A/C Recharge (without knowing proper quantity)

[DrBobD]

I have done automotive A/C for some time now but have started a business doing mobile work on heavy equipment. Approximately 70% of the time there is no tag showing the proper weight of refrigerant required. About 20% of those units have a sight glass which helps resolve the problem on those, to some extent. In some cases I am doing the work for a dealer who supplies me with the information.
My problem is with about 30% of the units that I work on. I have been charging to a high side pressure of about 225 psi on a hot day and around 200 when it is a little milder. I know an over charge is going to show high pressures, there is a the possibility that some of these units may be under charged. Most of us know that an under charge will affect oil flow and can be very damaging to compressor.
Here's my question, is there a better way to charge a system if you could not get the proper information on gauge readings or quantity of refrigerant specified by the manufacture?

I have been told by someone in the past to keep adding a quarter pound at a time until the high side gauge spikes and then remove a quarter pound. The problem is I have never seen the high gauge spike unless the fan is not working.

Any advice is much appreciated.

 

[Mjrdude1]

With our equipment, pressures at spec vary greatly. For example a Deere machine will usually run at much higher pressures than a Cat, just the way the system is designed I guess. Whatever machine we are working on I call the dealer if I don't have specs available. Cat is easiest, they have an A/C book that lists 90% of their models and what the amount of charge they take, including oil. The other mfg'ers are not that convienient.

 

[DrBobD]

Thanks but the local area Cat dealers are either to lazy to look this book up or are really having problems finding it. Could you provide a little info on the book.

 

[Chisumtrail]

The cat book number is SENR5664, All products air conditioning and heating R134a. Should be able to just give dealer book number.

 

[FSERVICE]

a little trick they taught us in school (several years ago) vacunm down. then charge liquid thru high side till pressure equalizes. this will get you very close on charge. try it knowing what it holds for total charge

 

[AndrewC]

That's a cool trick.
Another way I was shown was to check your vent temps and see if its good (40 F) and make sure the low pressure line at the compressor is as cool as a beer by touch. Again just another one you have to be careful with.
200psi sounds high to me but if it is working leave it alone.
What machine are you working on? Maybe someone will have the correct specs and can help you out.

 

[DrBobD]

I will try that FSERVICE. I have never put liquid in the high side before. I would imagine it would be recommended to manually crank the compressor before starting the unit. Would this work for both expansion valves and orifice tubes? The other question I would have with this is that I would likely have to heat the tank on some units depending on the temperature.

One of the challenges I had was on a piece of mining equipment that I worked on in the spring when it was cool out. There was no information listed for the refrigerant charge. I was working on the equipment in the cool weather and it was going down a couple thousand feet underground where to where it's hot. With that unit I put cardboard over the condenser to simulate a hot environment and charged it to 175 psi on the high side. Who knows where the vent temp would be in the hotter temperatures. It was only about 40 degrees outside at the time.

I currently do work for the local Deere dealership so finding information for those is OK for now, until they start doing their own work. I have found with their units the high pressures are around 200 psi on average. That's based on about 5 different types of equipment that I have done for them.

 

[mitch504]

That's the hard part of engine driven A/C work. With an electric unit you can charge it by the amperage draw on the compressor.

 

[td25c]

One thing I noticed about the few pieces of equipment whith A/C we have ( two trucks and a farm tractor) is they all hold around 4 lbs of refrigerant.If Im just adding refrigerant to the system I use a thermometer in the vent duct and when I see 50 degrees F I quit filling .

 

[FSERVICE]

i might have not made myself clear on the LIQUID CHARGE thru the high side port!! first vacuum down system to at least 500 microns THEN charge LIQUID Freon thru the high side port. till the low side pressure gauge comes up & the Freon stops flowing. no need to heat the tank as you are using LIQUID NOT VAPOR!!!! DO NOT open the low side port while u are doing this as it will mess up the flow/amount of Freon that enters the system, as the liquid goes in thru the metering device (TXV or ORFICE TUBE) it flashes off becoming vapor on the low side of the system filling it with VAPOR NOT LIQUID.

 

[willie59]

i might have not made myself clear on the LIQUID CHARGE thru the high side port!! first vacuum down system to at least 500 microns THEN charge LIQUID Freon thru the high side port. till the low side pressure gauge comes up & the Freon stops flowing. no need to heat the tank as you are using LIQUID NOT VAPOR!!!! DO NOT open the low side port while u are doing this as it will mess up the flow/amount of Freon that enters the system, as the liquid goes in thru the metering device (TXV or ORFICE TUBE) it flashes off becoming vapor on the low side of the system filling it with VAPOR NOT LIQUID.

To clarify, you're doing this with the system "not in operation", correct?

