need air conditioning specialist

[GTOSVX]

or a competent mechanic.
I have been using Gresham Subaru outside of portland Oregon, but they have been unable to repair my air condition.
It had a slow leak, so last year I had it converted to R134 and I thought repaired, but continued to not have cold air. So far I have a new evaporator, new evaporator sensor, new expansion valve, new engine control unit new air condition control unit, plus repeated orings schraeder valvem etc.
Compressor will not engage. Probably due to leak of freon, but I am not sure I have anything left to leak.
Any reccomendations on a shop in Portland Or area for air condition repair???

[dannmarr]

The compressor will not engage if you don't have enough refrigerant, or the condensor is so dirty that you have high head pressure and it will shut down the compressor. Try cleaning the the coil.

[GTOSVX]

Pressure was supposed to be good yesterday, that is when they finished the compressor install with a new unit (not rebuilt) The only reason I had the compressor changed was that it was showing a leak at the connections that they could not get to stop with new orings, etc. It held vacuum test for 2 hours, they filled it with R134 and did a leak test with a sniffer looking for any signs of a leak; none found. compressor was running for them. 2 hours later I pick it up, and driving home find no cold air. When I reached the house put it in diagnostic mode, and the compressor clutch will kick on, but immediatly off. I have them checking pressure this morning. did not think about the high pressure side of the problem, both will trip the trinary switch. Does anybody know the correct readings from the 4 terminals on the trinary switch? Is it open/close switch or does it use some reference voltage and comparator? I have never found cdocumentation on which terminal is high or low.

[mohrds]

When they originally converted the system from R12 to R134a, what did they all do?

They should have removed the compressor and drained the oil out of it, flushed all the lines, condensor and evaporator, replaced all the O-rings and the dryer, evacuated the air out, then pressure test with Nitrogen, evacuate again and then fill with R134a and POE oil.

There is a conversion problem called "Black Death" that occurs when shortcuts are taken during the conversion process. What happens is that there remains mineral oil in the system if not properly flushed and it can breakdown under the R134 environment and prevent the POE oil from doing its job of protecting the compressor, thus leading to metal shavings contaminating the entire system requiring a complete chemical flush.

If they install a new compressor without flushing the metal particles out of the system, it will clog and fail.

Another point to ponder. I would be suspicious of why a dealer wouldn't have the proper tools to resurface the ports on the old compressor instead of replacing the whole compressor.

[GTOSVX]

point well taken on the resurface of the connection surface and doubt of capability.
When they did the R134 conversion, they told me they had evacuated, drained oil, refilled with new oil, and charged the R134. invoice bears this out.
sytem is not currently leaking (which was the last round of issues) still has full charge, but compressor clutch momentarily engages. I do have a code 14, evaporator sensor open. this has been the case for as long as I have known how to check the codes. I had the sensor replaced, but it did not fix the code. I changed the amplifier (control unit for ac system) and it did not change the code. do not know if this is the problem or not. The compressor has run before with the code present.

[mohrds]

Originally Posted by GTOSVX

point well taken on the resurface of the connection surface and doubt of capability.
When they did the R134 conversion, they told me they had evacuated, drained oil, refilled with new oil, and charged the R134. invoice bears this out.
sytem is not currently leaking (which was the last round of issues) still has full charge, but compressor clutch momentarily engages. I do have a code 14, evaporator sensor open. this has been the case for as long as I have known how to check the codes. I had the sensor replaced, but it did not fix the code. I changed the amplifier (control unit for ac system) and it did not change the code. do not know if this is the problem or not. The compressor has run before with the code present.

It sounds like you know as much as the dealer about AC systems.

have you checked the pressure / vacuum throughout the system? You might have a clogged orifice tube which will trip the high pressure cutoff. Or pehaps the system is overfilled? Generally you need about 80% of R134a than the original R12. If the system is overfilled, it will cutoff after the compressor engages.

An overfilled system will usually run fine with the hood open while they test it, but as soon as you idle at a stop sign, the AC cuts out.

[dannmarr]

The evaporator sensor measures the inside temperature and signals the climate control to turn on the compressor or shut it down. If it shuts off, you might have an obstruction or non condensables in the system. This would not allow enough refrigerant to move through the coil. Now when the blower is blowing air across the coil without enough refrigerant in it, it won't take long for the coil to freeze. This maybe the reason it shuts down, it's a safety device to protect the compressor.

[Beav]

Why in the world would the evap sensor measure outside temperature? Wouldn't it be a lot easier to measure outside temp, well, outside?

Evap temp sensors shut down the compressor before the evap freezes over.

Excessive R134 cools about as well as too little R134. Start with 75% of recommended R12 charge and don't exceed 85%. R134 charge is hypercritical, you can't get away with guessing like you can with R12.

