Correct Pressure for Dash A/C?

[Bill C]

I'm not sure if I have the correct forum, or not. It says Air Conditioning, but I think they mean for rood A/Cs, but on the other hand, I did not see any forum for Dash A/C, so here goes.

I have a slow leak in my dash A/C, which is has been there for several years. I’ve had it looked at and the guys says “I replaced an O ring”, here’s a bill for $300, and the next year I have the same problem. I got tired of paying him $300 each year, so I started adding freon myself each year and problem solved.

I am by no means an A/C kind of guy, but I do have an engineering background and understand how it works.

Here is my question: After reading on the forum for several years I have picked up that there are certain pressure readings I should get when the A/C is functioning properly. So, I added freon until the compressor stays in all the time while running (it’s 98 deg here in FL) and the gauges read about 350 HP and 45 LP. See pic below. The compressor does cycle off a few times for maybe 5 seconds occasionally.

Supposedly, based on the chart below, this is close to within range, although if you read my gauges on the inner Blue scale (for R134a) the pressures are much lower. This confuses me.

 

Cooling is happening in the coach, but nothing to brag about (yet), 70 deg. I left the engine running for 10-15 min and when I came the gauges now constantly show 175 HP and 30 LP (on black R410 scale, much less on Blue R124a scale), and the coach A/C is now showing 60 deg, which I can live with.

 

What am I missing, if anything?

 

Thanks,

Bill

[jacwjames]

I am following. 

When I first bought our coach in 2008 the dash AC didn't work.  I added one can of refrigerant and it took off and worked for the next +10 years.  In 2020/21 when I was getting ready to think about doing some traveling I noticed the AC didn't blow cold.  So I bought a set of gauges and my neighbor (HVAC guy) sold me a large canister of refrigerant and helped me charge the AC and it's stayed up and still blows cold. 

From memory I believe the pressures were about the same.

If I can get by with adding refrigerant occasionally I'm good with that.  Chasing leaks in these beasts can be a problem, paying someone else to do it is worse.

[Rikadoo]

Bill so 350 is too high, the reason it cycles for such a short time is the the trianary switch is stopping the system from having a issue from too much pressure, out west here (less humidity) high side tends to be happiest 225 to 245 psi on the R-134 scale. Thats also with a normally functioning condencer fan, as for the low side i like to get on a system using the H valve style valve (looks like a block of aluminum) with 4 connections  and a top or dome looking thing 20 to 30ish psi on the R-134 scale. Thing to know about AC duct temps doesnt matter dash or roof the best running systems will be 25 degree difference from ambient temp to duct temps.

*Note, the reason to protect the high side pressure from going too high is that the oil in the system breaks down with high heat, when that happens bad things happen to the compressor which in turn does bad things to the rest of the system. Also if your condencer fan quits that will drive your high side thru the roof, which is another reason for the trianary switch.

Still not getting 25 degrees difference at the duct, consider your hot water valve for your heater may be bypassing and allowing hot coolant thru the heater core causing conflict with the ac, try turning off the hot water flow to the heater if the heater hoses are are getting hot (provided the engine is at normal temp), on that note dont use vise grips to chinch off the coolant lines as that can damage old heater hoses internally.

One more thing to add, with freon there is a temperature / pressure relationship generally speaking. 1degree pressure then =1 degree temp, if i remember right that changes with higher pressure as it increases, google freon / temp relationships.

[D.Ellison]

You can get a florescent dye to inject into the  system to help find leaks it works pretty good , One place on these MH  a/c systems that like to give problem is the compressor shaft seals, about 180.00  for a new compressor.  The press readings in the 1st pic are too high which is why its kicking in and out probably the high limit switch doin its job i think  it cuts out at 350 then press drops and  then the clutch kicks back until the press gets too high again,,,,based on your  cold air temp and the  2nd press reading give it a little more 134 and it will  cool rt down i  like to see about 25-30  and 150-200. hope this helps.

 

[Ivan K]

Some years ago I posted this and it still works for me. I don't pay as much attention to the low side as I believe it is effected by the long run back to compressor but rather go by the high side pressure.

[CAT Stephen]

Bill,

 

I have the same issue also.  This is caused by the orifice tube which works as an expansion valve to regulate the refrigerant amount flowing to the evaporator.  You can ignore this issue as it is not worthwhile to correct on the low side because you are very close to spec.  Set your high side pressure to specification and call it a success...

