HVAC REBUILD

[THEFERMANATOR]

GM's FIX for the liquid slugging was the elastic belt with no more tensioner. Telling people to have liquid returning to there compressors is a GOOD way of killing somebody's compressor. Rule #1 of A/C is to NEVER EVER allow liquid refrigerant to make it back to the compressor. it's no different than driving your vehicle through water, and ingesting some. It's HORRIBLE advice to reccomend overcharging to the point that liquid makes it back. Yes, oil WILL circulate with gas, so enough with that inacuracy. You've said in SEVERAL posts that oil will not circulate with gas, and now you say it doesn't circulate as well. So long as there is enough speed in the gas flow, it WILL circulate oil. I don't want somebody to read about charging to the point liquid makes it back to there compressor, they do it, and then send shratnel into there condenser now costing them a compressor, compressor manifoild hose set, condenser, orifice, accumulator, and a WHOLE LOTTA FLUSH to clean one out. I've seen people do it, and it's NASTY.

 

[WarWagon]

I meant to type A6 in my last post for the 1988 Suburban. My memory from 20-25 years ago may have faded or the 1988 had non-factory aftermarket air installed as AC wasn't standard back then. I checked our supporting vendor catalog Rock Auto to confirm that Factory Air did not use the A6 for 1987 or 1988.

The average backyard mechanic really shouldn't be messing with their AC system unless they have a $4000 Refrigerant recovery machine or hire the recovery step out to a shop or friend who does have one. Further the FOT (CCOT) systems are charged by weight only. Dropping a $50.00 fix it R134a can on their system leaves an unknown charge in the system. The correct way is to recover, vacuum, and recharge the system with leak check and/or repair somewhere in the steps depending on the situation.

The charge by weight is specific to the FOT system. Systems leak and the charge will leak out over time no matter what. So when in doubt, recover, vacuum, recharge, and leak check.

Now the graduate level course I am talking about here and this course is VERY specific as to why you charge the system by weight. This is not general theory! This is taking some little known information from the folks that developed the CCOT system, why you can't know this specific design system charge accurately enough by pressures and temperature, and failure analysis of the R4 vs. other compressors with a bulletproof reputation.

First off we need to understand WTH the suction accumulator does. It's location in the system is between the compressor suction hose and the evaporator. Contrast this to the expansion valve systems that have the receiver/dryer between the condenser and the expansion device.

The accumulator's primary job is to separate liquid and gas. It's name isn't so much from "suction accumulator" as it is from accumulating the unused system charge in liquid form. The system charge changes with heat load, condenser temperature, ect. It also holds a leakage reserve. Again I am repeating the cool-profit link.

The 5-10% of the total refrigerant flow out of the evaporator being liquid is when the system is properly charged by weight. The accumulator's job is to make sure this liquid refrigerant doesn't make it to the compressor, but, it has to return oil. The oil return is accomplished by a liquid bleed hole in the suction line pickup tube in the accumulator. Any system overcharge winds up in the accumulator, as well as the leakage reserve, and a varied amount for load changes. You don't get much error in overcharge that the accumulator can hold before liquid can reach the compressor - in theory as likely the high side safety switch would trip the system off first followed by the high side pressure relief venting the system. (Or the high side simply blows up like the 1993, Patch, was known for with a bad high side switch.)

The 5-10% of liquid refrigerant leaving the evaporator must evaporate as the book link explains the heat comes from gas leaving the evaporator. I'll mention some heat through the walls from the hot engine compartment, but, some are insulated from the factory so this isn't the primary heat source.

Now as to oil flow - think of drinking through a straw. It's all good till you get to the last of the drink and have to suck a lot of air to get the last of the drink out and hope you don't inhale it and cough. This be the two ways to move oil in the AC system. By liquid refrigerant or gas velocity carrying it along. The evaporator is "boiling" and uses a messy combo of both methods to move oil in the small tubes.

The accumulator uses the bleed hole to move oil otherwise the system oil charge would all wind up stuck there. If gas alone moved the system oil well enough there wouldn't be a liquid bleed hole in the accumulator as it allows the suction pipe direct to the compressor to fill up with liquid when the system is off.

To be clear for the quiz: The suction accumulator has liquid refrigerant in the bottom of in a properly charged running system.

And a good explanation with cut away picture is by cracking a book:
Fundamentals of Automotive Technology
By CDX Automotive

https://books.google.com/books?id=m...Accumulator ccot Accumulator cut away&f=false

 

[WarWagon]

Now that we have the above theory out of the way including very specific factors in oil flow lets revisit why the R4 makes noise and fails so often.

