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Working on your A/C system

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By: Hellknighthicks

Thanks to: Chevy 97

Apr. 20, 2012

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The purpose of this guide is to help fill the gaps in your knowledge about servicing A/C systems on your truck… The principals that will be covered here will apply to any car using R-134a refrigerant. So basically anything newer than 1995.
NOTE: assumption that you know nothing about A/C systems so I will cover all the little things that people forget to mention.

First you're going to want to buy some decent tools there are no substitutes.
*A good set of gauges 3 hose outfit
*Vacuum pump Air powered or electrically powered. NOTE: Mine is air powered
*Flush Kit Cylinder and Fluid
*R134a Refrigerant.
* Scale
* UV dye
* UV light

Visual inspection:
If the air in the cab is not blowing at all then the blower motor or related electronics my need to be replaced.
Check the clutch on the front of the compressor for heat marks… this may indicated that the compressor is slipping.

Possible causes
-Shorted Clutch switch wires.
-Bad clutch in compressor

Hose inspections:
Follow the hoses and check for oily spots or any places that are rubbed threw. Take special care in checking welds and transitions to rubber from aluminum compression fittings.
Check the top of the compressor where the lines come in for oils as well. If there is a leak here it would indicate that the seals need to be replaced at this connection point.
Take your valve caps off and check for leaks here as well if there is a blowing or hissing sound when you unscrew the cap your valve is most likely leaking and will need to be replaced.
Also check any connections going into the cab / out of and at the condenser for leaks.
Checking the condenser:
Take a flash light and inspect the condenser fro breaks and oil seepage. Also check the mounting points. The condenser should not be bent it should look like the radiator only thinner.
Note if you have to remove the condenser the connection on the lower side is most likely galled to the nut that holds it all together. Spray it with something like PB blaster and let it soak in. If that still doesn’t work you can try to drill a hole down to the threads and spray some PB blaster in there.
For me none of the above worked I had to throw some heat to it and it still pulled the threads right off the fitting… and I melted the little adjuster thingy

Checking the refrigerant:
So let’s hook up the blue hoes to the low side and see where we stand. Your gauge set if for R134a you won’t be able to plug up the wrong side… As a general rule the fat hose is low side… and the skinny side is the high side…

Engine OFF pressure tests:
First were going to check that pressure with the engine off.
Once you’ve plugged your line up you should be able to see what the current pressure is in the system

0 Low
0 High
There is no charge in the system.
This most likely indicates a leak. See charging a leak checking section.

100-150 low
100-150 high
It means your system is probably over charged. And you should recover some refrigerant to bring the levels back down to normal.
If the initial readings appear normal we can continue to the next step where we test the pressures with the engine on.

Engine ON Pressure tests:
First you’re going to want to start the engine
Turn the A/C to MAX and turn the Fan on high
And make sure you’re A/C system is cycling…
Make sure you’ve waited a couple of minutes before getting serious about your readings… The system needs to cycle and begin the heat exchange process before the readings will be useful for these tests.
The behavior you are looking for is that when the compressor shuts off the low side pressure will rise and the high side pressure will fall.
Your readings will come from the times when the compressor clutch is engaged.

20-30 low
200-300 high
Your system is properly charged.
For a more specific set of expected readings use the chart below to see what your pressure should be at in a given temperature. This will allow you to determine the approximate pressures that each side should be reading at a given temperature.

125 Low
125 High
This can indicate a fully open expansion valve or faulty orifice tube. It can also mean that the compressor itself or the engaging clutch is bad.

Very low pressure on the low side (in comparison to the normal 20-30)
High pressure on the high side (in comparison to the normal 200-300)
This indicates that there is a blockage in the system somewhere.
Possible causes
-Orifice tube
-Expansion Valve

High low side
High side
The system is overcharged and refrigerant should be recovered from the system to bring the pressures back down to a normal level.
Note if the needles are bouncing or vibrating in this scenario it would indicate a bad reed valve in the compressor.
If your gauges are bouncing around

Ambient temp is a reading of temperature approx. 2” in front of the condenser. Humidity also plays a role in what the pressures are in the system because it affects the ability of the air to move heat from the condenser.

