AC factoid

[joshua43214]

I am writing this since we are hitting AC season, and I have some to kill at the moment.

---components of the AC system
Compressor; mounted to the engine,
Evaporator; under the dash, basically a long tube coiled back and forth with fins on the outside, looks a bit like a heater core.
Condenser; in front of the radiator, same basic design as the evaporator, but much larger, looks like a radiator.
Receiver/dryer; under the hood near the condenser.
Expansion valve; mounted at the evaporator inlet.
Auxiliary fan; mounted in front of the condensor.
High/low pressure switch; on the receiver dryer.

---Basics of operation
For purposes of simplicity, I will use the word "r12" for all refrigerants and treat it like it is always a gas so I won't have to get into discussions of superheating and the like.

The r12 flows through the components like this; compressor > condenser > receiver/dryer > expansion valve > evaporator > compressor.

The low side of the system encompasses the following parts; compressor, evaporator, expansion valve. Low pressure (suction) hose.

The high side encompasses the following parts; compressor,condensor,dryer,expansion valve. High-pressure hose.

AC works on the principle of an expanding gas freezes. When the system is off, the r12 has a uniform pressure through out the entire system. When the compressor starts, it draws the r12 from the low side and compresses it into the high side. The expansion valve creates a restriction that the r12 has to pass through in order to enter the evaporator. Since the compressor is attempting to create a vacuum on the low side the r12 will "spray" into the evaporator and rapidly expand to fill the area cooling dramatically in the process. As outside air passes through the fins on the evaporator, it absorbs the heat and transfers it to the r12.

This r12 that has absorbed the heat is then drawn into the compressor and highly compressed and passed through the condenser, where its heat is then transferred to the outside air with the assistance of the auxiliary fan. The r12 then passes through the receiver/dryer where it is filtered of debris and any moisture is absorbed by the desiccant inside it.

The expansion valve has a plunger that varies the amount of r12 entering the evaporator.

On most BMW's the compressor is operated by the DME, this is done so that the compressor will not operate when the car is overheated, and so the compressor can be shut off at wide open throttle. The fan speed and vent positions are operated by the panel and climate control module.

---
In order for the system to operate, every thing must work together and be self-correcting to a degree. If too much r12 is allowed to pass through the evaporator, the fins can freeze solid from the condensed moisture in the air. If there is no air movement through the condenser, the highly pressurized r12 can make hoses explode and even make weak engines stall from too much load.

To prevent damage to the system, a high and low-pressure switch is installed that will only allow the compressor to run if the pressures are within a certain range.

---Diagnosing AC malfunction

This can be divided into two categories; compressor runs but does not cool enough, and compressor does not run.

If the compressor runs, but you are not getting enough cold air, most likely the car is low on r12. If the dryer is equipped with a sight glass, you can attempt to recharge it yourself. Be warned, more is not better when it comes to r12, it is easy to over charge a system trying to get all the bubbles out, this will at the least reduce performance and at the worst, destroy the compressor. With out hooking up a set of gauges, it is only guess work as to what is really happening. Also, r12 leaks often leak oil as well, large leaks always do. If you lose too much oil, the compressor will fail. If you add too much oil, the performance will be greatly reduced and you can also destroy the compressor. Any time a part is replaced, the proper amount of oil needs to be added as well(no oil for expansion valve replacement). Expansion valve failure is a very common cause of reduced AC performance, and can make using a sight glass inaccurate, easily over charging the system. Also AC dye's use oil as a carrier, so repeated charging with dye will reduce performance or damage the system.

If you are experiencing AC performance or other troubles and the compressor turns on, then the problem is almost for sure related to the systems r12 charge and is not electrical. Compressor short cycling is almost always from undercharge. A performance check should be run before any electronic diagnosis is performed.


If the compressor does not run, either the system is too low on charge to activate, or there is an electrical problem. To verify proper charge, one needs hook up a gauge(in the USA, its the law). Using a torx driver, briefly depress the shraeder valve, You should get a big blast of r12 in your face if it is properly charged, go to the doctor and get treated for frost bite, then return to diagnosis. Don't do this where it is illegal.

