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Old 04-26-2012, 12:42 AM   #1
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AC System Recharge - OTR

It appears that the 134a refrigerant has been lost after 5 years since the last recharge in our OTR air conditioning system.

My compressor is engaged and spinning but no cold air is produced at the Dash or in the Bedroom. The condenser fans are spinning madly.

I would like to add refrigerant, but do not know if the low and high side pressure readings on this system would be similar to that of an automobile.

Does anyone know what the pressure ranges need to be for this kind of Cummins engine driven system?
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Old 04-26-2012, 12:57 AM   #2
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proper pressures are dependent on outside ambient temps. Pressure will be like that of an auto with r134a
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Old 04-26-2012, 01:06 AM   #3
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Hey, thank you for the quick response SC3283.

I just found this info which if I am reading it correctly translates to about 216 to 264 psi on the high side or similar to an auto.

Here's a Rule of Thumb from Carrier Transcold which is a basic theory for proper maintenance of your air conditioning system. It will assist you in determining if your system is operating properly:
High side (discharge) pressure
o Use a digital thermometer and take the condenser inlet air temperature.
o Add 40 to this number and total
o Convert the total temperature to a pressure, using a pressure/temperature chart
o Compare this calculation to the actual system high side pressure.
o The high side pressure should be +/- 10 percent.
Low side (suction) pressure
o Use the digital themometer, take the evaporator return air temperature.
o Subtract 30 from this number and total.
o Convert the total temperature to a pressure, using a pressure/temperature chart.
o Compare this calculation to the actual system low side pressure.
o The low side pressure should be +/- 5 percent.
There are a host of other factors which may affect the above readings, and are provided simply as a guide.

That eases my mind a bit about recharging the system.
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Old 04-26-2012, 06:22 PM   #4
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Is the compressor turning?

Was it a factory R134 system or converted?

If it is spinning and you do not have cold air you may have a stuck water valve causing heater to heat the cold air, check your rear unit under the bed too.

If it was R12 that was upgraded to R134 then is could have had the gas transpire through the hoses, the molocules are small enough to go through the rubber, r134 has a liner in the hose.

If turning and low then you can add, and for a short period of time Sams Club currently is stocking the 30 pound containers of R134 for about $160 or so.

If you do not have a manifold DO NOT get the harbor freight one, DO go to an autozone or napa store and they have a delux one that comes with the charts, while there get a non-contact thermometer so you can take accurate readings to then simply look at the chart and fill to correct pressure.

Use the thermometer to troubleshoot, where the line is cold is where the liquid is turning back into a gas, old trick is to place gas into system until suction side of evaporator starts to ice up indicting boiling at that end.

Also can use thermometer to see indication of cooling at this point.
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Old 04-26-2012, 07:48 PM   #5
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I know a lot of folks do not agree that the best thing to do is to get a certified A/C tech to check the system for leaks and recharge it. It does no good to recharge it with out a leak check. Next is fact that not knowing what you are doing, you could wind up damaging the unit and you could do much more damage than the cost of a tech.

Ken
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Old 04-27-2012, 12:22 PM   #6
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Quote:
Originally Posted by TXiceman View Post
I know a lot of folks do not agree that the best thing to do is to get a certified A/C tech to check the system for leaks and recharge it. It does no good to recharge it with out a leak check. Next is fact that not knowing what you are doing, you could wind up damaging the unit and you could do much more damage than the cost of a tech.

Ken

Ken, that is good safe advice of course.

But, that could also be true of replacing a water pump, alternator, doing your own Allison transmission service, or coolant and thermostat change on a Cummins.

I also have replaced disc brakes, a leaking brake booster, and exhaust system sensors on my Jeeps.

I am not a mechanic and was a desk jockey as a career.

But I am having fun learning new skills. If I make a mistake and ruin something, hopefully the previousl savings will bail me out.

So, some of us like to learn about our rigs, accept some risk, and chase a few saved dollars.

YMMV
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Old 04-27-2012, 03:05 PM   #7
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I thought that usually the A/C has a low pressure cut-off that prevents the compressor from working without freon.

Deandec, I'm with you, I prefer to try it myself, I can always pay someone to fix my goofs, but mostly I get it done myself!
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Old 04-27-2012, 05:58 PM   #8
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Dean, servicing a transmission is not quiet the same as an A/C unit. Here you are dealing with a high pressure gas that cause injury if not handled correctly. If you have the right tools, gauges and know the pressures and the limits on the equipment, you can do it. But getting advice cousin June's hair dressers, flaky boy friend...well good luck.

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Old 04-28-2012, 08:33 PM   #9
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Today I put a gauge on the low side and the result was a very low pressure reading.

Then I installed most of 12 oz of 134a. The gauge does not show adequate freon yet but is much closer.

The temps at the vents dropped from 78 degrees on an 80 degree day to 52 degrees after running the system for about 20 minutes.

I will top it off on another day.

Thank you to all who responded.
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Old 04-28-2012, 08:44 PM   #10
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Thanks Dean...
Got me motivated to charge ours

Working great now

If you area cooling 20 or more degrees you may be good.

Get a chart and confirm pressure before you start, then verify pressure first.
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Old 09-07-2012, 11:28 PM   #11
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I have a 1993 Magna with a less than adequate dash cooling output. Does anyone know when they switched to 134A?

I am looking for a way to shut off the hot engine water from going to the dash heater in summer but still keeping the domestic hot water tank heated with the engine. Any ideas?

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Old 09-07-2012, 11:34 PM   #12
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Your rig is right at the change over time. I'd look for a label under the hood on the AC/Heater box or near the compresser on the engine. You could tell for sure if you check the fittings on the compresser, R12 and R134a have different fittings.

If your sure the water doesn't have to flow through the dash heater to your water heater, a simple valve can be cut and inserted into the input side of the heater.
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Old 09-07-2012, 11:40 PM   #13
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jofo, you probably have R12 based on the year....it is an easy thing to check.....look at your charge fittings....if they have threads on the outside it is R12...if you see no threads on the outside of the charge fittings but see a ridge made into the surface..you have R134a

If you do have an R12 system DO NOT...I repeat DO NOT let anyone convince you to switch it over to R134a. You will hate the conversion due to lack of cooling ability. Dash air in an RV is usually marginal at best...reducing the efficiency of an already marginal system would be a waste of your time and $$$

can't help on your hot water flow
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Old 09-07-2012, 11:47 PM   #14
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Where do you usually find the low side fitting?

Is any R-12 available?

What temp drop should you see after going thru the condenser under the dash?

Thanks for the quick replies, Joe
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