1987 converted to R134, Fl, 100 ºF days.

Ok...I'm at wits end.

I put in a VOV ( variable orifice valve ) to help low speed cooling, it works but not as well as I would like. I had to overcharge it to keep high rpm pressure from going too low and freezing the evap. I think I want to switch to a thermal expansion valve setup instead of an orifice.

I read where someone here had mentioned they did that...please tell me about it...PERTY PLEEEEEZ!

:(

 

Check out http://www.aircondition.com . Go to the bulletin boards. There seem to be several different refrigerants recommended over 134a. AutoFrost as a r12 replacement and Cooltop as a 134a replacment.

Sorry I can't answer the question about the orifice.

There server sems to be down at the moment.

 

i converted my '92, 9 mos ago, i replaced, weight wise, the amount of R12 with 134A and it works as well as the R12.
we hit 99 degrees on a regular basis here in south florida.
i really cant tell the difference.
actually if you look at a temperature chart, the 134A gets colder at the same pressure then the R12.
regards
doug :jester

 

I really want to try the thermal expansion valve. The way it is now, at idle ( 600 RPM ) in 100ºF weather, my vent temp is at best 68º, and driving at 2500 RPM, vent temp is 44º.

That is not unliveable...but I just want a temperature based valve so my idle temp is lower without having to worry about freezing the evap driving down the highway... inside temp/duct temp gets too hot when the evap freezes over.

I think the design of VOV is fatally flawed in two aspects:

1. At max flow ( about 2500-3000 engine RPM), it restricts freon more than at partial flow and at low rpm's, it does not provide enough restriction to develop a lower low side pressure. I tried re-machining the valve and got a little better result from it, but not so much different than teh standard orifice tube.

2. It is not externally referenced for pressure OR temperature...so it really depends on accurate orifice valve spring tension/linearity in the particular environment it is operating in. Again, it seems to be on par with ( it might be a little better..but not much more than ) the standard orifice tube.

I really think a TEV is THE way to go...since some designs are both actual pressure and temperature referenced. :yesnod:

 

Consider this - at idle, airflow over the condenser is at the minimum, limited to what the fan can pull through the radiator and condenser. Any obstruction between the two (birds nests, cig butts) hurts the airflow, and the seal along side the two probably isn't all that great anymore either. A poor seal lets the fan suck air around the condenser rather than through it. No airflow = no condensing, well, a lot less anyway.

Being too lazy to take my own advice, I wired the pusher fan to come on with the main fan. Good condenser airflow guaranteed. Outlet temp dropped from 59 to 50, although it creeps up to mid 50's at a long light (no VOV). Only drawback - it sounds like a hovercraft when the fans kick on.

 

Already did that.

 

Something is wrong here. I run R134a with the Smart VOV in my 85 and I have no problems at all. See here: http://forums.corvetteforum.com/zerothread?id=93384

I get high 40 degrees vent temps when idling during a hot & humid 95 degree day. I get lower temps when moving. I have a feeling that you may have foreign material (maybe water) in the system. You may not have flushed and/or evaporated correctly. You may have introduced air into the system.

You evap will not freeze over if your low pressure cut-off switch is working correctly. When running R134a the switch should be set to cycle the compressor when the pressure drops to 22.5 degrees. Overcharging the system is a bad move. Running with too much freon or too much oil will reduce the efficiency of your system. Adding more freon increases the pressure, which increases the temp of the freon.

I think you will need to evacuate and recharge the system. Make sure that you are not running with too much oil.

 

My problem was the VOV.

I replaced it with a standard orifice and it gives me about 54 degrees idle and about 48 degrees running.

They must not have good quality control on those things. Even after I modified it, same thing. I guess I knew something wasnt right when I ordered it, tehy returned confirmation of the wrong part, then I called them, they said they would correct the order and send the right one, then the one I actually recieved was even a different one than the order or confirmation.

I still want to try the thermal expansion valve.

BTW: When the low pressure cycling switch kicks in, it is already well below the freezing temperature. That means all the humidity in the air freezes to water and stops air flow...therefore thevent temp goes way up. You then have to turn off the AC until the core unfreezes before it will get cold again...so letting it cycle on the low temp switch has no purpose other than saving the compressor from burn up.

 

>They must not have good quality control on those things.

Mine works fine.


>Even after I modified it

:confused:


>I ordered it, tehy returned confirmation of the wrong
>part, then I called them, they said they would correct
>the order and send the right one

:confused:

There is only one orifice that fits the evaporator mounted C4. I do not know why you are having problems getting the right part.

>When the low pressure cycling switch kicks in, it is already well below the freezing temperature.

Not really.

>That means all the humidity in the air freezes to water and stops air flow

The warm car interior and warm under hood temps prevent freezing if the low-side switch is adjusted correctly.


>the low temp switch has no purpose other than saving the compressor from burn up.