There is a llot of variation in the AC wiring across the e34 line. So some quality time with a wiring diagram for your specific model may be required. The components listed above are common to all e34's. Some of the older models had 3 pressure switches on the dryer rather than a single high/low switch.

Simple things to check;
Check all of the fuses with a test light on the back of the fuse.
Check for corrosion at all the connectors.
Make sure the connector at the compressor is tight; it can rattle off, causing intermittent connection.
Using a wiring diagram, bypass the pressure switch and see if the compressor runs.
Verify you have voltage to the various connectors, do this with the ac on and the engine running.
Tap the top of the compressor relay with a screwdriver handler, if the compressor comes on replace the relay.
Check ac belt tension, it can slip and not make any noise. If it slips and the tension is good, check the aux fan for proper operation.
Make sure the compressor clutch is not slipping.
Verify the heater water valve is functioning properly.

Things to remember;
The blower is on a separate circuit, the blower can run with no compressor and vice versa.
The compressor will shut off after a short while if the blower is not working.
The compressor will shut off after a short while in the auxiliary fan is not working.
The auxiliary fan is pressure operated on some models, so it will not come on until the pressure is high enough.
Reduced air volume is usually caused by a clogged cabin filter.
Reduced airflow through the condenser will cause the system to leak faster.
Do not operate the system with components bypassed for any longer than absolutely necessary for diagnosis.
R134a and r12 use different types of oils.
If the system is empty, it's a very good idea to replace all the o-rings. Lube them with a small amount of r12 oil before installation, pag oil is not recommended for this.
Carefully clean any corrosion around the o-rings, use a wooden or plastic scraper if needed.
Slightly undercharged is better than slightly overcharged.
The under hood sticker will have all information pertaining to proper refrigerant and the amount to be used, ALWAYS use this reference first before all others.
Submerging cans of r12 in hot water will aid charging a great deal. I don't recommend a torch, no matter how tempting.
The system must be under vaccum before it is charged if it has been open to the air. If it has no pressure at all, it is best to take it to a shop for charging.
Make sure the service caps have the o-rings installed; the cap is the primary seal, not the schraeder valve. Get new caps if the old ones are missing.
When using a sight glass, check for bubbles with the idle at about 1500rpm. It is normal for few small bubbles to pass through when letting off the throttle, or even at idle in the right ambient temperatures.
When converting to r134a, it is best to replace the hoses since most r12 hoses will leak r134a. Drain as much of the old oil from the system(but not the compressor) as you can. And always replace the receiver dryer.
Never use AC sealer on systems with expansion valves or any other system with a capillary tube, it can clog the tube or cause the plunger to stick. This stuff is best used on orifice tube systems, and even there I don't recommend it.

Many AC problems can be fixed by the DIYer. Having someone walk you through the process of charging is a good idea if you have never done it before. Techs are often sympathetic to customers about AC since it is so expensive, and many techs prefer not to do AC service at all. They can be a wonderful source of tips and pointers, and can advise you on when its a good time to cut your loses and pay the shop for the service. Shops will sometimes even evacuate your refrigerant for free since its free parts for them, especially if you use r12, the stuff is quite valuable.

Having a professional service AC can get very expensive very fast. Many shops charge a premium partly because of the comparatively high comeback rate, and partly because both the equipment is extremely expensive and so are knowledgeable techs. It is common practice in many shops to simply service the AC, fix anything that shows up as bad, shoot in some dye, run it for a while and do a fast look over for leaks. Then they give the car back and tell you to bring it back if it stops working so they can check for dye traces. This method has proved to be more economical for the shop, but hurts the customer by having to pay for a service two times. Not to mention two visits to the shop for the same problem. Some shops will still use electronic leak detectors and actually have the patience to use them properly, these shops will charge a bit more for the service usually, but it is often money well spent if you are paying someone. Also, many shops will only cover warranty costs on hard parts and their labor, not the r12 itself or the labor of the evacuate and recharge.

Any corrections, feedback or flames welcome.

[Dave M]

Holy flying $hitballs. A 'bit of time to kill'?

I don't get that much work done in a day.

Since I removed my AC system, I didn't feel obliged to read it all, but i'm sure it was captivating.

