: An a/c question for all you experts...


krashDH
07-04-2009, 02:08 PM
So I picked up a can of refrigerant today (w/ pressure gauge) because I was thinking my system needed a little pick-me-up. I hooked on to the low side and the pressure was hovering around 50-55psi. I know that is a bit high. Who knows how accurate the gauge is though...

So, I went back to the Haynes to read what they suggested you check for low symptoms. One was touching the two pipes going to the compressor. There was definitely a noticeable change in temp between the two, so that checked out.

The second was the viewing test. It said that if while the vehicle was running the refrigerant looked foamy, then you may have low refrigerant...EXCEPT if the ambient temps are very hot (its about 80 today, within what they suggest for ambient outside temp). So this got me thinking there was a possibility it was low, because it had that white, frothy look to it.

But, it also said that "With the proper amount of refrigerant, when the a/c is turned off, the sight glass should show refrigerant that foams, and then clears." This was also my case, which contradicted above.

My a/c seems to run and work fine. It blows cold, but then again I have been in some vehicles that blow COLD. I'm wondering if I can get more out of it. I know it's bad to have low refrigerant and too high of pressures, so I don't want to risk anything. Also, I have owned the truck for 5 years, and I haven't noticed the a/c getting any warmer.

Does anyone have suggestions or is this something that a 2.7 is weak on?

firebox40dash5
07-04-2009, 03:04 PM
Mine was pretty weak when I lived in NC. I also tried one of those cans, but never wound up putting much in to get it in the green on that little gauge. It helped some, but what really made a difference was cleaning up the radiator and condensor. They were pretty well coated with mud and crap, and the engine had been heating up more than normal. A good, careful spray with a pressure washer with a wide nozzle (anything less than about 15 degrees will bend the fins way too easy) from the front and from the engine side helped both the AC temp and the engine temp quite a bit.

krashDH
07-04-2009, 04:56 PM
Ya I figured this much. I did take it to the wash and did a low pressure stream on it to get all the gunk off...so it's fairly clean. When I hooked the gauge up it getting into the yellow...so the pressure was at "warning level" Everything seems to be working ok though, just isn't as ice cold as I guess I'd like it for the summer heat, being a landscaper.

I guess since my refrigerant seems to be good, it's where it has to be for the time being...I don't want any higher pressure.
Any other input would be great.

Toyasaurus
07-04-2009, 05:06 PM
How is the fan clutch? When was the last time it was replaced? Mine was as simple as replacing the fan clutch to move more air over the condenser.

krashDH
07-04-2009, 05:21 PM
EDIT: Upon further investigation, it's not the fan clutch. when the engine is warm it is NOT able to spein freely, there is resistance.

It's not exceptionally hot here in Mt, maybe 95-100 tops in the middle of the summer, but I like to have ice cold a/c. I guess I can try to poke around a bit more...

Seeing that my vehicle is a 99 and I don't think any of the fan assembly has been worked on, I won't rule it out.

jski
07-05-2009, 11:04 AM
So I picked up a can of refrigerant today (w/ pressure gauge) because I was thinking my system needed a little pick-me-up. I hooked on to the low side and the pressure was hovering around 50-55psi. I know that is a bit high. Who knows how accurate the gauge is though...

So, I went back to the Haynes to read what they suggested you check for low symptoms. One was touching the two pipes going to the compressor. There was definitely a noticeable change in temp between the two, so that checked out.

The second was the viewing test. It said that if while the vehicle was running the refrigerant looked foamy, then you may have low refrigerant...EXCEPT if the ambient temps are very hot (its about 80 today, within what they suggest for ambient outside temp). So this got me thinking there was a possibility it was low, because it had that white, frothy look to it.

But, it also said that "With the proper amount of refrigerant, when the a/c is turned off, the sight glass should show refrigerant that foams, and then clears." This was also my case, which contradicted above.

My a/c seems to run and work fine. It blows cold, but then again I have been in some vehicles that blow COLD. I'm wondering if I can get more out of it. I know it's bad to have low refrigerant and too high of pressures, so I don't want to risk anything. Also, I have owned the truck for 5 years, and I haven't noticed the a/c getting any warmer.

Does anyone have suggestions or is this something that a 2.7 is weak on?

I just wrapped up my A/C install in my '03 yesterday, so I have some recent experience here.

At idle, my low-side was about 50psi. At the specified 1500 RPM, it drops to about 30psi. The manual says that at 1500 RPM, 86-95F abmbient, recirculation on, the low-side pressure should be to 21.3-35.5psi.

You say that the A/C is cold, but not COLD. What are your vent temperatures and ambient temperatures?

In my case, at idle I had vent temperatures of 60F. That lines up well with my low-side pressure of 50psi (54F) which should be the temperature of the evaporator. A five degree rise through the vents is believable with recirculation off and the fan on high.

