namasgt

Today I tried to charge the system with R12. I have installed a new re-manufactured Denso compressor, new AC lines, new front and rear expansion valves (made in China I believe), New drier, all new green O-rings. I have also flushed the system out with AC cleaner solvent. I drained the original oil in the compressor and flushed with new mineral oil ( not ester) then filled it up with 9.5 ounces of mineral oil. I ran nitrogen through the system and have pulled vacuum for as long as 45 min to 1 hour on two different occasions. It holds vacuum well pass the 30 inch mercury mark and it holds.

So I bought Four 12 ounce cans of R12. My car has rear ac and was built before Jun 88 so per the manual it should take 1200 grams or about 42 ounces (3 and half cans of R12 I bought). I tried to add the first 12 ounce can with a friends help. I did purge the air out before opening the valve on low pressure side, Engine bay was a little warm since I drove to his place. The can did not completely empty so started the engine with AC on and left it at idle. Didn't feel any cold air coming out of the vents. My friend thought it was odd and should feel some cold air, so we added another can, some freon did escape from the first can, so it wasn't fully empty. After adding the second can the air slowly got cold (rear ac on). But still not cold enough, outside temp close to 100F. I decided to add a third can. with the car seating in the drive way idling with front fan speed set at 2 and rear air set at 1 front air temperature was showing 59F and it took a while to get there then I turn the rear AC off and got about 52 coming out of the front vent. Was not impressed by the numbers total idle time was 5 to 7 minutes.
My friend thinks that there is something wrong with the new front expansion valve? Have you guys have had any problems with new expansion valves?


I did go for a drive and with rear AC off the front air set at 2 will get to 37-38F in normal city driving 40-42F in stop and go traffic and around 36-37F on the highway. Outside temp around 91F.

With rear AC on set at 1 the front air set at 2 will do 55F in city driving and about 48F on highway driving. Outside temp around 91F.

What I want to know is if you guys put the full amount of refrigerant suggested by the manual? Some are saying putting too much freon in the system will make it inefficient. I believe I have put less than 36 ounces of R12 in the system maybe around 34 since some of it did escape.

What are the correct pressure readings for the low and high side? I was getting about 35-40 psi on low side with only front AC on at idle and about 210-220 psi on the high side.

Is there a possibility that the expansion valve is bad? Could the valve be getting stuck?


I would appreciate any suggestions or insight you may have.
Thanks

dr bob

The system works by evapporating/boiling refrigerant in the evaporator. The lower the pressure in the evaporator, the lower the temperature of evaporation. It's the state change from liquid to vapor, the actual boiling of the liquid, that takes the most heat. That heat for evaporation is supplied by the cabin air. So two things are required to get max performance. First is a supply of liquid refrigerant to the expansion valve. To make liquid, you need refrigerant at a low enough temperature and high enough pressure to cause it to condense into liquid. The relationship between pressure and temperature is direct, with more pressure needed to condense at higher temperatures.

The compressor is a pump, and at any given speed the relationship between inlet pressure and outlet pressure is consistent. More outlet pressure means more inlet pressure and vice-versa. Going back to the heat balance, a lower evaporator/suction/compressor inlet pressure means lower evaporation temperatures.

So your goal is to have the lowest possible system pressures, while delivering liquid to the expansion valves under all operating conditions. That will get you the lowest evaporator temperatures, and consequently be able to get the most amount of heat out of the cabin air to boil the refrigerant.


Now to the real world, There's a sight glass on top of the receiver-drier, the cannister in front of the condenser on the passenger side. You should have enough refrigerant in the system so that the compressor can pump the high-pressure system high enough for the refrigerant to condense in the condenser, displaying a flow of liquid in that sight glass. Too much refrigerant makes pressures and temperatures higher throughout the system, therefore taking less heat to evaporate the refrigerant in the evaporator from the heat in the cabin air. Too little means that you aren't boiling (evaporating) liquid in the evaporator, since system pressure isn't high enough to condense the refrigerant after passing the heat out through the condenser.