Nice work

Dave M

Edit: Isn't the aux fan mounted in front of the condensor (rather than the evaporator)?

[Blitzkrieg Bob]

Expensive if you stick to r134, and aren't an environmentalist.

a manifold, thermometer, vacuum pump.

You'll end up fixing a bunch of A/C s for all you friends and family

[SRR2]

Heh heh. No explanation of what the expansion valve does. You chicken! 8-)

Your writeup would benefit, IMO, by including the notion of liquid refrigerant, the evaporation and condensation processes, and at least a mention of latent heat of vaporization. After all, latent heat is how liquid refrigerant at 110F can cool incoming air down to 40F.

[Blitzkrieg Bob]

That reminds of a Physics professor that could drone on for hours about refrigeration and how efficient the thermal transfer is, measuring the BTUs of each side, put hot food in, measure the BTU rise on the condenser…Etc. Etc…Etc.. and the only loss was running the pump. Or if you got him going on an L.P. gas refrig, then the whole lecture was shot.

[zygoteer]

Thank you .. a cogent explanation.

[joshua43214]

Heh heh. No explanation of what the expansion valve does. You chicken! 8-)

Your writeup would benefit, IMO, by including the notion of liquid refrigerant, the evaporation and condensation processes, and at least a mention of latent heat of vaporization. After all, latent heat is how liquid refrigerant at 110F can cool incoming air down to 40F.

LOL every attempt I made to write an acurate description of the expansion valve got really technical. I had started with a nice desription of the entire liquid to gas to liquid process, and how superheated refrigerant behaves. But it got all boged down with definitions and further explanations and the like. Since I was aiming this at the layman and the DIYer, I decided that it was beyond the scope of the article. The professionals already have been to the workshops and don't need my help.

I could picture peoples eyes glazing over and then skipping to the next article. It is interesting stuff, and I think most of the forum members here would enjoy learning it, but the article would have been 1/2 again as long.

[GoldenOne]

could you post expansion valve explanation anyway, im curious to see what it does...

[SRR2]

True, true.

However, you have those knuckleheads that tell noobs that "all you need to do is go get a set of gauges, and that will tell you how much refrigerant you have in the system" You must have seen posts like that. At least one pass through the expansion valve might convince both of these parties that they just don't know enough about what they're doing to attack a refrigeration system, much less one that's as touchy as recent R134a systems. Example: the charge in my E39's system is 750g +-10g. Yep, TEN. Now how is Clyde Clueless gonna manage to achieve THAT, with his set of gauges from Harbor Freight?

So that's what I'm suggesting: scare them off with a pithy paragraph on the expansion valve.

Related story: Due to a poor installation, I've had quite a bit of trouble with the Trane heat pumps in my home. I've going through three "authorized" dealer service departments in trying to get them working right. "Professional" #2 was here a few years ago and overcharged the systems so far that the compressors slugged under some conditions (When the reversing valve switched off defrost. It was an awful sound.) and eventually killed the valves. I should have known because I found him using charging curves (my system has expansion valves in both heating and cooling modes) for systems with capillaries. I kid you not. After "Professional" #3 came in and changed out the compressors I took nothing for granted. *HE* left both systems overcharged by almost 1.5 lbs. *sigh*.

[joshua43214]

could you post expansion valve explanation anyway, im curious to see what it does...

OK, the simplified explanation of what is really happening.

---Basic theory

When you raise the temperature of a liquid to its boiling point, you are adding energy in the form of heat from an

outside source in the form of heat. The point at which it changes state from a liquid to a gas is called a

transition point. A liquid has its molecules loosely bonded to each other, it requires energy to seperate thse bonds

allowing it to become a gas. So as you add energy to a liquid at its transition point, its temperature will not

rise. Rather that energy is used to seperate the bonds in the liquid allowing it to become a gas. Once the substance

has become a gas, you can continue to add energy and its temperature will contine to rise as normal. This bit of

energy that was used to loosen the molecular bond allowing the liquid to become a gas is called "latent heat". Any

time any substance changes state, this energy cost of loosening the bonds of the molecules to each other must be

paid.