Incorrect. The low-side switch regulates refrigerant flow and prevents evaporator freezing. You may need a refresher course in CCOT theory.
If your C4 system is operating correctly R134a with the VOV should net mid to high 40 degree temps while idling in 95 degree weather. You should also see low to mid 40 degree temps while moving in 95 degree weather.

 

Excuse me....but it looks like you are trying to pick a fight here.

Point for point?

Well...good for you.

There are two, one a severe duty one standard duty. I originally ordered the severe duty evap mounted VOV, they sent a confirmation of the standard duty condensor mount orifice. I called them, they said they would correct it and still they messed up and sent me the standard duty evap mount VOV.

Want proof? http://www.acsource.com/cgi-bin/Soft...6224+998562854

Actually...22.5PSI gives 22.5ºF with R-12, and even colder with R134. Thats definitely below freezing temperature of water at atmospheric pressure.
It is a wet to vapor phase change system, commonly referred to as a saturated system.

Do you know what that means? It means temp/pressure are directly proportional inside the evaporator...it's how AC works in the first place. If the pressure of the evaporator goes down, so does temperature.

Not in Florida...where humidity is darn near 100%. When you run low on freon, the damn thing gets real cold until the evap goes below freezing temperature, then the water vapor in the air solidifies and blocks air flow through the evaporator...at that point you get ice chunks flying out of the vent and eventually all heat transfer stops and no cold air comes out of the vent system at all.

You almost know enough to be dangerous. This car does not use the CCOT system.

The low pressure switch in this car is designed to turn off the compressor if you loose freon pressure by leakage or blockage. That is the reason it's cycle off point is at a pressure that puts the temperature well below freezing.

(edit...being nice now... ) :blueangel:


[Modified by BBA, 10:23 PM 8/23/2001]

 

Now...to my ORIGINAL topic...

 

As a matter of fact...I did a little readup right now.

The GM CCOT system uses a cycling clutch switch that has a capillary tube going into the low pressure side of the evap line or directly to the evaporator. This is adjustable and cycles the compressor to maintain relatively constant low side pressure/evap temp.

The GM Low Pressure Cutoff Switch, which the 87 corvette uses, is there to protect the compressor in the event of a low charge condition. The 87 corvette does not have a cycling clutch switch. Therefore it is NOT a CCOT system.

That is straight out of the GM manual. If you think the low press cutoff switch itself constitutes a CCOT system...you are conceptually mistaken.


[Modified by BBA, 11:21 PM 8/22/2001]

 

BBA,
Have you checked the evaparator to make sure it is clean? In residental cooling systems this will cause freeze ups from lack of air flow. Also one word of caution about over charging the system, if you dont boil off the liquid freon in the coil and bring back liquid to the compressor this will kill the compressor over time.
Good Luck
Richard

 

Yes, That was done a few weeks ago.
I pulled the evaporator completely out of the car and cleaned the heck out of it, made a big difference. Now instead of just feeling a little air on my face, it blows my hair up with force.

 

>but it looks like you are trying to pick a fight here.

I do not have enough free time to fight with you. This will be my last post on this subject for sure.

>There are two, one a severe duty one standard duty.

I am aware of that. When I said that 1 part fit I meant that there was only 1 physical design that fits. You are correct about the severe and standard duty. I am running th standard duty and I got mid to low 40 degree vent temps during the recent North East heat wave.


>Actually...22.5PSI gives 22.5ºF with R-12, and even colder with R134.

First of all that is not what I said to run, secondly you are dead wrong. The stock R12 switch switches off at 25 PSI (not 22.5 PSI), which is about 25 degrees F. R134a is WARMER at this pressure than R12, not cooler as you wrote above. I said that if you convert to R134a you should adjust the low-side pressure switch to cycle the compressor at 22.5 PSI. That will cause the compressor to cycle at the same 25 degrees F on the low-side. At low pressure R134a is warmer than R12. Your evaporator is not going to freeze over when the low-side is at 25 degrees F. If it did then every car on the road would be running with a frozen evap. :lol:

You need to read this: http://www.aircondition.com/pressure.htm

>You almost know enough to be dangerous. This car
>does not use the CCOT system.
>How dare you talk total BS like that!

Ohh please....

My 85 most definitely is a CCOT system. I've never worked on an 87 so I have to assume it is also a CCOT. Since you are so smart I pose this question:

If the 87 is not a CCOT then what is it? It uses a fixed orifice tube and a low-side switch to control freon flow by cycling the compressor clutch. Sounds like CCOT to me.

:crazy:

You need to read this stuff: http://www.delanet.com/~pparish/ac_works.htm http://www.forparts.com/techacglossary.htm (Look for CCOT definition) http://www.sunairusa.com/Term.htm http://members.tripod.com/acguy2/the...on.htm#Orifice Tube

You should probably bring this car into an AC shop.