On the road, I have vent temperatures of about 40F. That also lines up well with my off-idle low side pressure of 30psi or so (35F).

These were with ambient temperatures in the upper 70s.

Since my install was from scratch, I put in two 12oz cans of R134a. This is theoretically more than the system should have in it, but since you never get everything out of the cans and there is some lost to the hoses, etc, it works out about right. The system should have 19.4-22.9oz of refrigerant inside.

In operation my sight glass shows a cloudy liquid that completely fills the window. There is no sloshing or surging of the liquid. When shut down, the cloudy liquid bubbles then goes clear. I have heard from other A/C forums that R134a and PAG oil are normally cloudy in operation, so I believe this is normal. What should or should not be visible in the sight glass is a matter of much debate in the automotive A/C world.

Despite what the top-up kits would have you believe, there is no way to charge a system based on pressures only. They can tell you if the system is really, really low, but your low -side pressures will look "normal" from about 50% charge (mine looked "normal" with one can in the system) well up into over charged. As much as I hate to admit it, the only way to be sure about how much is in there is to recover the refrigerant and charge into vacuum.

I don't doubt that in many cases topping off improves the performance of the system, but since you don't know how much you need, can't control the temperatures and airflows enough to be able to use the pressures as any kind of a guide, etc. they are much more likely to result in an over-charged and poorly performing system.

In theory, A/C work is nicely scientific and precise. In practice it is almost black magic, even for the professionals.

krashDH
07-05-2009, 11:16 AM
^ Good information thank you.

I am going to check my temperatures today.

From what I have gathered, if anything I would need a slight top off. Since it's not going to do too much for me, I will probably leave it as is and save up my money for my next few repairs, since I don't believe it's anything wrong with the system, just not as cold as I like. Pretty much all of the things to check for a "healthy" system have checked out, so until something really takes a bomb, I guess it's on to other projects.

Thanks again

NorcalPR
11-09-2009, 02:05 PM
I don't doubt that in many cases topping off improves the performance of the system, but since you don't know how much you need, can't control the temperatures and airflows enough to be able to use the pressures as any kind of a guide, etc. they are much more likely to result in an over-charged and poorly performing system.

In theory, A/C work is nicely scientific and precise. In practice it is almost black magic, even for the professionals.

What about checking superheat temps? Won't this tell you if the system is over/under charged?

jski
11-10-2009, 12:10 AM
What about checking superheat temps? Won't this tell you if the system is over/under charged?

Sort of... haven't done too much with superheat and subcooling numbers before, so I may not have this quite right...

Toyota used a thermostatically controlled expansion valve (TXV) instead of a simple orifice tube. The TXV will attempt to maintain a preset superheat value by throttling the refrigerant flow. While this results in nice even evaporator temperatures under wildly changing conditions of compressor RPM, condenser and evaporator air flow, etc. it makes charging based on superheat difficult

An undercharged TXV system will be starved for refrigerant. This will cause the valve to open all the way, but the flow of refrigerant is so low that all of it boils off in the evaporator and the vapor temperature rises as it travels through the rest of the evaporator. This will result in a higher than normal superheat reading.

A properly charged TXV system will have enough refrigerant so the valve will not run out of liquid refrigerant. The valve will pick a nice setting that results in just enough refrigerant to fully evaporate as it goes through the evaporator. The TXV will do its best to maintain this condition even as the temperature of the liquid refrigerant and the evaporator change.

An overcharged TXV system will have more than enough refrigerant. The valve will simply clamp down on the flow and maintain the superheat that it is set for. Meanwhile, the liquid backs up into the condenser and reduces the effective area of the condenser. Up goes the high side pressures and down goes the efficiency of the system.

While superheat can work well for determining the charge in a stationary system with constant airflows across the evaporator and condenser and a constant speed compressor, I don't think that it works as well in an automotive setting with variable compressor speeds and condenser/evaporator airflows.

Subcooling supposedly can be used as a measure of the state of charge in a system as well, but since Toyota uses a receiver in the liquid line, you won't get an increase in subcooling until the system is severely overcharged and liquid is backing up into the condenser.

NorcalPR
11-11-2009, 06:11 PM
Time will tell, but I was undercharged.

It was about 70 degrees today and I was comming out with 39degree vent temps while driving, with the recirc off :)

Before charging, I was pushing 44-46degree temps.

Urf
03-09-2010, 09:37 PM
I was complaining to a service tech buddy of mine about my a/c being weak, he took it in flushed it and checked it all out. Everything was perfect, except for my vent temp, he noticed the temp control in the cab felt sloppy so if you follow that under the hood on your fire wall some times you can pull that little valve closed a little further. I ended up replacing mine, it was like $25 I think, got the center vent temp while sitting in the shade down to 36 degrees. If I have problems again I'm gonna put a little 90 degree inline T-valve to furthur limit or shut off flow to the heater core.