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clutch engages, no leaks, blows warm


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mccarthy_mg
New User

Mar 23, 2008, 6:49 AM

Post #1 of 7 (4037 views)
clutch engages, no leaks, blows warm Sign In

 My A/C stopped blowing cold. The radiator fans and evaporator blower work, the compressor engages. This wasn't the first thing I checked but eventually I got around to letting some R134a out of the system, and sure enough the compressor clutch stopped working. I added some back and the clutch started working again. So, there is at least enough pressure to activate the clutch cut-out switch.

It's a VW with a typical system layout The high side starts at the compressor discharge, then goes directly to the evaporator, then to the receiver-dryer (the r-d inlet has a blow-off valve, the r-d outlet is plain). Then, the circuit goes to the aforementioned cut-out switch and then to a "Y" where it forks to both expansion valves just ahead of the front and rear evaporators.

I'm going to skip describing the low side, because I have a question about the high side.

I have two similar VWs, one a '93 and one a '95, and they both have similar A/C systems, and it's the '95 that has stopped blowing cold.

On my '93, which works well, if you touch the lines on the high side they are all hot. The compressor discharge, the in/out thru the condenser, and the in/out thru the receiver-dryer...everything on the high side is hot.

On my malfunctioning '95, the compressor discharge line from the compressor to the condenser is hot, and the condenser is sorta hot, but everything after the condenser is cold. That is, the condenser outlet is kinda warm, and the in/out thru the receiver-dryer is stone cold. I think something is wrong right around there.

Could the receiver-dryer be bad? What are the symptoms of a bad receiver dryer? Could the compressor be bad?

It was working great the last time I used it, then all of a sudden it wouldn't cool. Thanks for any help.

(I have a cheap gauge, and I was hooking it up to both the '93 and the '94 to compare pressures, and in the process the membrane in one of the valves on the '93 tore and started leaking, so now I have to keep that system sealed with just the dust cap and will now have to replace the valve. It's been one of those days. It started with both A/Cs working and it ended with one not working, and one that could fail any second. I don't want to mess with pressure readings again unless I'm in a shop with better set of gauges. I'm assuming my cheapo gauge tore the membrane, but I'm not even sure of that.)


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Mar 23, 2008, 6:07 PM

Post #2 of 7 (4029 views)
Re: clutch engages, no leaks, blows warm Sign In

First - What is this vehicle? Two evaps is not typical of most VWs.

Know that 2/3rds of all A/C not blowing cold problems are low refrigerant and you let some out?? You added some back and now have no idea how much is in this system - too much or too little by a few OZs can stop the show especially with dual systems.

Pressures needed at a minumum or charge by known listed weight into a vacuum for further testing. If compressor was engaging why did you suspect the LPCO switch?

T



mccarthy_mg
New User

Mar 23, 2008, 7:11 PM

Post #3 of 7 (4027 views)
Re: clutch engages, no leaks, blows warm Sign In

I should have mentioned it's a Eurovan. They all have two evaps (except the quite rare pop-top camper, which only has one). They were sold from 1992 to 2005. Even though they were sold for 14 years there are not very many of them on the road (a poor seller) but the vast majority of them have two evaps. (And, btw, when the A/C is working, it really works well, front and rear -- big evaps, huge fans and blowers....they move a lot of BTUs).

I mentioned in my original question that I released some freon just to see if the system had pressure and was reacting to low pressure correctly. Remove a little, compressor should stop. Add a little back, compressor should start up again. It did. It was just a test to see if the system was responding to changes in pressure.

Now, all things considered, and as you say, I think the situation is simply my charge is too low. I'll wait for warmer weather and add a little back. I'll have to look for leaks, too. If it gets low it's leaking somewhere.

I don't have a good set of gauges, or a way to pull a vacuum, so I am forced to pay a technician to do that part of the work for me. What I am unhappy about, and why I am reluctant to employ a tech until I'm sure exactly what work is actually required, is how techs will charge you by the hour for very little hands-on work.

For example, after the last time I replace a device on the A/C on another vehicle I own (I replaced an o-ring on the compressor discharge line that had bulged out and was leaking) I then needed to evacuate and charge the system, so I took it to a tech. The tech attached his equipment and started to draw a vacuum, then he walked away and worked on other cars. He came back an hour later, added the correct amount of R134a to my A/C (there's an underhood label so the info is right there and didn't need to be researched), checked that everything was working correctly (it was), and then wrote up the bill: $145. He spent, at most, 15 minutes under the hood. He charged me for the time spent pulling a vacuum, while he was working on another car.