Now take the same liquid and place it in a vacuum chamber. As you lower the pressure (raise the vacuum), the

molecules of the liquid will be mechanicaly forced appart into a gas. The energy required to change state is taken

from the liquid itself. This will dramaticaly lower the temperature of the liquid as it gives up its engergy. If you

take this same gas and compress it, the energy used to change state back into a liquid be taken from the gas's own

latent heat, leaving you with a liquid of the same temperature as what you started with. In theory, this works out

as a zero sum for energy gain/loss, since all the energy was provided from out an outside source.

---Operation

The r12 is compressed into a liquid on the high side of the ac system. As the pressure on the low side is decreased,

the liquid passes into the evaporator and begins to change state into a gas. The liquid r12 becomes very cold. Once

the temperature of the liquid r12 falls below the ambient temperature, it will begin to absorb ambient heat, or

energy. This energy is then used to to pay the energy cost of changing state from a liquid to a gas and is "carried"

along the r12 molecule as latent heat.

If the system is properly balanced, almost all of the liquid r12 will have become gas as it reaches the end of the

evaporator. this is important since the gas will not carry oil with it, The compressor depends on the small amount

of liquid r12 to carry oil into its suction side. When raw liquid enters the compressor, it is called slugging and

can destroy it. Car compressors can tolerate a bit of abuse in this department, but they are really only designed to

handle the relatively small amount of liquid in the low side when you first turn it on.

What happens of course, is that on hot days the r12 becomes a gas before it reaches the end of the evaporator and on

cool days, there is still alot of liquid at the end.

Normaly with any gas, you would cool it to make it into a liquid. and heat a liquid to make it gas. The AC system

can compress a gas into a liquid that is far above its boiling point, or convert a liquid into a gas far below its

boiling point. Regardless of how the change in state was achieved, energy must be provided from an outside source.

---Enter the expansion valve

Since the energy absorbtion of latent heat is so efficient, idealy you want any heat absorbtion of the r12 to be

used in converting liquid to gas. In practice of course, the r12 gas is absorbing energy at the same time the liquid

is as it is changing state. Once the r12 has boiled off into a gas, any energy it absorbs is called "super heat",

since there is always a certain amount of energy absorbtion by the gas, there will always be a certain amount of

superheat. The expansion valve is calibrated to react to this superheat.

The thermostatic expansion valve is an alluminum block mounted on the inlet and outlet of the evaporator. it has

what looks like a small flying saucer mounted on its end. this saucer is a thermal diaphram. At rest, the expansion

valve is open to a set point that is calibrated to the system size and the refrigerant used. when the system

operates, liquid r12 on the high side forces the valve open, this is countered by the thermal sensor in the flying

saucer. When there is too much r12 passing into the evaporator, liquid will exit the suction side. This low

temperature liquid will cause the diaphram to pull down lowering the flow rate on the pressure side via a plunger.

If there is not enough r12 entering the evaporator, the high temperature gas will cause the diaphram to move up,

allowing more r12 to flow on the pressure side.

To a limited degree, one can test the reaction of an expansion valve by heating or cooling it with a hot gun or a

cold gun (yes, there is such a thing, the old timers who worked on carbs have them rotting in their tools box's).

These valve are sensitive to contamination from debris and commonly get clogged by deteriorating hoses or

reciever/dryers. It is also important to keep them insulated from ambiant air, this is why they are covered in

dumdum. Since they are calibrated for the refrigerant used, converting a r12 system to r134a does not always have

good results since r134a has different thermal saturation characteristics. r12 expansion vlaves can be used on r134a

system, but often require a fair amount of "tweaking" the charge for best operation.

---The other end

After the r12 is drawn from the evaporator, it is forced into the condensor where the thermal exchange is reversed

and the latent heat that was absorbed from the evaporator is transfered to the condensor and radiated to the

atmosphere. The liquid r12 is far above its boiling point even after leaving the condensor, the only thing

preventing it from cavitating is pressure, this is why it is normal to see a small amuont of bubbles in the sight

glass at idle.

Hope all this makes sense, I just fired this off since it was requested and didnt proof read it or anything. For

those physics nuts out there, I appologize for my over simplification of thermodynamics, it should be correct on all

the important parts. Any corrections are greatly appreciated, I only know this stuff accidentaly since I never took

physics.