 

The main cause of leaks on the older coaches are schrader valves everywhere.  My 2000 HR Endeavor 38WDD has six leaky schrader valves that I had to replace twice since I have owned the coach.  

Edited August 14, 2023 by CAT Stephen

[Scotty Hutto]

Just a quick note about refrigerant gauges… maybe everyone already understands this, but I’ll repeat for the sake of clarity.

Pressure is absolute, regardless of the gas. The big black numbers on the gauge always tell the pressure. 

The other numbers on the gauge (blue, red, etc.) are the corresponding *refrigerant* temperatures. Not the ambient temperature, the air coming off the condenser, or the air coming off the evaporator. They represent the actual temperature of the gas at that pressure. If you had perfect heat transfer, they would represent (and *do* approximate) the evaporator (low side) and condenser (high side) temps. 

Hope that helps. 😉

Scotty 

[PeterSchweizer]

First off you have to make sure you have the correct amount, weight or ounces, of refrigerant in the system. The pressures will neve be right if you just keep adding refrigerant , especially every year without fixing the leak source.  that will also damage the system. The amount of refrigerant is listed on a label on the Ft dash A/c unit evaporator cover inside the front cap (generator area)  passenger side. this refrigerant amount typically in oz's will vary per coach length / length of refringent lines. 

Pump down system with vacuum pump (or have a professional do it?) check for loss of vacuum over an hour +.  if ok change dryer and charge system with Nitrogen n check for leaks  its a loooong system so lots to check. I usually test it with nitrogen at 200 lbs if it holds for over an hour it usually good to go and then add the proper weight oz's of refrigerant. Note if the system has been closed, NO bokken lines,then you may not have to add any oil note the amount of compressor oils is a set amount per they system you have and size of rig.  

once you have the basics done then run the system and see if any other issues exist. these RV systems are big and tricky I find its not as simple as an autos A/C system and the pressures are way different and takes patience.

Keep COOL!

[Bill C]

Thank you everyone for your assistance. I am still left with a couple of questions:

1. Why are these two temp/pressure charts so different?

2. Why are my readings so high for a while and then level out much lower? I mean it takes a while, and while running, the High Press stays around 350 without the compressor cutting out much, if at all.

3. Should I be reading the blue scale on my gauges, since it says for R143a?

Thanks

[cbr046]

As @Scotty Hutto said, pressure is absolute.  I always read the pressure scale, not red-green-blue-purple-whatever. 

I have a slow leak also and need to add freon usually once / season . . . . . but I add 2-3 cans (12oz) somewhere each season.  I usually stop around 30-35psi (low side).

If you have the time, dexterity, flexibility and willing to crawl around everywhere go ahead and add some dye and find those leaks.  Me?  I'll just add a few cans each season.  Sorry environment. 

You can tip a can upside down and, as a liquid, get it into the system much faster, but if the liquid ever gets to the compressor it'll kill it.  Literally dead compressor.  Fortunately the lines are so long it'll most likely boil off before it gets there. 

And no, I'm not a professional HVAC mechanic but I knew one once 😉

- bob

 

[Scotty Hutto]

I chased a leak in my dash air for almost 4 years. Three different AC shops, including one who has a reputation of being the best in the Atlanta (Smyrna) area. Heaven only knows (and I really don’t want to know) how much money has been spent…

The leak was bad enough that I would lose a complete charge in 4-5 days, so it had to be fixed. Shops tried dyes and sniffers and could not find the leak.   

MTR/Fleetpride in Cumming, GA took a different approach. The evac’d the system and pressurized it with 300 psi dry nitrogen.  Found the leak in about 10 minutes when a hose-end fitting blew apart. Apparently the leak was intermittent, which is what made it hard to find in the shop. As soon as I headed down the road and things started shaking, it would dump the charge. MTR repaired the leak with a new fitting crimped on the hose.  

It’s been a month and it still cools great!  Maybe not long enough to know for sure, but I’m happy because I have dash air for the first time in years.  

PS - I now have a spare, almost new, completely operable Sanden 4288 compressor that one of the previous shops replaced.  

[1nolaguy]

1 hour ago, PeterSchweizer said:

First off you have to make sure you have the correct amount, weight or ounces, of refrigerant in the system. The pressures will neve be right if you just keep adding refrigerant , especially every year without fixing the leak source.  that will also damage the system. The amount of refrigerant is listed on a label on the Ft dash A/c unit evaporator cover inside the front cap (generator area)  passenger side. this refrigerant amount typically in oz's will vary per coach length / length of refringent lines. 

