Basic Principles of Auto AC

To Flush or Not to Flush, that is the question.

Another controversial subject. GM recommends against flushing, many AC shops swear by it.

Flushing is the act of flowing a solvent through the system to dissolve and remove any debris or contaminants. The entire system cannot be flushed as a whole, and some components cannot be flushed, period. You cannot flush a compressor. You cannot flush an orifice tube. You cannot flush a parallel flow condenser.

You can flush lines, you can flush the evaporator, you can flush a linear flow condenser.

Let’s start by looking at the arguments in favour of flushing: any debris remaining in the system after a failure, such as bits of metal shed by the compressor, will eventually make their way through the system and damage the new compressor and/or plug up the orifice tube. Getting them out will ensure longevity for the repaired system. All true.

Arguing against flushing, if the flushing wasn’t perfect, all you did was concentrate the debris and make it more likely it will cause problems as it travels in one large mass. If the flushing chemicals were not completely compatible with the refrigerant, there could be a chemical reaction that will cause even worse problems (eg: R-11 is often used as a flushing agent, it’s OK with R-12 but will react with R-134a). So it is vitally important that all the flush be completely removed from the system and that is a difficult thing to assure. Also all true.

GM’s position is that an inline filter is the best option. An inline filter is considered a warranty approved alternative to flushing when replacing a failed compressor*. In fact, there is one installed as part of the orifice tube, but it’s possible to add additional ones. There are filters available that can be cut into the line (eg: GM part number 89016656 / AC Delco part #15-10413 or GM part #88960977 / AC Delco part # 15-1747). Early factory service manuals also reference an aftermarket liquid-line filter. Note that if you install a liquid line filter, it may include an integral orifice tube. If so, you must remove and discard the original orifice tube for proper cooling. Also, a liquid line filter comes after the compressor, so it does NOT protect the compressor from debris.

The simplest and least likely to leak is a filter screen that fits into the suction line at the compressor (and therefore also protects the compressor). Simply disconnect the block from the compressor and insert the screen. These were not developed until 1998 -- after the last B-body was built -- so the various FSMs make no mention of it, but they are completely compatible with our cars. GM of course offers a special tool (J-44551-5) and mandrel (J-44551-3) for the task but it’s a clumsy excuse for a tool and with some ingenuity you can press the screen into the block without it.

A wide variety of screen sizes are available to fit various vehicles. I have yet to find a definitive list of what vehicles require which screen size. I do know that a 1995 Roadmaster takes size “A” – 0.510” diameter, to fit lines with an ID from .499” to .507”. GM Part # J44551-10, AC Delco part # 15-21186 (not all sizes are available from Delco). I strongly suspect all B-bodies from 94 to 96 use the same, as they all use the same compressor. For the sake of possible others, here are all the available sizes:

J44551-	Size"	Range”	AC Delco Colour, #	Notes
-40	.395"	.384”-.392”
-60	.471"	.460”-468”	Blue, 15-21185
-70	.492"	.480”-489”
-80	.500"	.489”-.497”
-10	.510"	.499”-.507”	Natural, 15-21186	Originally size “A”, J44551-15
-90	.521"	.510”-.518”	Yellow, 15-21187
-110	.531"	.520”-.528”	Green, 15-21188
-120	.538"	.527”-.535”	Purple, 15-21189
-20	.552"	.541”-.549”	Black, 15-21190	Originally size “B”, J44551-16
-130	.561"	.550”-.558”
-140	.579"	.568”-.576”
-30	.595"	.584”-.592”	Natural, 15-21191	Originally size “C”, J44551-17
-150	.600"	.580”-.597”
-160	.618"	.607”-.615”	Green, 15-21192
-170	.649"	.638”-.646”

AC Delco sells a set, including the tool, under part # 1521184. The original GM kit, J-44551-A, includes the tool, mandrels, etc. and sizes A, B, and C. The rest of the sizes come in the update kit, J-44551-50. There is also a J-44551-800 kit, BUT it is Saturn specific and comes with the –90, -120 and –20 sizes only so it probably won’t work with a B-body car.

So much for the inlet filter, let’s look at the failure mode:

The most likely failure is the compressor. When it fails, debris flows out into the condenser and may make it as far as the orifice tube. All the lines between the compressor and the orifice tube must be considered compromised. The line to the evaporator, the evaporator itself, the accumulator and the lines back to the compressor should be protected by the orifice tube filter.

Since the compressor has to be replaced and can’t be flushed anyway, we don’t need to worry about that. Same for the parallel flow condenser fitted to the B-bodies – compressor failure means the condenser must be replaced. In theory, it might be possible to clean the orifice tube and screen, but for 93¢, is it really worth the effort? We will be replacing the accumulator as part of the repair – after all, it is designed to capture and hold any contamination or debris that develops as part of the normal operation of the system, and replacement is a requirement of any replacement compressor warranty.