 

( edit...I was not being nice )

I won't even ask what you mean by the phrase "you should adjust the low-side pressure switch to cycle the compressor at 22.5 PSI" when it DOES NOT EVEN HAVE AN ADJUSTABLE switch.




[Modified by BBA, 10:20 PM 8/23/2001]

 

I guess I can spare a few more seconds.

>you seem to be a "I know it all your stupid" person

I am just stating how a C4 CCOT system works. Sorry if my abruptness offends you. I guess I do not have enough time to write politcally correct postings. I just get right to the point and post the technical facts.

>I won't even ask what you mean by the phrase "you should adjust
>the low-side pressure switch to cycle the
>compressor at 22.5 PSI" when it DOES NOT EVEN HAVE AN ADJUSTABLE switch.

85-91 C4 Vettes all have the same low-side switch and it is adjustable. Remove the 2-wire connector from the pressure switch after the evap in the low pressure vapor line. You will see the adjusting screw. The stock R12 setting is 25 PSI, which is 25 degrees F. The evap will not freeze over at 25 degrees F. R134a is slightly warmer than R12 at the stock 25 PSI low side cycling pressure. To get the same 25 degrees F with R134a you need to go down to 22.5 PSI. 23PSI should also be ok.

--rb--

:blueangel:

 

Hmmm...looks like I should be apologizing to you. I did find the adjustment screw.

I am impressed with your positive attitude in responding to my unkind remarks. Thanks :cheers:

I wonder if I sshould put the VOV back in now and play with it. Right now it only gets about 62º at idle ( 600 RPM ) with fan on high on 95-100º days, and goes down to 45-48 at speed. ( I am measuring with a K-thermocouple inside the vent. )

Or, I could just lower the system pressure until idle temp comes down some, then adjust the low pressure switch to keep it from freezing.

What I was doing with the VOV was trying to find the right pressure spring to let the valve regulate low side pressure, to keep the compressor from cycling at all, by maintaining a more consistant pressure. I machined the valve orifice to close a little tighter to drop low side pressure better at idle, and loosened the spring to get it to actually regulate, but I could not find a lighter spring small enough to fit inside the valve tube. The best result I got was idle pressure was dead stable giving a 45º temp, but any high speed driving froze the core solid because the pressure switch was not cutting out at high enough pressure to prevent freezing.

It really sucks being the kind of person who always has to make things work better than they were designed to :D


[Modified by BBA, 10:30 PM 8/23/2001]

 

You are gonna have to excuse my abruptness with this post. I am going to try and post as much help and as many facts as possible before my wife kindly asks me to get off the computer.

>I wonder if I sshould put the VOV back in now and play with it. Right now
>it only gets about 62º at idle ( 600 RPM ) with fan on high on 95-100º days,
>and goes down to 45-48 at speed.

I am not sure if you should use the VOV since you said you made changes to it. Your cruising temp seems good, but the idle temps are way off. You should be able to beat 62 degrees with a new & clean stock orifice. If you are only getting 62 degrees then you could have the following problems:
*Low-side pressure is too high (and hot)
*Bad freon or oil mix
*Damaged evaporator
*Blower/heat exchanger ducts are not working correctly.
*Other things that may be slipping my mind right now

You need to run some tests and list your results before any advice can be given. As a start try the following with the stock orifice:

* Warm car up, open doors and windows, connect a manifold gauge set to both the high and low sides, put the AC on max with the blower on max. Let the car idle during the tests.

* What pressures do you see on the high & low sides? Are the pressures steady or do they fluctuate. If they fluctuate describe in great detail how they fluctuate (speed of fluctuation, range of pressure, etc..)

* Does the compressor cycle or does it run continuously? If the compressor is cycling then describe in great detail what happens to the low-side pressure as the compressor cycles. How long does the compressor run before it cycles off? If the compressor is cycling does the low-side pressure drop immediately when the compressor engages? Also describe the high-side pressures. The high side can cycle the compressor if the pressure gets too high.

* If you run the AC with the settings I stated above you should have fairly steady pressure on the low side, with the pressure in the range of high 20s to mid 30s. The pressure may drop a bit over time and cause some slight cycling, but the compressor should be running most of the time. My R134a equipped 1985 AC system does not cycle when running these tests. My low side stays rock steady at about 32 PSI. The high side is also steady at about 210 PSI. Pressures change if I start to rev the motor or I turn down the blower motor. If you lower the blower speed the pressure should drop a bit because the interior heat will not be heating up the freon & raising the low-side pressure.

Your pressures will tell us a lot about what is going on.

:seeya

 

I'll do that if today is hot enough...yesterday was so cool it barely worked the AC at all. I also have to get a fitting to connect my R12 gauges to the R134 hi side port, since I changed the fittings.

One question is where do you live? North or South? The reason I ask is the temp can be the same but humidity makes an extreme difference on AC performance, and it is always near 90-100% humidity here on the dog days.


[Modified by BBA, 11:56 AM 8/24/2001]