this another try once everything has warmed up again. Would it help to put the refrigerant tank in warm water?

Gerald

PS: I just re-read the Prevost Maintenance Manual section on A/C. It recommends putting the R134a tank in 125 deg F water to accelerate charging. As I charged, the container got pretty cold. At 125 deg F the saturation pressure is 185 psi, at 60 deg F it is only 62 psi. I bet this would make a lot of difference

[Tim]

I would wait till tomorrow, then check the vapor pressure reading against the table for the ambient temp. That will give you an idea where your at.

I assume your filling from a 30Lb tank? If so is the tank shutoff valve getting COLD when your adding Freon? It should be.

To get it to fill faster you need to add the Freon in a liquid state. If it was mine, I would open the suction king valve no more than 1 turn to add the Freon in a liquid state(faster) without hurting anything.

Heating the tank does force the Freon out, BUT I wouldn’t do it that way.

[Tim]

Gerald you wrote: “The bus interior was cooling down nicely (air at vent was 58 deg F) and the compressor started unloading”

By unloading, do you mean dropping pairs of cylinders? If so, how could you tell?

[Jerry]

How did I know the compressor was unloading (i.e. taking 2 or 4 of the 6 compressor cylinders off line)? Well, I saw the high end pressure go from 200 psi to 150 psi and the temperatures of the left and right cylinder heads went from 150 deg F to 105 deg F. The compressor clutch did not cycle. I think this behavior is normal but when it happens you are pretty much done charging using freon vapor.

Gerald

[Per]

Don, yes, I’m using a 30lb refrigerant tank ($140 delivered by Amazon). The tank did get pretty cold and started collecting condensation. I’ll try it again using a warm water bath for the tank as the manual suggests. I have not worked up the courage to go liquid on the suction side. I’m almost there pressure wise- I think I only need a couple more lbs.

It will be very convenient to do my own top offs once a year. I’ll also make sure the king valves don’t get over tightened. The high side was hard to open, the low side king valve was replaced three years ago and opened fine.

Thanks again for your A/C posts

Gerald

[Archived]

Very dangerous to introduce liquid at any place but the receiver and that is usually done on a system that is in a negative pressure.

The tank is getting cold because it is acting just like the evaporator and is giving up heat to vaporize the freon being introduced into the system. Warming the tank offsets that and increases the pressure differential between the tank and bus system.

[Gerald]

I have read about adding liquid at the receiver service valves but wonder how do you do that? Turn the tank upside down? I have replaced all my hoses and valves and about ready to recharge so I will starting with a vacumn.

[Archived]

If you pull a vacuum on the system and it hold it verifies the system is leak free. At that pont you can add liquid at the receiver. Some bottles have two valves, one for gas the other for liquid, some have one only and require you to invert the bottle. You should use a scale and get close and then verify the charge with pressure gauges.

Topping off and refilling are two different procedures which is why when refilling it usually involves a new dryer as well as pulling and maintaining a vacuum.

[Archived]

Just to close out the original thread. Putting the refrigerant tank in a 125 deg F water bath makes all the difference in the world. I was fully charged in another 20 minutes. I updated the original item list in first post to reflect this.

Gerald