This leaves the hoses. To replace them with new is under $100. To flush them requires a flushing machine and an hour per item (15 minutes of flushing followed by 45 minutes of trying to get all the flush chemical out). The cheapest warranty approved** flush system is around $400 from Hecat, and uses your shop air supply. Add another $25-$50 for the flushing solvent itself. Approved** solvents are the Hecat’s “Safe Flush”, Four Season’s “Dura-Flush” (R-141b), and Cliplight’s “Diamond Flush” but only when used with the respective manufacuter’s equipment.

Now, if there is reason to suspect contamination of the evaporator (for example, if a sealant chemical was added to the system), flushing becomes a more attractive option. The evaporator itself is less than $100, but replacement is intimidating. The B-bodies are not as bad as some cars where the entire dash must be removed down to the firewall, but it is a cramped and awkward job that will occupy several hours.

What to use as a flushing agent is the next debate. GM approves closed-loop flushing using R-12 only (for flushing R-12 systems) and R-134a only (for flushing R-134a systems) as a condition of warranty. On the other hand, Four Seasons says exactly the opposite: use of a closed-loop flushing machine with refrigerant as the flushing agent specifically voids their warranty!

Both agree that the following methods will void any warranty:

• Open loop flushing (hand held flush gun) with terpene based flush.
  
• Any flushing chemical that is oil, alcohol, mineral spirits, isopropanol, acetone, toluene, or heptane based.
  
• Any flush chemical that has a known health hazard (1.1.1. trichloroethylene, trichloroethene, tetrachloroethylene).
  
• Any other cleaning agent such as brake clean, degreasers, carburetor cleaners, etc. that can deteriorate rubber components and reduce lubrication.
  

So to recap:
The only universally accepted and warranty approved method of flushing is not to flush at all! Install a filter in the compressor suction line instead. Replace the orifice tube, the accumulator and the compressor hose. If there is a sealant in the system, you are better to flush the evaporator since the use of a sealant in the system will void any warranty by anyone anyway. If you are going to flush, don’t bother wasting your time and money on one of those cans of self-propelled flush – they may do more harm than good. And finally, make sure absolutely every drop of flush is cleansed from the system. It takes about 15 minutes to flush a component with the proper machine, and about 45 minutes to get the flush out of that same component.

*by Four Seasons, a major aftermarket AC components supplier and by AC Delco
**by Four Seasons only

Troubleshooting (or why do I need gauges if they won’t tell me the amount of charge?)

Refrigerant is considered a condensable, not a compressible. That is to say, the more refrigerant you put in a closed system, the more liquid you get. The pressure does not increase unless the temperature increases. Each refrigerant has a very specific relationship between pressure and temperature. Knowing this, the pressures can tell us a lot about what is going on inside the system.

Under normal conditions with the system not running and everything at an equalized temperature, the static pressures at both the HP port and the LP port are equal to expected pressure at ambient temperature as indicated on a temperature/pressure chart for that specific refrigerant.

Once the system is running, as described above, the system divides itself into two halves: the High Pressure side and the Low Pressure side, each with a port to monitor the pressure. Once running, typical pressures for R-134a are LP =30-31psi, HP 204-210psi. R-12 has a narrower range, typically LP = 32-33psi, HP=185-190psi.

The above is rule of thumb only. Actual normal pressures may vary by ambient temp and humidity. Reference should be made to the R-12 performance chart or R-134a performance chart for greater precision when deciding if a pressure should be considered low, normal or high under the exact circumstances. The same goes for the vent temperature – a system in perfect working order might blow near-freezing air out the centre vent when ambient temperatures are 75°F and RH is 40 or 50%, but might only achieve 70°F when the mercury climbs over 90° and humidity approaches 100%.

LP low, HP normal to slightly low.
Probable causes:
1. Incorrect adjustment of LP switch (note: some vehicles use a  TXV valve control to perform the function of the orifice tube used in the B-bodies. The symptoms associated with the orifice tube would apply, with modification, to the TXV valve in those vehicles)

2. Restriction in the low side of the system. EG: plugged orifice or screen.

3. Moisture in the system (may freeze at orifice inlet, causing very cold inlet).

To verify between 2 and 3, turn off AC and allow to stabilize for 15 minutes, then turn back on. If gauge reading immediately goes to abnormal condition, the screen is probably clogged. If the gauge readings are normal for a few minutes, then goes abnormal, it is probably excess moisture.

LP very low to low, HP low
Probable causes:
1. Low refrigerant charge
2. Clogged inlet screen
3. Defective or plugged orifice tube
4. Moisture in the system (as above)
5. High-side restriction in the high side before the orifice tube (eg: crushed condenser tube).
Loss of refrigerant is usually due to a leak, although all systems leak a tiny bit as a normal condition. R-134a systems are usually more sensitive to the correct charge than R-12 systems. R-12 systems usually have a sight glass, and low charge will show up as bubbles in the sight glass. The compressor will usually cycle rapidly, as there is insufficient liquid refrigerant available to supply the compressor intake. The evaporator outlet line may also be warm, as there is insufficient refrigerant to keep the evaporator full of liquid.