 

[FSERVICE]

yes its NOT running... the compressor is NOT operating

 

[AndrewC]

I love it fservice. Next time I do one Im going to try it.
Do you have any other tips for topping a system off without knowing how much is already in the system? I usually fill it until I see on the high side 150-175psi on a warm day here. I usually put my bottle on a scale and don't put more than a pound in depending on the size of the system for example I would feel comfortable putting in more in a loader as it takes five to eight pounds than an excavator that usually takes two pounds. And the fact with the loaders most of them are undercharged after they are worked on because certain people don't own a scale. I found this out a few times changing failed compressors but not leaking compressors as I scale it out and after I do work I vacuum test it with a micron gauge to confirm there are no other problems.

 

[cutting edge]

Andrew C, have you ever seen a book or list of AC charge weights for current Deere machines?

Sure would be nice to find one seeing as most of the specs are wrong in Service Advisor

 

[DrBobD]

This is great guys, please keep the information coming. It is very much appreciated.

I do use a micron gauge on about 50% of the units I work on but here is the problem. I do field service which is 1 to 3 hours away. I advise the customer that the system should be drawn down to 500 but in most cases they let me run the pump for 1/2 an hour. Obviously they don't want to pay me for standing around. I do recommend that when the unit is down for service etc.. to have it done properly. Whether I see it back or not time will tell. The other 50% of the calls are local and the unit is shut down for a period of time so it can be done properly. On these jobs I can let the pump run longer. I use my good equipment on these, Robinair 6 cfm dual stage pump and yellow jacket brute 2 gauges with a 3/8 vacuum line but it still takes a long time to reach 500 microns. For example I had a unit with a leaking fitting so I replaced the fitting and the receiver dryer. Checked for other leaks with nitrogen, everything was OK. The system had not worked for a few weeks so I let low pressure nitrogen flow from the high pressure side to the low side to help dry the system. It took 4 1/4 hours to get the system to 500 microns. Any advice on how to draw the system down faster. My next route was to remove the shredder valves to speed the process.

The other issue is that I too have seen a lot of compressor failures, don't know why some of this is new equipment with a full charge. Here is how I handle this problem and please let me know any other recommendations. I change compressor obviously, replace receiver dryer and flush the line from the compressor to the condenser. That is as far as I go, I have tried in the past to flush the condenser but have never had any contamination come out. I think it would cost as much in flush agent to clean a condenser properly then the cost of a new one. I know that there are proper machines that would do it but who has that kind of money. Any other recommendations?

FSERVICE great info. I am going to try that on a few different size systems.

Thanks, Guys

 

[DrBobD]

cutting edge, as I said in a previous post I do work for the local Deere delership. Just yesterday I asked the parts salesman to look into it. I have ordered the Cat book and when it arrives he wanted to see it so he will have a better idea what to look for. I will let you know what becomes of it.

 

[AndrewC]

Cutting edge. I haven't found other weights other than the ones in service advisor. There are a few dtac cases about low charge from the factory but usually they were ag cases. The weights I have found are usually pretty accurate unless there is another problem in the system, they seem to work. There are a few cases that are good troubleshooting guides. Hittachi case 3 2011 0110. 69491.

John deere fos book about ac gives some high specs dependent on temp

80 f 150-170
90 f 175-195
95 f 185-205
100 f 210-230
105 f 230-250
110 f 250-270

Low 1-30psi
vent temps below 75 outside 20f vent
75-90 outside 25f vent
above 90f outside 30f vent

To me these vent temps seem too low. If it is between 30f-40f at the vents I would say there is nothing wrong with it unless it takes a while to get there.

 

[AndrewC]

Deere says genesolv 2004 (TY16134) works well. I have never used it though. They say you can flush it through 6-8 times before disposing of it.

 

[Birken Vogt]

One way to check it is to measure the temperature of the condenser outlet, and how much cooler it is than the pressure indicates...this is called "subcooling" and tells you how much extra liquid you have in the condenser. Once you start to see subcool you know the condenser is full and it should be solid liquid reaching the evap. inlet.

However, it seems that no one method is fool proof, and on an unknown system I like to just pick a good hot day, run it and "titrate for effect" as they say in medecine...let a little more in at a time and monitor the effect until it is cooling right, then try to resist the temptation to add a little more.

 

[DrBobD]

Hey guys, keep in mind that if you have a low pressure going below 6 psi there is no oil movement. The minimum pressure no matter what system you have must stay above 6 psi. I have seen the pressure get low on a couple pieces of equipment but below 6 psi on the low side usually means an overcharge. Please correct me if I'm wrong. On a couple pieces of equipment the pressure stayed close to the minimum threshold but when rpm's were brought up it usually climbs. Feller bunchers for example usually run at max rpm. It all depends on the piece of equipment.