$30 or $40 (max, probably less) of supplies and 15 minutes of labor equals $145?

In short, I want to make sure I have all the bases covered before I go to a shop and have them hook up their gauges and start billing me for their "time."


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Mar 23, 2008, 8:46 PM

Post #4 of 7 (4025 views)
Re: clutch engages, no leaks, blows warm Sign In

Ok: $145 is a deal. Just the know how and equipment needed to propery vac and charge a system is worth all that. Some charge that just to take a good look!

You simply can't play with letting gas out and some back in without unreal experience and still need some tools. Gauges are primal and guessing doesn't work well or at all. If you forget to purge lines of however you add refrigerant in you have added air which contains moisture which will turn oils for 134a into acidic oil and may have done about $4,000 bucks in damage! That won't show up right away but will just start new leaks all over the system one at a time - game over for that old a vehicle at least for A/C!

You can feel for pressures! 134a will just by coincidence be about the same in temp F. as the pressure is in PSI - surprise - you just learned something!

There is no dipstick for the exact weight of refrigerant. This is not unlike guessing how much engine oil is in an engine without a dipstick. Ex: Take or let some out that you didn't measure, add some more that you didn't measure and you are lost without emptying system and put in known amount. It's similar with A/C - you let refrigerant out or oil out till there's no oil pressure (engine) and add some back in and get some pressure but you are lost. The bad news with A/C is if system is low it can't carry the oil to lube the compressor so low pressure switch shuts it down and that's normal for when it's cold to prevent liquid refrigerant to return to compressor which will lock it up. Then if that happens (overcharge could do that) little particles go thru the whole system and must be flushed out and any TXVs replaced and most condensers won't flush out debris well - all new parts could be needed!

Pressures alone are marginal with dual air for diag as one compressor is feeding two evaps and how do you know which is getting what - - best pressure tests are when using just front at full request, windows open at 1,500 RPM. This is a whole science to do this - not a passing whimsical hit and run guess.
http://www.autoclimas.com/...igerant%20charge.php <<<< Click that as it describes what you need to do and know just for a charge! Any hair left?

Ok again: I'm guessing since this worked with the known right charge that another leak happened or went unoticed. Holding a vacuum for an hour usually indicates you are free of leaks - at least for that time. A new one could happen a mile down the road. Finding those is an art. My silly old electronic sniffer is almost $400 bucks which is so sensitive you could squeeze some unseen but known gas between two fingers - walk around the block one hand open the other closed - come back with both still tight and I can tell which hand you did that with!

You can rent gauges and a vacuum pump for free at some large chain stores - full deposit for safe return. You could weigh in the charge with little 12oz cans to a known amount and weigh the can bit by bit down to last even oz with even a good postage scale. You still have to bleed air out of the hose each time you disconnect it before you reconnect it or in comes the air.

Read that info above and decide what you want to do. If it's less than 70F ambient temp out, testing is not dependable or accurate. Better to wait even if you send this out as you can't test performance well even in a heated garage,

T



mccarthy_mg
New User

Mar 24, 2008, 5:53 AM

Post #5 of 7 (4019 views)
Re: clutch engages, no leaks, blows warm Sign In

A few things. I completely understand the issues with keeping oil at the correct level, and keeping air out of the system. I know why you pull a vacuum. I know why you flush air out of any hoses you attach.

Second, I completely understand the concept of a business owner charging enough to cover the costs of tools, overhead, etc. I'm a cost accountant by profession, and a small business owner by occupation. I also understand when someone double dips. It's no different than a lawyer taking a call during a client meeting, and then billing for both the call and the meeting. It's unethical, in some situations even criminal. It's why defense contractor sometimes go to jail when gov't auditors catch them doing such things. If you charge me for an hour of your time, you better be working on my car for that entire hour, not someone else's. And the $100/hour you charge me covers the cost of your time and equipment and overhead, plus you bill me for supplies, and parts, and disposal fees, and more. But you don't double-dip. If you only spend 20 minutes working on my car, you only charge me 20 minutes of your time.

