Ninja tips: AC systems Options

[technicalninja]

This thread is my base AC thread for suggestions, questions, whatever...

I run
https://granburyautoac.com/

I specialize in automotive heat and air conditioning systems.

I've been doing AC work since 1983 and have seen all of the changes, good and bad, that have occurred over the last 40 years.

I'm keeping the first post short as they always show up at the top of each page and if long, they clutter up the thread...

[technicalninja]

Strangeness of having both liquid and gas in a sealed system and the use of gauges...

Folks believe that the gauges alone can be used to correctly charge an ac system.
This is FALSE!

I use the gauges to let me know how the system is working. When a sight glass is available the gauges and the sight glass can be used together to determine if the system is low or high.

AC systems have a range of fill where they function properly. Many systems will have a minimum/maximum charge specification. The specification range is conservative. Most systems will work within a 20% range and the specs are usually 5%.

You CANNOT use pressures alone to correct a system that you didn't prep first.

It is VERY common for a car to show up with what look like good pressures (200/30) and have piss poor cooling. After service that same car (which was 60% low on refrigerant) will show 180/24...
WTF!
How do I end up with LOWER pressures after adding .9lb to a 1.6lb system!

It's dirt simple...
I removed excess air from the system.
1% air will screw up the gauge readings, 4% air will make the system not function.

This is assuming you are using R134A. R12 is far more forgiving regarding both air and moisture.

A better indicator of system fill is the temperature of the low side return hose.
When stuff is right it's like grabbing onto an ice cube. It should be stupid cold and wet.
You might even find frost!

I use the temperature of all of the hoses to INSTANTLY tell me what is going on in a system.

The discharge line is the beginning of high side from compressor to condenser and is the hottest in the system. At 70 degrees ambient it should feel like a hot bath.
At 90 ambient it's pretty hot. I wouldn't hang onto it for more than 10 seconds.
At 105 ambient you should treat it like a cloths iron. Just touch for a milli-second. It can burn you pretty good.

Next is still on hot/high side. The liquid line is coming from the condenser to the expansion device. It should be substantially cooler than the discharge line but still above ambient.
70 degrees you can't really feel heat. 90 warm bath, 105 hot bath but not blister creating...

Moving onto low side. This is past the expansion device. This device can be different things at different mounting locations.
What most of us are familiar with is a TXV.

Thermostatic Expansion Valve. This valve's orifice is controlled via the outlet temperature of the refrigerant from the evaporator core; the colder the refrigerant is the smaller the orifice gets. The old school ones had a coiled probe or a capsule that was tightly clipped to the return hose at its exit from the evap core. These were usually covered in "stick-tite" tape. This is a thick tar like insulating tape that you can form together with your hands. These probes have to be well insulated to work properly.

A modern expansion valve is a simple block that has both sides running through. Often the outside of this block is covered with the sticky tape. Insulating these are far less critical and almost all OEM installations no longer uses insulation.

TXV valves are usually directly before (or bolted to) the evaporator core.

Another expansion device is the orifice tube which is a simple restriction made in the liquid line via a plastic insert that has a smaller tube in the center of it.
This can be installed anywhere in the liquid line past the drier assembly.
I think they work best in the entrance of the evaporator, but they can be a good distance from the evap and still function.
Early chevy trucks have them mounted behind the right front turn signal.

2000 up have them mounted in the liquid line about 10 inches away from the firewall/evap entrance.

This is usually where the cold is the very coldest.


We want the suction line from the evaporator to the compressor to be as cold as possible its entire length.

When the gods are in your corner you will see the wet shiny tube suddenly get "flat" looking and frost will appear. At this point the compressor cycles off and the tube turns back to shiny from the compressor end first. It "defrosts" SO FAST that it looks like a "special effects" in a movie. The frost retreats up the tube 6" per second...

As the ac charge weight is dropped the suction tube will gain temperature from the compressor end FIRST!
A little bit low-and the temp difference is small.
A medium bit low (25%) and the tube will be noticeable cooler (10+degrees) near the evap exit (firewall in most cars but not in a 914) versus at the compressor.
Significantly low will feel cooler than ambient but not cold; maybe 60 degrees at evap exit and above ambient at compressor, maybe a bit warm.
Worn out compressor feels much the same.