The system operates in a dynamic balance, where heat and energy movement ALWAYS balance out. So as one parameter changes, the other parameters will follow. Your system pressures will vary based on ambient conditions, and more importantly on how much heat can be removed in the condenser. At the same time, the amount of heat absorbed in the evaporator depends on the temperature of the cabin air, the total mass of refrigerant passing through the expansion valve, and the pressure/(temperature in the evaporator.




So, got liquid in the sight glass? With the proper amount of refrigerant charge, you have just barely enough to maintain high-side system pressure at the bubble point of the refrigerant as it leaves the condenser and passes through that sight glass. Under all operating conditions. Since you can't see it "under all operating conditions", charging just to what you see in the sight glass at low RPM's and non-typical system air flows is a bit of a crapshoot. That's why charging by weight is the safest method. Somebody did all that heat-balancing prediction work for you!


The results you see now are the expected numbers for your slightly undercharged condition. With the front system only, the refrigerant is condensing OK after taking cabin heat through just the front evaporator. Turn on the rear system, and the added heat passes to the condenser, making it hotter. Adding the rest of the charge will bring the pressure up to the condensing pressure at the higher temperature, and once again deliver condensed liquid liquid to the expansion valves. Remember, without liquid there's no boiling in the evaporator and therefore no heat recovered from the cabin air to boil the liquid refrigerant in the evaporator. No liquid means vapor is just passing through on the way baclk to the compressor, with just a few percent of the normal heat movement available.


For those playing along at home, the 'solution' to an underperforming system is not always 'add more'. It's easy to get carried away, and again the ambient conditions really do affect the system more than is readily apparent. Any dirt that affects air flow and heat transfer through the condenser or evaporator will skew your performance to the poor side. Compressor worn? Any air in the system never condenses to liquid, so it robs the capacity that liquid refrigerant can deliver at the expansion balve and evaporator, as well as taking up valuable condenser space and capacity. The pressure of any air in the system is added to the refrigrant pressure, so hotter at the condenser and hotter at the evaporator, and the compressor is doing work compressing the air that might otherwise be compressing the refrigerant.



Whew!! I think that's enough spewage for now...

namasgt

Thanks Dr Bob.
The sight glass on the drier showed white liquid passing through when I added the 2 cans, once I added the 3rd can it got a little clearer but still some white liquid flowing. I also forgot to purge the air out on the 3rd can so some air did get in the system.
I will empty the system again next weekend and buy 3 cans of 14ounce R12 which is about 1190 grams (10 grams less than the factory recommendation) Some freon may escape but it should be close enough.
I think its safer this way than buying 4 12 ounce cans and try to only use 3 and a half of them.
I need to read more on how to put freon in the system since it took too long to add the 3 cans. Some spray the condenser with water to help the freon get in???, I may do that next time.

Thanks again.

Bill Ball

Not after just one can. Dr. Bob explained that in detail.

Several things...you were sitting at idle with no air flow other than the AC fans. Things are going to get very hot. And the compressor is not as effective as at higher RPM. And 5-7 minutes just idling in a heat-soaked car in 100 degree weather with only a partial charge is not going to do much to cool down the car. I'm surprised it was cooling as much as it did at that point.
If the expansion valve were bad you wouldn't have gotten as much cooling as you did. Another clue would be the low side pressures would stay very low while the high side would be much higher than expected due to the blockage IF you were not running the rear AC (assuming it had a good valve). If the front never got cold while the rear AC chilled well, then a bad front expansion valve would be a possibility, but you don't report anything like that.


That's pretty reasonable, especially with an insufficient charge.

Dr. Bob covered this - inadequate charge.

Yes, put in the factory recommended charge if the system is fully evacuated to begin with.

I'll guess you measured these at idle. You should measure them at 1500 RPM, which would probably put the low side around 30 and the high side maybe 250. There is a temperature/pressure chart in the shop manual, since this is greatly affected by the ambient air temperature. Also, if the engine bay area gets very hot from long term idling with no air flow other than the radiator fans, the high side pressure can REALLY go up. It helps to have extra fans blowing over the front of the car or check pressure later after the engine compartment has cooled down.