My A/C system was brand new, solvent flushed clean and dried, empty (except the 4 ozs PAG oil shipped with the new compressor), no leaks, no malfunctions, no issues at all. It just needed to be evacuated and charged: 3lbs R134a and 4 more ounces PAG oil. 20 minutes is all it needed, and 20 minutes was all it took. I was billed for an hour (plus the supplies, etc.). That's wrong. I didn't argue. I expected it. But it's still wrong.

As far as the harm in guessing charge levels, the system has both high and low pressure cut off switches to protect the compressor, so what harm can be done, on the low side especially? Unlike your engine oil analogy (where overfilling the crankcase can pound the oil into foam, then pump foam into bearings, then harm bearings; and underfilling does nothing more than blink an idiot light, which the operator may or may not know requires the engine be stopped immediately), the A/C system will not allow itself to be damaged, or will it? My low pressure switch works (I tested that) so that protects the compressor from lack of oil and liquid refrigerant, right? No harm is going to occur just by losing a little refrigerant. After all, if it did, then every little leak that eventually caused the low pressure switch to function would typically lead to compressor damage (and debris in the system, that may or may not be removable, and thousands of dollars in new parts)? And I don't overfill. If it's not working right, I don't keep adding and adding. I know that once the low pressure switch tells the compressor to engage that I should be getting some -- even a little -- cooling. If not, I don't just keep jamming in refrigerant. My high pressure switch has never been called to duty.

In my case, in the 15 years I've owned the van, I've had to open the system a few times. Once, around 1998, the low pressure switch just failed without warning. A big woosh, a cloud of refrigerant, a messy spray of oil in the parking lot at the Post Office. My wife called to ask if it was safe to drive home, and of course it was. I installed a new switch and a new receiver-dryer (and new o-rings), took it to a shop, they pulled a vacuum, guessed how much oil might have been lost and needed to be replaced, and recharged it. It was fine for another couple years when it suddenly stopped cooling. If found a leak, this time at the high pressure switch. I bled off the refrigerant (slowly, so as not to lose any oil) and replaced the switch. Back to the shop to pull a vacuum and recharge. Again, the tech wanted to add a couple ounces of PAG oil....again just guessing. I talked him out of it since I hadn't lost any oil this time. We were both guessing.

Finally, in year 12, the compressor fails, with some drama. Totally overheats, melts the serp belt. I buy a new one. I rent a solvent gun and buy all the right o-rings (a dual evap system has a bunch of them and most of them were unique). I buy two new expansion valves. I disassemble the entire system and flush and dry each line, both coils and the evaporator, individually. It took two days. I didn't see a spec of debris. The old expansion valves were sanitary. I assumed when the compressor failed it dumped junk in the system, but it looks like that didn't happen. Oh well. In any case, my system was clean and dry with a new compressor, new expansion valves and new o-rings. I took it to a shop, they pulled a vacuum, added the correct amount of oil (finally, no more guessing) and refrigerant. It's worked fine for three years.

Then the heater coil starts to leak. The only way to remove the heater coil on this particular van is to remove the entire airbox from under the dash, and that means the evaporator has to come along for the ride, so again the system had to be opened...a couple new o-rings, recharge, etc.

Anyway, the point is I know about cleanliness, and oil, and water vapor, and cut off switches. I never overcharge. If the system loses a little (and it will....a system with a real slow leak that loses a few ounces in a year is the perfect candidate for topping off) then bringing the system up to the correct charge from a low level is harmless, isn't it? If there's any harm then please explain. A can or two every year costs $10 at the autoparts store. Finding a tiny leak, replacing parts, recharging....that's going to cost many hundreds, maybe a couple thousand. The math just doesn't work. You live with tiny leaks, you top off conservatively, you fix it when the big stuff breaks. And when you do need a tech, you don't let them say "I'm going to add a couple ounces of PAG, OK buddy?" every time you need service or you end up with way more oil than you should, and that reduces cooling and risks harm, right?

Otherwise, thanks for the tip about borrowing gauges and vacuum pumps at autoparts stores. I didn't realize any of them rented those. I'll look into it. I've have a lot of tools myself, and I've borrowed certain special tools from autoparts once in a while, but I didn't realize they rented vacuum pumps. If they do, that's great.


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Mar 26, 2008, 11:01 PM

Post #6 of 7 (4000 views)
Re: clutch engages, no leaks, blows warm Sign In

What do you want to do?