LP low, HP high to extremely high.
Probable cause:
1. Restriction in the high side of the system
2. excessive oil charge
In the case of a restriction, it could be anywhere from the compressor outlet to the fixed orifice tube. The closer to the compressor, the higher the high-side gauge pressure will be (because the refrigerant has less time and space to cool).  Probably a kinked or bent line or tube. A marked temperature change will often mark the location (as it acts like an orifice tube) with the cooler side downstream of the restriction.

Excessive oil charge may result in vibrating or pulsating hoses. The oil acts as both an insulator and takes up space. The compressor must work harder against higher pressure and the lines may vibrate as the refrigerant is pushed through a pool of oil in a line. The outside temperature of the high-side line might be cooler than the temperature/pressure chart would indicate as the oil is preventing heat transfer from the refrigerant to the line.

LP high, HP low
Probable causes (electrical):
1. compressor LP cycling switch
2. PCM pressure sensor
3. ambient air temperature switch
4. engine coolant temperature sensor
5. throttle position sensor

(mechanical)
6. defective/misadjusted clutch
7. defective compressor

The compressor may or may not be running – look closely at the centre hub. If it is not turning, or not turning continuously with the pulley then there is an electrical problem or a clutch problem. If it is turning, it is probably the compressor itself or a mechanical problem with the clutch.

Disconnect the LP cycling switch connector (on the accumulator). With a piece of jumper wire, briefly short the two wires and verify if the compressor starts. The switch is adjustable, using the small slot-headed screw inside the connector. The switch can also be replaced without evacuating the system as it has a Schrader valve beneath it.

Using a scan tool, verify the AC pressure as seen by the PCM and compare the reading to the actual measured pressure. If the indicated pressure is much higher than the measured pressure, the PCM may be cycling the compressor each time the sensor hits the high limit. Similarly, the PCM will shut down the compressor if it detects an overheated engine, full-throttle operation, or low ambient temperature.

If the clutch fails to engage, only engages when travelling uphill or only engages when the centre is tapped then the clutch gap may be excessive and should be reduced. The clutch coil may be burnt out or shorted. A shorted coil will draw excessive current and usually blow it’s fuse. An open coil can be determined with an ohm meter. A slipping clutch is rare, but can occur if a rivet or arm has broken and it will make a racket. You won’t need gauges to figure out what’s wrong if the clutch is slipping.

Once you have ruled everything else out, the only thing left is a worn out or damaged compressor. Unless someone else worked on the system and forgot to reinstall the orifice tube.

LP high, HP high to extremely high
Probable causes:
1. Air (or other contaminants) in the system
2. Overcharge of refrigerant
3. Excess oil in system
4. Condenser fins clogged or obstructed or debris trapped between condenser and radiator
5. Defective cooling fan(s)
6. Overheating engine
7. Incorrect refrigerant

An overcharge of refrigerant will often result in a cool to warm evaporator outlet pipe and while cooling will be poor at idle, it may be OK on the highway. Air in the system can be very similar, but can sometimes be identified by turning the system off while watching the gauges – the pressure will drop 20 or 30 psi very quickly, then taper off very gradually as the two sides equalize.

Poor cooling of the condenser (fans, obstructions to airflow) can usually be temporarily corrected by misting the condenser with a garden hose and sprayer. The HP will usually plummet to near-normal almost immediately when the water hits the condenser.

An overheating engine causes additional heat load, keeping the condenser warm by radiation.

Air contamination can result from improper charging practices of course. However, one Florida study suggested that as many as 25% of commercial AC shops have contaminated recovery equipment, probably by servicing vehicles after the owner tried and failed. Similarly, the owner may have tried to recharge the system with some other refrigerant. If the shop does not test the system contents with an expensive refrigerant identifier, the contaminants get spread to successive customers.

Exactly what I said!Bet you are a blast at a party.Lot of effort went into this and its easier for me to understand than the manual,thanks a bunch Dwayne!

Flasheroo wrote:

Exactly what I said!Bet you are a blast at a party.

Psst. See that guy all by himself in the corner, the one that came in the old station wagon? Don't make eye contact unless your AC is broken. He'll go on about it all night.  

Actually, most of the work seemed to be getting the tables to format properly. Maybe the moderators could investigate TinyMCE for the WYSIWYG text editor at the next upgrade? Please?

I don't know if this is forum legal or not but Here is a great reference from www.AutoAcForum.com on what type of oil, and how much refrigerant is required for vehicles. It appears to be all inclusive. I used it as a guide when re-doing the retrofit on Ms. Roadie this summer.

http://www.autoacforum.com/speclisting.pdf

Mods if we can't link this info please just delete my entry on this thread thanks.

They got that listing from Four Seasons -- the numbers in the 2 right hand columns are Four Season's part numbers for oil. So they don't hold any copyright on the material.

A very useful forum though.

Thank you! Making a Sticky!

Chris

Informative thread, thank you for all the work!

As to the editor updates, those aren't controlled by us since this software is provided free.

so if my sponge is absorbing and is not compressed, do i need to flush?

great thread, I only read a couple parts though as I've done at least 5 or 6 systems now. Interesting reading on the HC stuff...