Flushing almost never cures the effects of black death on OEM R134 condensors. If you find black death has occured on a R134 car plan on compressor, drier, condensor, o-tube and add on a couple filters after flushing. Hopefully you won't need to change the evap also. What has happened is that R134 requires a much larger/high exchange rate condensor. Since there is usually only so much space available they make the condensor with more, but much smaller, tubes. They tend to plug much easier than the old R12 types. Sometimes the same space restrictions apply to the evap, but usually the o-tube will catch the garbage before it has a chance to plug up.

[dannmarr]

My mistake, I rewrote my reply. Thanks Beav.

[Beav]

Keep working on it. Evap sensors are strictly for cold sensing. It is a probe-type thermistor that signals too-low temps to the controller which will turn off the comp when the evap coil is approaching freeze-up. Absolutely nothing to do with refrigerant obstruction or whatever non-condensables may be. Too low refrigerant certainly won't cause the coil to freeze...

O.K., A/C primer: Most of us have experienced using spray paint and the effect of prolonged spraying causing the can to chill. It's simple physics. High pressure escaping through a small orifice into a low pressure area chills. Why my fanny doesn't freeze after eating Mexican food is beyond me. Same goes for everything shrinks when it gets colder - except ice - it expands. I guess that proves there's always exceptions..

Anyway, back to a/c. So, the compressor compresses low pressure gas into high pressure/high temperature gas (pressurization causes friction and friction causes heat, remember?) and sends it forward, to the condensor. The refrigerant then condenses (duhhhhhh) into a high pressure liquid as the heat it contains is removed by the lower temperature ambient air rushing over the condensor coils/tubes. From there the refrigerant is collected in the receiver/drier where it is filtered and moisture is removed. Onwards to the evaporator the refrigerant first passes through a throttling device, whether it be an orifice tube of fixed size or an expansion valve that has the ability to throttle by pintle and seat. From there it enters the evaporator, expanding and chilling. Back to physics: heat is ever present. Cold is the absence of heat. The expanding refrigerant absorbs the heat and leaves the evaporator.

Now, before it reaches the evaporator, the refrigerant has options for temp control. And I don't mean for cabin temp control, but rather for evaporator temp control - they are definitely two separate issues. You see, the refrigeration cycle continues forever without control, but just like Granny's claw-footed, Iron Man GE 'fridge, it ain't worth a dang if it freezes-up. (Good Lord, I'm getting old...) The ice is cold but nothing else is. Old cars used several methods: 1) they froze up. You'd get p.o.'d, shut it off and it would work again in about 15-20 minutes (this was a long time ago and only Ron, Larry and a few others might recall trunk-mounted, electric a/c with plexiglass tubes that passed the chilled air through the monkey seat into the car. ('monkey seat' is the shelf behind the back seat and below the rear glass - the smallest kid in the family would sleep there at drive-in movies while Mom and Dad acted like there was nothing going on in the front seat)

Next came making the system small enough that it wouldn't freeze, but it also wouldn't keep up with July and four screaming kids and a post-partem Momma on the way to the fairgrounds for fireworks.

GM finally came up with the STV, or Suction Throttling Valve. Someone finally convinced a bean-counter that adding a valve on the exit side of the evap would keep the evap pressure high enough to not allow freezing. You see, R12 pressure yields about 1°F/1lb. pressure on the low side. So if you keep the low side above, say, 26# pressure and the fan moving 'x' amount of air across an evaporator of 'y' size the evap won't freeze up. Later GM changed from the STV (easily identified by a vacuum hose to the engine) to the POA or Pilot Operated Absolute (there's a 15¢ acronym) valve. It did the same thing but saved them the cost of a vacuum hose. Four more lobster lunches per year for the schmuck at corporate that approved that innovation!

Now that covered GM - what did the rest of the world do? (I promise, there's a reason to reading all of this) Well, there still needed to be a throttling device before the evap to change the high pressure liquid into low pressure gas. Long before there were orifice tubes (or 'o-tube') there was the TXV or 'thermal expansion valve'. A valve that had a pintle and seat operated by a thermal bulb which was placed in the fins of the evap coil. The evap got cold, the mercury in the bulb reduced, the diaphragm released a bit (about .007") and the refrigerant flow reduced - decreasing the cooling power - simple enough. Why GM felt it need a suction throttling device is beyond me. It was a PITA at times to determine which valve was defective so most customers ended up replacing both. pssst... I wasn't the only one that subscribed to this program. It was an industry-wide issue. Actually, it boiled down to a push - take a chance and probably blow-off a fresh charge of R12 to replace the other valve or just include the price of both to begin with.