1. A shot gun approach - hit or miss - without proper equipment or.......

2. Buy a few thousand bucks worth of equipment and I can easily walk you thru this.....

3. Pay up and let a pro with equipment do this properly and perhaps save you tons of $$ in the long run.

Note: 134a and dual TXV systems will fail if not real close to exact charge by weight. For diagnosis for the leak which about all systems will have at some time would help to measure how much comes out of it now with it's current behavior and pressures could lead to the real problems.

Hey - there are "flat rate" charges for certain procedures on a car that is not measured with a stop watch. Unseen time caring for equipment, maintaining it, depreciating it, cleaning up the place your vehicle was worked on, static costs for the space, storage, taxes among other costs just for being able to do this.

It's up to you........

T



Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Mar 26, 2008, 11:11 PM

Post #7 of 7 (3997 views)
TXV systems and how SUPERHEAT is captured by them explained Sign In

Note: Understanding and applying the knowlege of just this is a college course of study for MVAC. Not easy, not cheap........

BTW, this author of this is a friend of mine who offered this tidbit for the world to see for FREE!


What is Superheat? ACProf was kind enough to explain the meaning of Superheat for us. The following text originated from the A/C forum. The original message and replies appear here.

This question comes up every so often on the board when someone uses the word SUPERHEAT in a response about Thermostatic Expansion Valve ( TXV)operation. Unless you are in the field of Thermodynamics, or went specifically to an Air Conditioning School, what superheat is is generally a mystery. I was asked in another posting by a frequent contributor to explain in some detail what superheat really is.

If you are not familiar with the basic names and operation of an AC system or don't care to know what superheat is, STOP HERE and go on to another post. Knowledge at this level is not necessary to diagnose and repair an AC system. I've condensed this tutorial down to as few words as possible and still present as simply as possible in general terms what SUPERHEAT is and why it is important in the d esign and operation of AC systems..
-------------------------------------------------------------------------------------------------------
Superheat measurement is the only completely accurate way to determine if an airconditioning system is charged with the exactly correct amount of refrigerant and is operating at peak efficiency. Superheat measurements are most commonly used on HVAC home and commercial systems to determine the correct refrigerant charge. HVAC systems have a known speed and volume output compressor, permanently adjusted orifice system (called a capillary tube system), known heat load, and constant airflow over the evaporator and condenser. The constant superheat temperature value will be determined mainly by the refrigerant charge in the system. None of these are true on an automotive system making superheat determination more difficult, and not widely used. . Although not complicated to determine, it does require a few laboratory quality pressure and temperature measurement devices.

Superheat is very near and dear to me because it is the reason, as a teenager, that I got the left side of my face and ear pretty severely burned by steam expelled from opening an overheated car radiator.

Because everybody knows abo ut water, I'll use water to explain the principle of superheat and then apply it to AC evaporator operation.

Everything exists in one three states, solid, liquid, or gas (vapor). Water can exist in all three states, depending on its temperature and pressure. Let's start with water as a liquid. How hot can water get? Does boiling water get hotter if you turn up the stove heat? Answer: NO. WATER (liquid) at sea level, can only get to 212 degrees F. If more heat is added it starts to boil and turn 212 degree water (liquid) into 212 steam (vapor). Turning up the heat more only makes water boil FASTER into steam, but as long as both, water and steam exist, they both stay at 212 degrees.

Water requires a lot of absorbed heat energy just to turn into steam at the boiling temperature. The heat absorbed by the water to change into steam (vapor) doesn't raise the temperature of the water or the steam. This is called LATENT HEAT (hidden heat) because in spite of all the heat a bsorbed no temperatures were raised. This law of thermodynamics is what AC systems are designed on.

Once ALL the water (liquid)) has turned into steam (vapor), and ONLY steam exists, if we continue to add heat, the STEAM temperature will rise above the boiling point. This amount of this temperature rise above the boiling point is called SUPERHEAT! The very presence of a temperature greater than the boiling point (that's the superheat temperature) means that all liquid has been converted to a vapor. Obviously (but VERY IMPORTANT) then, if a vapor contains superheat, its guaranteed to all be in the vapor condition!