Charging the system:
Assuming you have resolved your issue or your at 0 charge and were unable to locate the leak this is your next step

First things first you need to flush the coolant system

Your flush kit should come with everything needed

- Your going to remove the valve cores in over top the compressor or where ever they are. (Assuming they system is empty)
- I attach a hose to the low side nipple and point it into a 5 gallon bucket to keep from making a mess
- Then you’re going to put your pressurized solvent nozzle down in the high side hose
- Blow it in till it comes out the other end and stop.
- Let it sit for a few min Approx 5
- Then with an Air nozzle blow it all out

Repeat if necessary.

First thing you’re going to want to do is use your Vacuum pump to pull a vacuum on the system.
You’re looking to pull approx. 29 hg on the system. This will vaporize any water that’s in the hoses. It’s recommended that you pull the vacuum on the system for 30-45 minutes to ensure that the system is completely water free.

Once that’s done you can remove your vacuum pump. Make sure that you turn you ports off before you disconnect… because if you don’t you’ll let air back into the system…

Personally I add dye to all of the systems I charge to check for leaks… You can use the 4oz bottles of dye and a syringe to squirt some in the high or low access ports whichever is in the proper position to hold the dye and then press the valve down just enough to suck the majority of the dye in… Be very careful not to suck in any air. If you do suck air you might want to pull a little more vacuum back on the system again.

There are other methods for injecting dye into the system that have not been listed. (Other options please do tell)

Ok now were ready to charge the system. Hook you hoses back up with your valves off and hook up you can or tank.
First, identify how much coolant (R134a) your system takes. This should be under your hood somewhere.
Once you’ve identified that you’ll need either enough cans to do the job or a scale for your tank so that you can measure the amount you put in.

Using yellow hose to your tanks and your scale either do the math or some scales will tell you the amount of weight the tank has lost. Whichever you chose you should put your tank upside down and turn it on.

Hook the yellow line up to your can cracker. Turn the can upside down.

Note. Red/High side is not open at this point.

1.Loosen the yellow line at the manifold (the part with the gauges on it) until you here it hiss for a couple of seconds then seal it back up. This gets the air out of the manifold line and prevents air from entering the system.
2. Start your engine and turn the A/C on max
3. Turn the low side valve on
4. Watch your scale
5. If you can empty’s turn off your blue valve and repeat step 1 with the next can
6. Once you’ve put the proper weight/amount of coolant in shut all of your valves off.
7. Check your pressures
8. Check for cool air blowing into your face

If you have a leak you should be able to locate it with a black light.
To check for Condenser leaks you should take your black light to the drain. If it’s leaking refrigerant then the dye should show up in the water that’s coming off of the condenser. It should also leave a residue.

Addendum - Further instruction on AC systems:

Contaminated Systems:

1) Have repair parts on hand.
2) Flush system to remove contaminants and oil
3) Complete repairs.
4) Drain oil from compressor.
* Note: if replacing compressor, is it dry of oil or does it contain oil and how much and which type.
5) From service manual obtain amount of oil system requires and which type.
* Note: (thumb rule)
1oz for dryer
2 oz for evaporator
2 oz for condenser
6 oz for compressor.
* Note: for 4 cyl radial GM only 2oz.
6) Install all oil into compressor.
7) Replace receiver dryer.
8) Evacuate for many hours. (Overnight.)
9) Recharge system and fully test.