In the early '70s, GM (who else?) designed/introduced the V.I.R. system - Valves In Receiver. Basically the POA, TXV and receiver/drier were all in one unit. Each portion was available separately - yeah, right. I'll wager they sold 50 more complete units to any single component. Beginning of the end for auto repair techs actually 'rebuilding / repairing' cars. Also around this time was the introduction of the 'thermal limiter' by.... who can guess this one?....GM.
Too high pressure = too high temperature = baked A6 compressor. Basically a 'superheat' switch was added to the rear of the compressor which would short to ground and blow a fuse - thermal limiter - and protect the compressor. Phhhbbbllttttt! Most were wired past until word got around that you could rape a customer that had three screaming brats and a post-partem wife on vacation in July. (just had to throw you conspiracy theorists a bone) Add another $12.95 if a menopausal mother-in-law was also present.

Next came orifice tubes. A fixed diameter restriction in the high pressure line to the evap. Cheap. Twelve more lobsters per year in the executive dining room. Plus a golden parachute for the big boss and golden handcuffs for the putz at Frigidaire that designed it. O.K., how do you keep it from freezing up? Simple - evap temp sensor OOOORRRRR a low pressure sensor on the evap exit tube (remember - 1# pressure = 1°). Low pressure/temperature = evap freezing = compressor 'OFF'.


Now that you have a grasp of the refrigeration side, you should understand that anything else is merely to control the (drum roll please) air temperature side. First, the heater control valve was left in place to shut off the hot water but now it's commonplace to abandon that (lobster for lunch, anyone?) and simply use a blend or 'stratification' (add one more place to the lunch table) door to mellow da chill. Refrigeration pressure/temp switches, no matter what their name is, still serve the same basic purposes - keep the evap from freezing or keep the compressor from grenading due to too much/little refrigerant.

[dannmarr]

I work on building A/C systems and I was taught that freeze ups are caused most of the time by a low refrigerant charge. Dirty filters, dirty evaporator coil, defective blower motor can also cause freeze ups. Freezing up can also be caused by air and moisture (non condensables) in the system that allows ice to form. I have taken these words straight out of my notes while I was trained on A/C systems in Oklahoma. If auto A/C are different from building A/C, then I apologize for any errors I have made.

[dannmarr]

GTOSVX,
To confirm my original reply regarding your problem, here is a quote I found in my study books an A/C; "On many systems, the compressor won’t turn on if the refrigerant is low because the "low pressure safety switch" prevents the compressor clutch from engaging if system pressure is low. This protects the compressor from possible damage caused by a lack of lubrication."

Beav,
Here is a quote to back up my reply about low refrigerant; ''If the coil in front freezes up, it is likely low on refrigerant." Regarding obstructions; "System suction pressures can fall to a level such that coil freeze up can occur.''

Also, evaporator sensors also regulate the operation of the compressor.

I hope this helps

[Beav]

Geez, I apologize if I said something that made you feel like you were being attacked. Just trying to pass along some info. No reason to apologize to me, your intentions were proper. I just think there is some confusion between your HVAC experience/training and the way auto a/c operates. HVAC air handling depends on a lot of planning and engineering and size is of little importance, so the refrigeration end is pretty archaic. Auto applications need super-high efficiency refrigeration with simple air handling.

There is a big difference between the low pressure cut-out switch (included in the trinary switch of the SVX) and an evaporator sensor. The evap sensor isn't there to protect the compressor but rather to cycle it before the evap freezes over. The trinary switch will protect the compressor from high or low pressure damage, however it isn't part of the climate (read:temperature) control system, per se.

Not that I have any training in regards to HVAC in buildings but I'll gamble a guess that the reason for low pressure freeze up is a result of the low side not having adequate controls. The engineering of air flow would keep the low side above 26psi or so and prevent evap freeze-up. Lower pressure could possibly equal lower temperature and freezing. My minuscule experience with HVAC evap freezing reflects your statement: it has been the result of insufficient airflow over the coil, normally a result of clogged or non-existant filters, collapsed flex ducting, closed registers or just plain ol' crappy engineering.

Now, on my side of the fence, non-condensables is a term that we don't use. Nevertheless, the presence of air or moisture inside the refrigeration system inhibits cooling and will never allow freeze-up to occur, in fact you'd be lucky to get a 20° drop with any appreciable amount of either in the system.

I don't recall obstructions being mentioned but I'll agree - anytime an obsruction exists in the high side there will be cooling immediately past the obstruction. In effect the obstruction acts as an additional orifice tube or expansion valve.

[dannmarr]

I'm always willing to learn something new, Thanks Beav.

[Trevor]

Originally Posted by dannmarr

I'm always willing to learn something new, Thanks Beav.

Agreed and written well enough to be included in a text book.