Not based on what you report.
Complete the charge. You mention in your reply to Dr. Bob that you might start over from scratch. So, I gather you would be wasting all the Freon you have put in. I know you may be concerned about the air let in when you forgot to purge the lines with your last can, but this is not a good solution unless you have a Freon recycling machine. "Top off" the current charge using the sight glass as Dr. Bob described. You're almost there.

namasgt

Hi Bill,

Yes the pressures I mentioned were at idle. I went for a 10-15 minute drive on the HWY tonight with only the front AC on and temperatures got as low as 25F with the temperature **** moved all the way to the left, all the temperatures I have mentioned above were taken at this setting. I guess as you said I can add a little more to the system. I was actually thinking to take the car to a shop and have them evacuate the R12 if they have a R12 recovery system. The car has not have had AC since I bought it in 2008, definitely going to be a big change, once I get it right.

Thanks for your reply.
Ali

FredR

10 out of 10 to Dr Bob for stamina writing the initial spot on response!

The a/c system will blow colder if you only have the front a/c system selected- overall performance is constrained by condenser capacity so the rear a/c unit simply robs Peter to pay Paul as it were.

Did you mean to say 25F or did you really mean 25C for the air temp? 25F is well below freezing-indeed your nuts will drop off at that temperature. 25C would suggest something not right and can be expected if you have air in the system- relatively small amounts really do kill a/c performance which is why I inspect for and check that a full vacuum is present before any refridgerant is bled into the system.

Another thing you may check- indeed such expansion valves are available from China but unless you purchased it from a worthy source such as King Roger of Dallas you would do well to be a bit suspect. Everyone thinks these knock offs are a standard [i.e. correct] size but I have reason to believe that is just not the case. These are available in our market. Indeed I believe the main expansion valve on the front may be a different spec to the one on the rear auxiliary unit- someone chime in who knows better here- but they sure look physically similar and can be mounted in either position.

A/c techs invariably throw some water over the condenser to check the system is working correctly when the engine bay is heat saturated and agree with the chaps that you need to put some modest revs on the motor [not idle] when checking to see if it works.

As Dr Bob says getting the right weight amount of refridgerant is critical to good performancep- non of this "bang a bit extra in for luck" nonsense- it just does not work! With ouyr high ambients I have had plenty of trials and tribulations with 928 a/c performance.

Presumably you have inspection checked the evaporator for cleanliness?

namasgt

Fred,

It was 25 Fahrenheit and yes it was to the point that I wasn't comfortable, too cold. Outside temperature was in the high 80s. My last number of 36-37F was at only 2 to 3 minutes of HWY driving, this time was like 15 minutes.
I flushed everything out with AC cleaner several times and blew out with about 100 Psi of shop air when I was going through the whole system. I bought the expansion valves from Roger. I didn't think there was anything wrong with the front expansion valve and was telling my budy its going to need more freon per the manual (that's why I bought 4 cans in the first place). But he was sure that something is wrong with the system and more freon is not going to fix it. If I would make a guess its probably a little over 8 ounces short...

marine928

Ali, where did you order the R 12 from. I need to have an extra can just in case I need it.

Thanks

Lon

FredR

If you have 25 degrees F what makes you think there is anything wrong? Crikes your anti icing device must not be working- that is 7 degrees below the freezing point of water [32 F] and you will get severe icing if that is a correct statement unless you are in a very very dry [low humidity] area-quite amazing if I am understanding you correctly- that defies the spec sheet performance curves I suspect.

When it is 38 degrees C outside here I am happy if I can get 20 degrees C or lower air temp out of the grill. On a good day I think I get about 15 degrees C but I have never measured it. Fortunately this summer has been one of the mildest I remember over here so have been driving more as a consequence.