About the car radiator. This particular car radiator had a 13 pound pressure cap on it. This added pressure will cause the boiling point of water to rise, and the water to remain a liquid well above its regular boiling point temperature. THAT's WHY pressure cookers cook food faster. The food inside is in water that is actually much hotter than 212 degrees. Same in the radiator. The 13 pounds of pressure causes the water in the radiator to stay liquid at temperatures well over 230 degrees.

Not many teenagers know about superheat or raised boiling points. Soooo, the car was overheating and I slowly removed the cap to take a look. I knew that sometimes the radiator spews a little steam and hot water as the pressure is relieved so I wiggled the cap first. - nothing happenned - good. I twisted the cap the rest of the way off and as I lifted it up, the cap seal to the radiator broke loose. Uh Oh!

Well, with the pressure in the radiator now gone, the water reverts to boiling at around 212 degrees. However, the water in the radiator is still 230 degrees. This water is SUPERHEATED 18 degrees above its new boiling point!!! WHAT HAPPENS??? It wants to be ste am! ALL the water in the radiator IMMEDIATELY turned to steam and blew out of the radiator like a jet engine. I jerked away, but it still got me!! LESSON LEARNED THE HARD WAY!!! As mentioned earlier, (and getting more IMPORTANT) SUPERHEATED substances MUST be in a vapor state.

Finally, how does this SUPERHEAT relate to air conditioning.

The whole reason air conditioning works is because liquid refrigerant with a very low boiling point entering the evaporator is made to boil to vapor by absorbing heat from the air passing over the evaporator coils. The air forced through the evaporator is now leaving with less heat (say-cooler) than when the air went in.

At this point, as mentioned earlier with water, the temperature of the vapor will be the same as the boiling liquid it came from (remember latent heat). This also means that the temperature of the evaporator output tube will be at the same temperature as the input tube.

However, a serious problem may still exist, if not all the liquid actually gets boiled into vapor, and some liquid leaves the evaporator. The liquid will damage the compressor. This is where SUPERHEAT comes in. The TXV will insure that all the liquid is boiled by limiting the incoming refrigerant flow so that the outlet temperature is higher than the inlet temperature. When the evaporator output tube is slightly warmer than the input side, this INSURES that there is a SUPERHEAT temperature and everything leaving the evaporator WILL be vapor. HOW does the TXV know how to do that?? Oh Yah, the TXV temperature sensing bulb that is attached to the evaporator tubing near the discharge end!

The temperature bulb "reports back" to the TXV what the outlet temperature of the evaporato r is by way of pressure in the bulb's tubing. The TXV already "knows" what the boiling point temperature of the refrigerant is (by way of the low side entry pressure). These two pressures are on opposite sides of the diaphragm that operates the flow control valve. The TXV diaphragm movement is factory adjusted to allow just enough enough refrigerant into the evaporator to guarantee complete boiling to vapor and an additional amount of temperature rise above boiling (THE SUPERHEAT!!) guaranteeing all the liquid refrigerant has boiled and no liquid to escape remains. The TXV then, as a side benefit, will always allow in as much refrigerant as possible, while maintaining the superheat temperature to prevent liquid discharge. That's MAXIMUM EFFICIENCY of the evaporator. Too little (starved TXV) or too much (flooded evaporator) refrigerant charge will interfere with the TXV's operation and results in less than perfect operation.

Typical SUPERHEAT for auto systems is 3-5 degrees. On Home and Commercial Units, it can be over 10 degrees.

Too bad Detroit replaced this little jewel with an orifice tube that does NONE of these things. THAT'S WHY t here is an accumulator on the end of the evaporator on an O-tube system -- to catch the liquid that gets in but doesn't get boiled because there is no SUPERHEAT control.

Side note 1- for you Suburban and most DUAL-AIR system owners. The REAR system generally has a TXV and the front system has an O-tube. THAT'S WHY the rea r system seems to work better all the time, compared to the front system. It IS working better!!!!

Side Note 2 - (A CAUTION ACTUALLY) I have heard that there have been several cases where water put in a glass measuring cup and heated in a microwave didn't boil and became superheated. When the person disturbed the cup, the entire contents of the cup turned to steam and severly burned the person's hands and arms. Sounds plausible. To avoid this, it is recommended to put a spoon (metal's ok) in the cup before heating. It will cause boiling to occur instead of superheating.

The article above originated from the A/C forum. The original message and replies appear here.

Thank you ACProf!







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