Compressors, as with so many things, lose the ability to suck FIRST.

I always touch suction first, then liquid, then discharge. Without spending the time to hook up gauges I have a quick assessment of how the system is moving heat.
Cool to ambient suction, ambient liquid, center of hell hot discharge is restricted condenser.
Cool suction, ambient liquid, warm but not hot discharge. Either low on refrigerant or compressor efficiency (gauges will make that determination).

If hoses all feel right, you should have good cooling coming out of the vents. Just after feeling up your hoses, I'm going to hold my grubby hands in front of all your vents.

Ice cold suction and pissy vents (55 degree) you have evaporator housing airflow issues/ blend door/ restricted airflow through evap/bad cabin air filter/blower issues/heater bleed through.

It took far longer to describe this as to do the testing. In 5 seconds I'll have a pretty good "feel" for what your system is doing and where I need to look first...

All temp testing is done at the metal sections of the lines. The rubber portions insulate well enough that they cannot be used.

Enough on system evaluation. Back to some basic physics.

Pressure cannot be used to determine weight. You can take two 30lb jugs of refrigerant in which one of the jugs is full and one is 90% empty. If the jugs are at the same temperature, they will BOTH read what I call "static" pressure. This number can be found on a refrigerant chart. These charts show temperature vs pressure. They are accurate!
If you take the 90% empty jug outside and leave it in the Texas sun in July an hour it will most likely show nearly double the pressure as the full jug that is at 75 degrees.

Your vehicle will show proper static pressure at rest (off the temp charts) if it is 300% overfilled all the way down to not having any liquid at all in the system (95% empty)
The ONLY time it will show more than the charts predict is if it is either hotter than you are estimating or it has a bit of air in it. Air makes the pressures go UP!
The only time your car will show below static is when nothing but vapors exist in the system and you are almost empty.

Everything has an exception.
2011 Camary, Complaint AC goes away after driving for a bit. Car shows up after being driven 20 minutes. 70/0 Vacuum on low side! No temp difference in high/low lines, no cooling. Static pressures should have been 90+ This puppy has less than static on high side and vacuum on low side. Looks empty as shit!
1.1 lb system which I pulled .95lb out of.
Almost completely full!
?????
The expansion valve was too sensitive and was completely closing off sealing the high side from the low side, but it was functional for the first 10 minutes. This allowed the refrigerant to get super cooled by the condenser and just fill it and the liquid line completely up with liquid refrigerant. Way weird!

If the expansion valve did not allow the cooling to happen and was closed on initial start up you might see 450/cut out/450cut out, possible vent refrigerant, damage compressor, a heart attack, system death!

SO, gauges are used to show performance, not weight or charge level.
I have multiple fancy pants recycling machines that have scales that show to the third decimal point. I use these exclusively. I have enough experience and feel that I can get the job done without the machine but I believe the machine is more accurate than I am.
I do want to see what the gauges show after I inject the correct weight of refrigerant.

Only use the cheap 12 oz cans for DIY use. DO NOT use anything with extra oil or especially sealant. I can pretty accurately gauge how much I've let out of a can.
I'm betting you can too...

There are always exceptions to the "rules" Some late model 134 cars and almost all 1234 cars have a secondary cooler which wraps the warm/hot liquid line around and through the ice-cold suction line. This assists cooling of the less efficient new refrigerant but it kills the above novel for a method. It's even more important to use a proper recycling machine with the new crap. It's also slightly explosive so you CANNOT use your old fashion vacuum pump. A new-fangled "sparkless" vacuum pump is a requirement to the new stuff.

They bitched at us for three decades to not use hydrocarbon based refrigerants and now that Dupont/Honeywell says so we all are...

Enough brain dump, should have chopped and channeled this but you're getting it raw.

DO NOT QUOTE THE WHOLE POST PLEASE!

If you have questions, comments, chop and channel appropriately.

More later...

Like I can vary the low side readings in your car by 25 psi by messing with how the system is working. I can vary the high side 100+ using same methods. I can make the gauges read what I want them to without changing the charge level.

Gauges cannot be used to properly fill a system by themselves.
Gauges and basic logic/understanding how the systems work is what's required.

Rick