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I need help with the ac system for my 2002 blazer

Reefgeorge on Sat February 05, 2011 9:00 PM User is offline

Year: 2002
Make: Chevy
Model: Blazer 2wd
Engine Size: 4.3l
Refrigerant Type: 134a
Ambient Temp: 75f
Pressure Low: 13
Pressure High: 135
Country of Origin: United States

I need some serious help. I am an epa certified tech with no experience on a CCOP system.

Is this a CCOP system and is it supposed to cycle even at the target charge levels and the target system evaluation levels of 80F ambient, 1500 rpm, max blower, etc. Or only when the system limits are approached such as low ambient, cold cabin, low charge, etc.? If its supposed to cycle, how do you measure low side pressure when it is constantly rising and falling?

I have been told that the target low side pressure is 28-32 psi and 37-42 psi. Which correct? Also confirm high side psi and vent temp.

Can this system be charged by measurements of pressures, temps, etc or can it only be done with the measured charge of 28 oz of 134a?



Dougflas on Sun February 06, 2011 3:54 AM User is offline

It's best to measure the charge by weight. At the ambient you have (75º), the system will cycle 2-3 times per minute. Leave the doors/windows open when checking.

Reefgeorge on Sun February 06, 2011 7:16 AM User is offline

How do you measure the low side pressure with the value ramping up and down constantly?

mk378 on Sun February 06, 2011 3:46 PM User is offline

Bottom line is the only way to know the charge is right is to charge it by weight.

The quoted pressures are more for stress test conditions, hot weather with the interior fan on high. In cooler weather it's going to cycle. The range of pressures achieved while cycling will be determined only by the switching points of the switch. Even if you measure them exactly it isn't going to tell you anything about the charge level or anything else.

So again, especially when you can't test in hot weather, the only way to be sure of the right charge is to take all the refrigerant out and weigh in the amount you put back.

Reefgeorge on Sun February 06, 2011 4:37 PM User is offline

Got it. Evacuate, vacuum, and weigh the charge in.

I understand the concept of the system running always on with no cycling in extreme hot conditions. Ambient is high so pressures are high. Latent and sensible load are high so the evaporator is providing maximum load and low side pressure should remain above the 22 psi low side limit of the cycling switch. I also understand that at the opposite end where the low side switch will trip as the evaporator heads towards freezing with reduced system load.

Are the conditions stated for testing - 1500 RPM, blower on high, high initial cabin load, 80F ambient what is considered "hot" and therefore the compressor should not cycle or are these ideal test conditions such that the compressor will still cycle?



NickD on Mon February 07, 2011 9:38 AM User is offline

Could be CCOT for Cycling Clutch Orifice Tube system, not exactly sure what CCOP stands for. But also here to learn.

If we are dealing with a CCOT system, no doubt, charging by specified weight is the easiest, fastest, and safest way to do it, but with production tolerances, may not be optimum for your particular system.

Charging by pressures is best done on a 85*F day also noting the relative humidity that equally plays a role and trying to get a hold of a PT chart for your particular vehicle. GM isn't particularly great on this. But some manuals will show a range of acceptable pressures, shoot for the mean value. This is done at 1,500 rpm, AC on, blower at maximum, doors open. But there is a tendency to undercharge at this stage, you would like to be closer to that 30 psi range for far superior cooling, but will pay the price in excessive cycling at lower humidities and ambient temperature. At 95*F would have to be around 50 psi. Just works the opposite with CCOT systems, the more you need cooling with hotter ambients and humidity, the less cooling you will get. Put briefly, CCOT systems suck.

Even normal cycling puts a severe strain on the system, belt drive train, all the components within that drive train, the clutch, and the compressor as it is expected to accelerate from zero to engine rpm in practically an instant. Look at the reliability for 1/4 mile dragsters to get a hint. Excessive cycling expedites this wear and tear at an exponential rate. This was started by GM to get rid of that relativity expensive POA valve and TXV. Hell, working for GM even 3/4 of a cent is too damned expensive. A couple of bucks is like millions to them, actually it is considering the volume, but the consumer never sees these price breaks, but the stockholders do.

