OT: Can someone school me on Automotive A/C?

[Cougar281]

As the title says, can someone school me on automotive A/C? My wifes Jetta started having issues with it not cooling off right - took it to a friend of mine who's a certified A/C tech and we found the charge was a little low, but the system wasn't behaving as he expected. The Jetta has an expansion valve, as opposed to an 'old school' fixed orifice.

If I understand the way it works, with a fixed orifice, only a certain amount of refrigerant is allowed past, which is what created the high side/low side pressure differential, which in turn is what creates the cooling effect. So as the compressor draws more refrigerant out of the low side bring it lower, it increased the high side, thus increasing hte pressure differential and increasing the cooling effect.

With the expansion valve, if I understand it correctly, it varies the size of the orifice based on the temperature of the refrigerant (or something to that effect). If that's the case, what would the purpose be? To keep the pressure differential at a specific range? What's the point of over complicating the system with another moving part to fail as opposed to a fixed orifice that worked just fine for many years?

 

[catman3126]

So what is your question? yes that is the just the type of system the VW went with and yes it is a over completcated system that can adjust as demand for cooler air increases or decreases.

 

[Cougar281]

So what is your question? yes that is the just the type of system the VW went with and yes it is a over completcated system that can adjust as demand for cooler air increases or decreases.

Basically, what does the expansion valve do that the fixed orifice does not (besides just quit working)? Is it meant to keep the pressure differential in a very specific band or something else?

 

[Chevy1925]

with a fixed orfice, the compressor must be cycled on and off to control pressure. with an expansion valve, the compressor can stay on and the expansion valve adjusts size to control pressure.

there is more to it than that depending on vehicle but that is the jist of it

 

[68skylark455]

Yes the expansion valve is over complicated but when working actually can give you temps cooler than a fixed orfice. Best way to diagnosis one is to look at the pressures. With the gauges hooked up observe the low side at idle and on accel and at 2000 rpm. If it has some trash in it or is starting to fail normally it will go into a low reading and in worst cases it goes into an actual vacuum. The other thing is with R134A freon it has to be very specific on amount, one ounce over and it will not cool as well. Too much oil in the system will also mess with the expansion valve. Best thing to use is an AC machine that you can program the amount but you have to add for what gets left in the hoses so it can be tricky. Like today, I did a caravan and it calls for 2.88 so we set the machine to 2.94 because you will never get it all out of the hoses:thumb:. Also expansion valves hate humidity and plugged cabin air filters when one is present and dirty. If you need anything else I believe you still have my number and I can answer any questions you might still have. Hope this helps

 

[mike diesel]

Does the heater/cool selection knob work? Is the ac stuck on one temperature? The arm controlling the blend door falls off causing the heat or ac to be stuck where ever it fell off. Usually happens on mid travel when adjusting so you could easily assume the charge is low or the heater is weak, depending where it fell off. Not saying this is your issue, but it is something you may want to look into as well.

 

[THEFERMANATOR]

Orifice tubes are basically are a trade off. The orifice size is set to allow for OK cooling at idle, keep the pressures out of the danger zone at high RPM's, but works best at the RPM the engine will spend most of it's time at. An expansion valve varies the refrigerant flow to allow the system to work over a wider range of RPM's and conditions. When the expansion valve senses the suction side is warm, it opens and allows more refrigerant to pass and cool the system down. When it senses it gets cold, it closes off the flow to keep the A/C from getting to cold. They still use some sort of sensor to cycle them off when they get to cold, normally a temp probe, but some systems used a low pressure switch. Many systems with an expansion valve still have a sight glass for charging the system as the best way to check the charge in one is to check the sight glass and look for a solid stream of refrigerant flowing from the condenser to the evap.

And if you really want to get confused, you can throw variable displacement compressors into the mix. Or the variable displacement, expansio nvalve equipped, with a thermal sensor to cycle the system. Or the classic dual air system with either dual orifices, orifice/expansion valve combo, or dual expansion valve systems.

 

[Cougar281]

Thanks guys. The blend door is fine. We did find that the cabin filter was dirty, but more importantly, not installed properly. The cover was off and the filter had dropped like 1", and it was leaking like crazy. Put a new filter in and sealed it up right, but no change. The symptom is the car takes FOREVER to even start to generate cool air - when I took it out today to test and see if the charging last night did anything, the behavior was the same - it took about 2.5 miles at 1800 RPM to just start blowing cool air. EVENTUALLY it will get cold - such as the 45 minute drive to my friends house. My CTS, for comparison, starts at least feeling a touch cooler 30 seconds or so after it's started - granted, not ice cold that fast, but you can feel the difference.

I don't recall the numbers on the Jetta, but I know the pressures were not behaving properly - the low side was dropping and the high side was increasing, but not the way it should have. Since it does eventually get cold, it seems most likely to be the valve, not the compressor.

We hooked the gauges up to his Vue, which has a similar expansion valve setup, and as he increased the engine speed, the low side dropped and the high side increased - settled around 25 on the low side and 250 on the high side and was blowing COLD.

While on the topic, how long is it safe for the drier to keep the system open? It's hard to say for sure, but I suspect that from the time I crack the tubes to the valve to the time I have the valve replaced, sealed up and starting to draw down the system will be less than 20 minutes. A drier is only like $20, but if I can safely avoid replacing it, I'd rather do so.

 

[Chevy1925]

Sounds like the valve is stuck on the Jetta. I've ways been told and gone by the "one hour" rule for accumulators or receiver/dryers. How old is the Jetta?

 

[Cougar281]

That's exactly what my friend said. It's a 2010 TDI with about 84,000 on it. So if I can have it opened, replaced and sealed in 20 or so minutes (or less), it should almost certainly be fine?

 

[Chevy1925]

Of your opening the system, replace it. If your doing more testing and diag before opening the system, leave it.

Is the pump noisy at all?

 

[Chevy1925]

Have a fresh dryer or accumulator in there is like changing the oil after doing a headgasket on a truck even if water didn't go down the oil passages. Cheap insurance

 

[THEFERMANATOR]

Do you know what the pressures were on yours? Feel the lines at the expansion valve and see if it is cold. The line from the compressor to condenser should be hot, you should feel a good 20-50 degree temp drop from the line going into the condenser VS coming out(the temp drop across the condenser is what allows for the temp drop at the evap), and then feel the line coming right out of the expansion valve going into the evap and see if it is cold or not. Normally a stuck expansion valve will cause icing right at the expansion valve outlet, and not much furthur past that. From what your describing, it sounds like overcharged by your saying it takes so long to cool down. Either that or grossly undercharged. Expansion valves can fail, but in a system that hasn't been opened it is rather uncommon.

 

[68skylark455]

The dryer/accum is also susceptible to humidity, if it's opend and very humid is draws in moisture quicker than brake fluid as it is designed to absorb moisture, most often we just change them, 4-5 years old has been a rule of thumb if it's opened and in that range of age replace it. As said before , cheap insurance:thumb:

 

[68skylark455]

one other area always overlooked is the space between the condenser and radiator. we find a bunch of them filled with grasses and wheat stalks from the farm roads around here.