The R4 lives on a razor's edge of oil flow. There is a point where the accumulator runs out of liquid refrigerant due in part to the many ways the R4 compressor leaks. Body shell expanding and the orings leak, shaft seal problems from day 1 of production, and it goes on and on where all the R4's can leak at. (The A6 also had many seal designs including if I recall correctly a revised seal for R134a PAG oil .)

When the level in the system is below a critical charge oil flow is not steady. Compressors like the A6 that have a sump and pump are very tolerant of short interruptions in oil flow. The R4 depends on a steady mist of oil by design. So every time the oil flow is interrupted by low refrigerant levels the R4 suffers where an A6 survives off the sump oil level.

This critical charge is where the system is running but the evaporator is not returning the 5-10% of the system flow as a liquid. Somewhere here is a point that the systems will run and cool without cycling the clutch on a low charge. It still cools and may or may not be noticed by the driver. The lack of good cooling can be ignored because it cools some. This low charge is returning oil, but, not steadily returning oil. Any lower and the system rapidly cycles the clutch where all compressors eventually fail, but, more importantly the system isn't cooling worth a damn so it will get serviced.

This specific chain of events and the R4's dependence on oil flow is the entire point I make here: and that be you have to charge these systems by weight especially the R4 system because it's THE MOST sensitive to the correct refrigerant charge due to oil return.

Other compressor designs without oil pumps may be relevant.

 

[dangerousdave]

Thanks. 4 seasons are made in Texas still?

 

[THEFERMANATOR]

So which is it? FIRST you say oil will NOT move with gas, then you say it can, now it does so long as there is velocity(which is what I have said all along). There is ZERO need to spend $4000 on a refrigerant machine. Commercial grade machines used in dealerships can be had for less than that. And they CAN be charged by pressure and temp if you know what you're doing. What do you do when you have a modified system that no longer holds the factory amount, the oil has been changed out because of a bulletin(since GM has superceded PAG 146 for PAG 46 now), orifice tube changed out because of a compressor or oil change? You charge it via pressure and temperature until you find the system volume. An accumulators job is just that, an accumulator. It has a desicant inside to capture any moisture in the system, and acts as a liquid break to let the refrigerant boil off back to a gas, and stores an excess charge so the system can work at varying RPM's. You can charge an A/C and make it ICE cold at low RPM's, but at higher RPM's the air will warm up in a CCOT system that utilizes a low pressure cycling switch because the compressor sucks the accumulator dry. This oiling hole you talk about in the accumulator is just that. It allows oil that gets trapped in the bottom of the accumulator where circulation is minimal to get pulled back into the system. And I guess me and THOUSANDS(if not MILLIONS) of people who do A/C work WITHOUT a $4,000 machine are just screwing them all up? I haven't charged a system with a scale in over 10 years, and I ALWAYS get ICE cold air, and have yet to have a compressor failure(have had better A/C performance chaging my way than I did by weight anyways). It isn't rocket science doing HVAC like some try to claim it is. There are some who have no business doing it, but not all of us without a $4K machine don't know what were doing. And from my experience, MANY WITH those $4K machines and all this tech training SHOULDN'T be working on them either.

Since you keep saying the R4 compressor, what makes it ANYMORE susceptible to lack of oiling than any other modern compressor. I guess I should have been more specific early on and said MODERN A/C system since you keep wanting to bring up the 50's technology of the A6. The A6 had it's own sump and pump for lubrication because it was largely used in POA throttling valve systems. They ran 100% of the time that the A/C was engaged. They could just about block the suction line going back to the compressor off during normal operation, hence the need for it's own oiling system. When the throttling valve would close off, gas return would drop to next to nothing, so when that happened, no oil got returned as there wasn't enough velocity for it to carry the oil back. It wasn't a great system, hence why many have done away with them and switched to CCOT systems with SANDEN compressors. The R4 gets oiled just like the HT6, the DENSO 10S series, SANDEN, and so on. Most all of them now use a CCOT and the majority use a low pressure cycling switch to activate them(but not all as some were smart enough to use a temp probe in the evap). Thereason you can get colder air at slow speeds while undercharged in a CCOT system is teh fact that at slow speeds undercharged, it is actually properly charged. At higher speeds, the compressor will suck the accumualtor dry because of the greater cooling across the condenser, and higher engine RPM's pulling more gas through. So a correctly charged system is a trade off of high speed and low speed performance, while balancing the charge to a point that liquid will not make it back to the compressor.

 

[ak diesel driver]

And where do I find the ignore this thread button?