Testing for cycling is an entirely different procedure, here looking more like a 70*F day, vehicle in the shade, engine at idle, AC on, but blower at minimum and primarily watching the action of the cycling switch, compressor should kill at around 24 psi, and start up again once that pressure climbs to around 42 psi. What is equally important is the time delay, should be around 45 seconds, if less, system has internal leaks, typically the reed valves are not sealing properly. Here, there is a tendency to overcharge to reduce the cycling rate, the greater the charge, the longer between cycles. But doing this will certainly blow the system at higher ambients.

So are you getting the picture by charging by weight?

Variable displacement compressors were a great, but inexpensive solution to cycling, run all the time like the old POA systems provided the high side pressure is in range. These are extremely difficult to charge by pressures, as they are all over the place. But again experience helps a lot to save recovery and recharging by weight. Many factors involved and don't care to get into all of it.

If your pressures are jumping all over the place at a constant engine rpm, you have other problems to address.

Reefgeorge on Mon February 07, 2011 10:37 AM User is offline


Everything that you stated makes sense and is at the crux of my issue. Leak test, evacuate, vacuum, weighed charge is still a great starting point but we still need to verify functionality on occassion and you don't want to dump the charge every time there may be a problem. They publish many pages of target operational specs and diagnostic flow charts for this reason.

Some sources have said that this system is a clutch cycling system (CCOP I guess) and some sources have said that it is not suppose to cycle, under most condiitons. How do I figure out which one is correct? I have opinions and documents that say both - many of which are supposed to be originally sourced from GM. It has a strong bearing on verifying correct functionality. One of the target low pressure specs that I read was 28-32 psi, lets call it 30 psi which was your number and a common number in automotive AC charging. Well, my current low pressure with the compressor forced on with a jumpered low side switch is 15 PSI. If this is a non-cycling system, I am grossly under charged (high side is 135, 75 ambient). Obviously I am cycling continuously because after the compressor comes on I will always be heading below the 22 psi switch limit. Some have suggested that the way you measure low pressure in a cycling system is to take the average of the ramp which is 22 - 52 psi in my case, or an average of 37 psi. That happens to be the target of another source that I read but is too high with a 30 psi assumption. In addition it looks like my low side switch may be out of bounds on the high limit but I have read both 42 psi (as you have suggested) and 53 psi, which I have now and supports a 37 psi target. At this point I have no idea where I am because just taking the average of the range will give the same answer over a wide range of charge levels, all the way down to a system so low in charge that you are just above 42/53 psi at static for start up. No system is at the proper charge if static is below the ambient temp/psi chart for 134a. If this is a cycling stystem then an "always on" pressure of below 22 is actually necessary for normal functionality. If this is supposed to be a non-cycling system then I am grossly undercharged and will kill my compressor. In this case I would need to charge at least until I get above 22 psi low side and then watch the compressor run constantly at the target test conditions. Obviously you look at other conditions to make sure that you don't see anything else wrong (freezing lines, high head pressure, noises, vent temps, etc.) because low side pressure is not the only measure of correct functionality.

Also I agree, a system that is supposed to cycle continuosly for 200,000 miles at mostly 2,000+ RPM engine rpm, with a higher compressor rpm is destined to wear out fast. I understan why. You get a cheaper system with less moving parts that hoveres just at evaporator freezing over a wide range of conditions. That said, this compressor is starting to sound bad at 170,000 miles but still cooling, until recently.

NickD on Mon February 07, 2011 6:54 PM User is offline

Really don't brag about my 609 certification, didn't have to know a single thing about MVAC to pass that kindergarten type test. But needed it so I could buy a can of R-134a in Wisconsin. Yes you need a 609 certification to buy a can of R-134a in Wisconsin.