Pump down system with vacuum pump (or have a professional do it?) check for loss of vacuum over an hour +.  if ok change dryer and charge system with Nitrogen n check for leaks  its a loooong system so lots to check. I usually test it with nitrogen at 200 lbs if it holds for over an hour it usually good to go and then add the proper weight oz's of refrigerant. Note if the system has been closed, NO bokken lines,then you may not have to add any oil note the amount of compressor oils is a set amount per they system you have and size of rig.  

once you have the basics done then run the system and see if any other issues exist. these RV systems are big and tricky I find its not as simple as an autos A/C system and the pressures are way different and takes patience.

Keep COOL!

Thanks for that information Peter. I have a few follow up questionsfor1) If a system holds a vacuum, say -3psi for an hour without change, what is the benefit of charging to 200 psi with nitrogen?  Can you have a leak that only reveals its self under positive pressure?

2) In this exchange I have not read anyone mentioning the low pressure switch. Assuming that motor coach systems are similar to automotive systems and have a low pressure switch a bad one may be the cause of short cycling. I had this issue with a car earlier this year. I did not discover it was an issues until after I had replaced the condenser and dryer (twice). In a car the LPS is generally located near the accumulator/dryer. Is this the same for the RV and if so is that back near the condenser for a DP?

[Scotty Hutto]

Not Peter, but the short answer is yes, you can have a leak that only reveals itself under pressure.  One of the frustrating things about my leak was that it would hold a vacuum.  Once as long as overnight.  But when pressurized with refrigerant, it would leak.  

I’m guessing that the vacuum actually closed the gap and sealed the leak. 

[PeterSchweizer]

The system runs on the high side in excess of 225 lbs so you want t o have the system tested in that range best with nitrogen as it is less expensive then testing with Freon and its a dryed air.  On my Cayman the low and high switches where in the front cap the low was on the dryer attached my data plate and some generic RV cab a/c stuff 

Dash A_C Charge Amount 2003 Knight_ - AC, Heating, and Cooling - Bill D’s Monacoers.pdf

[vito.a]

Subscribing.  Owning a motor home is forcing me to learn automotive A/C systems.  

 

[Bill R]

10 hours ago, Bill C said:

2. Why are my readings so high for a while and then level out much lower? I mean it takes a while, and while running, the High Press stays around 350 without the compressor cutting out much, if at all.

Does your High Pressure occur during a cold engine start up? I am not an expert HVAC guy but I will take a stab based on my coaches system.   On a cold engine start up my hydraulic stack fan will run at a low rpms to start.   I do not have an AC switch that speeds up my hydraulic stack fan.  As the engine warms the hydraulic stack fan rpms pick up.   As cooling increases across the condenser, Pressure will decrease.

https://www.heatcraftrpd.com/resources/heatcraft-tv/did-you-know-29-why-head-pressure-changes-as-condenser-fan-speed-changes

[cbr046]

24 minutes ago, Bill R said:

Does your High Pressure occur during a cold engine start up? I am not an expert HVAC guy but I will take a stab based on my coaches system.   On a cold engine start up my hydraulic stack fan will run at a low rpms to start.   I do not have an AC switch that speeds up my hydraulic stack fan.  As the engine warms the hydraulic stack fan rpms pick up.   As cooling increases across the condenser, Pressure will decrease.

https://www.heatcraftrpd.com/resources/heatcraft-tv/did-you-know-29-why-head-pressure-changes-as-condenser-fan-speed-changes

The AC condenser is cooled by a separate electrical fan.  The engine fan has nothing to do with the AC system. 

- bob

[Ivan K]

That depends on the coach, my condenser is cooled by hydraulic fans just like Bill R described.

[vanwill52]

Not trying to hijack this thread, but if you are anywhere close to Atlanta, GA, MTR Fleet Services in Cumming, GA is an EXCELLENT repair shop for just about ANYTHING large-vehicle related.  Jason, the owner, is a friendly, compassionate fellow and an excellent diagnostician.  Even Paul Whittle, the most DIY guy I know, takes his coach there for service.

[Scotty Hutto]

2 hours ago, vanwill52 said:

Not trying to hijack this thread, but if you are anywhere close to Atlanta, GA, MTR Fleet Services in Cumming, GA is an EXCELLENT repair shop for just about ANYTHING large-vehicle related.  Jason, the owner, is a friendly, compassionate fellow and an excellent diagnostician.  Even Paul Whittle, the most DIY guy I know, takes his coach there for service.