Regards

Fred

namasgt

Lon

I bought the R12 from Roger. But I also did buy two extra 14 ounce cans for 25$ each from someone on Craigslist. You can actually find a lot of R12 for sale (or at least I have) on Craigslist for 20-30$ a can from individuals.

Fred,

I don't know what to say. But I will add more freon and see what happens. Like I said there is less than 34 ounces in system now.

FredR

Ali,

If you are seeing such temps, and to my way of thinking that is very cold air, why add more refridgerant?

Just took a look in my manual for R12 systems and at 80F air temp [27C] you would be struggling to get down to 5 degrees C so I am totally perplexed as to the temps you report.

For the 38C ambient temps I see it suggests temps in the 13C to 15C range which is what I expect to see and that if everything is working perfectly which more often than not it isn't..

If the expansion valves came from roger no need to worry about them I suspect.

Regards

Fred

namasgt

Fred,

I may just leave it alone. But with only 34 ounces in the system will it cause anything to fail or wear out the compressor?
I would think being slightly under charge is better than being slightly overcharged? I can leave it alone for a few weeks and see what happens or wait until next year.
Again the last 25F was with front AC only and fan speed set at 2.

928mac

Ali, I suspect your measuring device that you are using may be showing lower temp then what it is. Verify your tool by measuring the temp of ice in a glass. 32F or 0C
The reason I ask is because I have never seen a system give colder then 36F at the front vent, even on a new car.

about your charge,
If you only have 2 or 3 bubbles in the sight glass a sec then I would not worry.
If it foams up (lots of bubbles), when you rev it up to 1800 rpm, then you do not have enough charge.
According to the pressures you quoted in that ambient temp, sounds low to me.
Also dont worry about the air in the line charge as I assume it is not that much BUT, dont do it again because air and the moisture in it damage the system, Always purge the line.
1st can goes in under the 30Hg so no air and water get in.
2nd can when connecting should purge, release(loose some) so your safe but I guess it depends on your connections. There should be valves in the end of the hose.

so if you see lots of bubbles put more in, but only enough to have the bubbles just about stop as it sounds like you are almost there.

FredR

Ali,

I am all out of ideas as to how you can possibly beat the performance curves specified in the manual unless the refridgerant is propane or you tested in a deep freezer!

How confident are you that you indeed do have R12? Hate to ring alarm bells but you simply cannot beat thermo dynamics and I would bet my pension fund on the accuracy of the charts in the manuals. Are you sure some scheister did not fob you off with a can of propane or butane labelled as R12?

Hopefully someone can tell me why my thinking is at odds with what you are reporting.

Your initial temperature figures are consistent with what I might expect to see. If the system is undercharged then the suction pressure will be down a bit and so will system performance but the same happens if over charged thus why charge figures are specified in weight and you appear to understand the quantities correctly. If you have good thermal performance you cannot go far wrong I suspect. If you have any air in the system there will be a performance penalty. The folks who do not have a weight scale make judgements based on suction pressure and discharge pressure. However my system has always worked best when the correct weight measure technique is used- surprise surprise!

The oil is the main protection for the compressor assuming your gas pressures are there or thereabouts.

Regards

Fred

Bill Ball

The freeze switch is SUPPOSED to shut off at about 32F and back on at about 40F, I believe, but I also see as low as 28F at the vents in my car measured with an accurate AC thermometer probe inside the vent. Note that IR temp guns will give spurious readings against shiny black surfaces like the vents. I've had IR guns report -30F.

If you are seeing 25F with an accurate AC thermometer probe in the vent, hallelujah. Still, go ahead and look at the sight glass. It SHOULD be clear. 928mac's comments are germain. The system will cool well with less than a full charge, but I'd make sure it is very close to full.

I didn't know Roger sold R-12. Perhaps Fred's alarm bell should be investigated just to be sure. The are all kinds of R-12 "replacements" around under names like "Freeze-12", but name brand, legitimate R-12 is still out there. I buy some occasionally off eBay or Craigslist now that my 70 lb canister has gone empty supplying R-12 to my local 928 buddies.