For one thing,

Ambient Temp: 75f
Pressure Low: 13
Pressure High: 135

Your pressures are way too low explaining why you have to bypass that cycling switch just to get it running. Don't run it very long in that state, low pressures like that inhibit oil circulation. Tim has a book on MVAC, suggest you buy and read it to get up to par. We can answer questions on this subject, but really can't write a book, although Tim and others have accused me of that.

Reefgeorge on Fri February 11, 2011 3:01 PM User is offline

I picked up more information from an AC tech that worked for a GM dealership and has worked on thousands of GM systems over a long time period.

GM has at least 3 different types of AC systems. One of them is a CCOT system but it is used in a small number of luxury models such as Cadillacs. The 2002 Blazer can have 2 of them (neither CCOP). You cannot describe the function of my system unles you first ask if I have manual controls or digital controls. I have manual.

System 1, manual controls:

There is a high pressure and a low pressure pressure switch in the refrigerent circuit. High pressure for severely out of bounds condiditions for protection. Low pressure to protect at the limits of operation (cold ambient, low cabin load, etc.), or for out of bounds conditions (very low on 134a, etc.). This system regulates cabin temp by using a blend door to mix cold from an all out compressor/evaporator with heat. There are no thermistors and the compressor is not designed cycle under most conditions. At the target conditions for testing (1500 rpm, max blower, high cabin load), the compressor does not cycle and you can get steady state readings.

System 2, digital controls, no CCOP (found as an option on the 2002 Blazer)

Besides the pressures switches there is temperature sensing and a digital control head. This system also will not cycle at high intial loads but rather will run all out until you approach the target cabin temp and then will cycle the compressor all on and all off with a fairly wide range (+/- 8F). The evap coil still heads towards near freezing while on.

System 3, CCOP - not found in the Blazer:

System 2 plus additional temp sensors, line temp sensors for the refrigerant, and a more elaborate cycling algorithm. This system will cycle often because it attempts to control the cabin temp to within +/- 2F and cycles the compressor in such a way that as you approach the cabin target, the evap temp is much closer to the cabin temp than a normal system will produce. If say you are near your cabin temp target of 75 F, the evaporator may be held to 65 F through cycling to provide better cabin comfort.

bohica2xo on Fri February 11, 2011 4:08 PM User is offline

I prefer the CCCP system.

In Soviet Russia refrigerant charges you!

"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

iceman2555 on Sat February 12, 2011 1:12 AM User is offlineView users profile

Good gracious, why is it so damn hard to understand that the only true method to diagnosis a system for operational conditions is to insure that the system is fully charged. Get the refrigerant out....evac the system to completely de gas the system...recharge the system utilizing the correct equipment and then test the damn thing.
The reason the compressor is beginning to be noisy is that the system is undercharged and the flow of lubricant to the compressor is below the system requirements. It is seriously doubtful that the system has ever been properly serviced. A undercharge of several ounces in this system may result in a undercharge condition and this will result in a loss of lubricant flow.

ATC AC system use a blend of heat from the heater core to maintain a given cabin temperature not a cycling of the compressor. Spent several hours looking over the my ATC system today attempting to locate all these inline sensors....heck...GM really screwed me....could not find one of the darn things....did find this little switch on the accumulator.....wonder what it was for.....ah heck.....maybe I can find those sensors tomorrow.

Q]Everything that you stated makes sense and is at the crux of my issue. Leak test, evacuate, vacuum, weighed charge is still a great starting point but we still need to verify functionality on occasion and you don't want to dump the charge every time there may be a problem. They publish many pages of target operational specs and diagnostic flow charts for this reason.

No.... one does not want to 'dump' the charge every time there is a problem....that is why the EPA requires all licensed AC shops to purchase a machine called a recover/recharge machine to prevent us from 'dumping' the charge. And yes, there are dozens of operational specifications and diagnosis charts available...but the correct diagnosis procedure requires first and foremost that the system must be completely charged. If this means 'dumping' and starting over...this is what it takes. Everything else is merely an ASSUMPTION.
Why make this more complicated...this is a very straight forward evaluation...not rocket science....charge it correctly and evaluate the system.