Purging the System:

The use of a recovery system is required for purging refrigerant.
1) Install the manifold gauge set.
2) Run system. (If possible)
A) 10-15 minutes
b) Engine at fast idle. (1200 -1600rpm)
c) A/c system at maximum fan and cold.
3) Shut off the system and engine.
4) Connect manifold gauge center hose to recovery system inlet fitting.
5) Open both manifold hand gauge valves. (HI & Low)
6) Use recovery system as instructed by decal. (Depends on R12 or R134A)
*Note: do not mix the two in the same recovery tank.
7) When system is empty close manifold gauge hand valves before removing center hose.
8) Drain and measure oil recovery unit removed from A/C as per decal. (Sticker on truck) If excessive replace and using new oil only.
* Note: only if your recovery system has the oil separator on it. Some machines filter all the recovered refrigerant.
9) System is now ready to open for repairs.

Evacuating the System:

A good vacuum pump is required for this procedure.
1) Install manifold gauge set.
2) Repairs have been made.
3) Connect the manifold gauge center hose to the vacuum pump inlet.
4) Uncap the exhaust port if equipped.
5) Start vacuum pump.
6) Open both manifold gauge hand valves.
7) Evacuate until lowest gauge reading is obtained. (30-45 minutes)
8) Close manifold gauge hand valves first.
9) Close the inlet valve of vacuum pump.
10) Shut off vacuum pump.
11) Remove manifold gauge center hose from vacuum pump.
12) System is now ready for Charging.

Charging system with Refrigerant:
These steps are for liquid charging.
Procedure for charging by weight using:
A) A charging cylinder
B) A scale
1) Install Manifold gauge set.
2) Look up amount of charge needed. (lbs/oz or kilograms) Use Decal (sticker on truck) or manual.
3) Charging cylinder- fill to more than system requires. (Do not over fill). This is if you have a recovery machine.
4) Connect center manifold hose to container so liquid is withdrawn.
5) Open container valve.
6) Scale- with container on scale calibrate.
7) Open high manifold valve and allow charge to flow into system.
8) After charge has entered system close all valves.
9) If total charge will not enter system follow these steps.
* Note: Be sure high side manifold valve is closed.
A) Start system run engine at fast idle. (1200-1600rpm) A/C max fan and cold.
B) Now continue charging through the low side of manifold gauge set.
* Note: Do not allow low side pressure to exceed 40psi with the system running.
C) Continue charging until the proper weight is installed.
D) Close all valves.
10) Remove container from manifold center hose.
11) Performance test. (How cold is the system putting out?)
12) Stop system and engine.
13) Leak test high-pressure side.
14) Cross bleed.
15) Leak test the low-pressure side.
16) Remove gauge set and cap the fittings.

Charging the system with refrigerant
These procedures are for liquid charging.
B) Procedure using site glass only.
1) Install manifold gauge set.
2) Connect center manifold hose to container.
3) Open container valve.
4) Place container so liquid is withdrawn.
5) Open high side valve for 10 seconds and then close.
* Note: This is for initial charge.
6) Start system, run engine at fast idle (1200-1600rpm), A/C at max fan and cold.
7) Now continue charging through the low side of the manifold gauge set.
* Note: do not allow low side pressure to exceed 40 psi with the system running.
8) Charge until sight glass just clears.
9) Close manifold valve and watch to be sure sight glass stays clear.
10) Now we add an additional 4oz (thumb rule) or as per service manual.
11) Close all valves and remove container.
12) Perform test.
13) Stop system and engine.
14) Leak test high-pressure side.
15) Cross bleed.
16) Leak test the low-pressure side.
17) Remove gauge set and cap the system.
* Note: If you are using propane you will have to use a scale. (What they mean by propane is Duracool or Red Tek.)

Safety precautions when working with refrigerants:
1) Do not purge to atmosphere (recover)
2) Do not expose eyes to liquid.
3) Do not Subject container to high temperatures (120F 50C max)
4) Do not carry container in passenger compartment.
5) Do not fill container completely full. (Leave an air space 80%)
6) Do not expose to a flame. (Becomes poisonous gas)
* Note: If refrigerant should contact the eyes, splash eyes with cold water to thaw them out & raise their temperature.
Consult a doctor or eye specialist as soon as possible.

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