I second Van’s comments. 

[Gerry f]

If you are running 350psi at an idle then you are much higher a faster engine speed.  That is MUCH too high for the compressor!

The high pressure switch will shutdown the compressor to try and protect it.  Pressure that high will cause the compressor shaft seals to blow out.  The short cycling of the compressor will damage the compressor also.

I had exactly the same problem.  Very high on the high pressure side and low on the low pressure side.

First check to see if the condenser fan is running at full speed.

If it is then if you have a TXV at the evaporator then it has most likely failed.  That was what happened to me.

If you have a orifice tube and the condenser fan is operating properly then the orifice tube is probably clogged.

Just be aware that any time you open up the system that you should replace the receiver/dryer and you must pump down the system with a vacuum pump for at least 30 minutes and insure that it will hold a -29.9 level for at least 30 minutes with the pump off.  Charging with 150+ psi of nitrogen and seeing that it holds the pressure is also a very good test for leaks then pumping the nitrogen out to -29.9 again before adding R134a.  Be careful to not overcharge the system.

Just my experience.

Gerry Faulkner

2007 Executive Sandia IV

[David Pratt]

 

The Dash AC system in our Coaches hold a lot of Freon. They are the approximately the size of a 1.5 ton AC Unit. and hold anywhere from 40 to 60 ounces of Freon. You have hoses that run the entire length of the coach. Most hose failures occur in the area of the fittings unless the hose has been rubbed through or chaffed by a chassis component or improper installation. another area of a potential leak is the AC compressor shaft seal.

It is not uncommon to have to add a small amount of Freon annually to keep the system fully charged and operating at it's best. Check the hoses for any chafing and inspect closely the area around the fittings. If you detect any sign of an oily residue it is a sign of a leak. The older AC hoses had a tendency to become porous as they age and will show these signs. The newer hoses are manufactured much differently and prevent the hose from becoming porous. Seals in the compressor will also leak slightly as age sets in. It is very difficult to prevent this and the best way to stay on top of it is to annually have the system inspected, the pressures checked and add Freon as necessary to top the system off.

AC systems should be exercised as much as possible. They do not like being OFF.

As long as the AC system stays closed and is operating normally just add Freon as needed. You may also want to add a dye into your system the first time you have to add Freon. With the Dye in the system you will be able to detect a leak or potential failure using an Ultraviolet light. The Dye used for the AC systems is a very small amount and will not hurt the system.

There are also some very good Sealers on the market today that will seal off any small leaks that may show up in the AC system, such as Evaporators, Condensers and other components. When properly added they live in the system, will not harm the system and if a small leak were to develop may and can stop it.

I have four Coleman MACH8 Roof Top AC Units on my ih-45. The rear unit failed due to a leak in the evaporator. I installed a schrader valve in the system and recharged the unit. It leaked down again within about six months. I decided to evacuate and flush the system with Nitrogen, recharged it and added a stop leak into the unit. That was three years ago and the Unit is still working and cooling great and no leaks. The sealer is used through out the commercial refrigeration Industry and is called PROSEAL XL4. It is compatible with R134A and R410A and other Freon systems. You can purchase in various amounts to service AC units from 1.5 tons to 5.0 tons. When used properly it will not harm the system or the compressor.

The receiver-drier, expansion valve, pressure switches and thermostat are all installed outside of the Evaporator case and normally in plain site.

I keep a  Thermometer stuck into my center AC outlet in the dash closest to the Evaporator so I can observe the temp coming out of the duct. The outlet temperature will vary based on the ambient temperature inside the coach on recycle mode. I travel with the front and rear roof airs operating and the thermostats set at 76 deg. The dash outlet temp is always between 49-55deg when the outside temperature this summer was 105+ deg.

I added approximately 4.5oz of Freon to my system when I serviced it this year. This was a bit less than what I added to the system when I serviced it last year. This is another indication that the system is still tight and operating normally.

[Bill C]

My coach A/C seems to be working pretty good now, so I don't want to mess with it, but I am still confused about this:

Red High pressure: 315-350 on black outside line – but after a while the reading is: 175 and it cools even more, to 60 degrees

Blue Low Pressure: 35-50 on black outside line - but after a while the reading is: 30 (at the same time the high pressure went to 175) and it cools even more, to 60 degrees

My question is: is does the system take 10-15 minutes to stabilize to the best cooling condition? After a while the high pressure came down.