Oh, yes...what is a CCOP system?

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

NickD on Sat February 12, 2011 7:02 AM User is offline

We can really get into the differences in MVAC systems and also toss in a bit of ideology and misinformation. Differences in compressors are either the vane or piston type, fixed or variable displacement, or whether the compressor has an oil sump in it for continuous lubrication or relies strictly on refrigerant flow for proper lubrication.

Refrigerant flow is either of the fixed orifice type or of the expansion valve type, whereas the former works extremely well with a variable displacement type of compressor, but the later is somewhat of a compromise to control refrigerant flow in an attempt to reduce cycling. But are also systems that use a pressure regulator such as a POA valve before the fixed orifice or preferably the expansion valve type that maintains an almost identical low side pressure over the entire operating range of the MVAC system.

Automatic temperature control as opposed to a manual control in it simplest form, controls both the blower speed or compressor cycling, or even a blend door to maintain a preset temperature control by employing thermistor. But can be done by a thoughtful operator with minimal adjustment. But are adaptable to any system configuration in terms of climate control.

High pressure cut off switches in practice should never cut off, more of a safety device, prior to their application, a high pressure safety refrigerant release valve was used, but a one time affair. Once you blew your refrigerant, your AC system was dead. Most typical causes would either be a debris ladened condenser or a broken fan belt with a side effect of an overheating engine. Adept drivers would instantly turn off the AC system, but the vast majority of inept. EPA was not really concerned about your compressor or engine seizing, but the release of refrigerant into the atmosphere. Thus use of the high pressure switch became mandatory.

Low pressure cutoff switch is far more known to those that know AC systems as a cycling switch where this switch serves both functions. But some systems incorporate a true low low pressure cut off switch in conjunction with the cycling switch or even alone, in TXV or variable displacement systems. Primarily to prevent compressor engagement at low ambient temperatures.

So you do run into a lot of variables in MVAC systems. One thing they all share in common is inoperativeness if the refrigerant is lost. So is the first thing to check. Charging by pressures or other means, requires a great deal of skill, practically all OE's recommend by charging by weight.

Since we are dealing with GM, was extremely difficult to charge by pressures with the then POA system as pressures would vary all over the place depending on AC load and ambient conditions. That is when they came out with the charging system. Would draw out all the refrigerant in the system into a viewable container, you would add the required amount, and put it back in. Since port connections were not tampered with, wasn't a need to be concerned about introducing air into the system. But any lost of refrigerant is caused by a leak, finding those is a different subject.

mickel on Fri March 04, 2011 6:10 AM User is offlineView users profile

I am experienced with residential and boat HVAC systems but have never worked on an automotive cycling system so I need some help setting the charge level. I know that the fool proof approach is to identify and fix any leaks, evacuate, pull a vacuum and charge with 28 ounces of 134a but I want to identify the best way to "sneak" up on the correct charge.

My refrigerant charge level fell below the low pressure cutoff limit and my compressor would not cycle on. I put a set of gauges on the system and shorted the low pressure switch to get some measurements. Low side was 0 PSI, high side was 70 PSI. I put in 12oz of 134a. Ambient was 75 F. With the compressor forced on full time I got 15 PSI low side, 135 PSI high side. In normal compressor cycle mode the low side cycled from 52 PSI on to 23 PSI off, with high side PSI at 135 PSI maximum , 46 F center vent output. There were 4 cycles per minute (6 seconds on, 8 seconds off).

I realize that the spec is 28 - 32 PSI on the low side and 160+ on the high side and this system is probably still undercharged. Do you set the targets with the compressor cycling and look for the cutoff to fall within the 28 - 32 PSI range (with an eye on head pressure maximum) or do you set the charge with the compressor forced on?



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