Automotive A/C Diagnostic Checkout Procedures

Please send comments, corrections and additions to: PParish@delanet.com

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Static Inspection (Vehicle Engine Off)

A/C System History: It is very important to get the history of the A/C system from the customer! You need to understand the customer's complaint.
Visual Inspection: You can see signs of most leaks if you look very careful. Refrigerant oil leaks out with the refrigerant and leaves an oily film that collects dust leaving a build up of dirt.
Compressor seal leak: Look at the compressor clutch and surrounding area for signs of oil and dirt. The clutch will sling oil if there is a leak.
Refrigerant Hose Leak: Look at the hoses for oily spots.
Evaporator Drain Oily: We try to look at or fell for the evaporator drain. Look at the area right below the drain and touch the drain hole.
Any Other Obvious Leaks: Look around at the other components for oily spots and a build up of dust, as on the condenser. We will sometimes remove the blower resistor in order to see the evaporator core.
Fan Clutch Normal: Look at the front of the engine fan clutch. A build up of grease on the thermostatic coil is an indicator that it may be bad.
Vee Belts: The belts should be tight, not cracked and fit in the belt groves of the pulleys properly.
Static Pressure Reading: The system should have a pressure that corresponds to that of the pressure temperature relationship chart. For each temperature there is a different pressure for each refrigerant.
See Pressure temperature relationship table.
Identify Refrigerant Type: If you have any suspicions or you are going to recover the refrigerant make sure you test the refrigerant with an identifier. I use a Neutronics refrigerant identifier.

Hooking up and using a Gauge set

It is standard practice to use a gauge set to service or check out an air conditioner. I started using one at in the mid 60's.
There are three hoses attached to a gauge set (manifold gauge assembly):

A red hose that is to be hooked to the high pressure side of the system. The red hose is on the right side of the gauge set. Just above where the red hose attaches to the gauge set there is a red gauge that reads the pressure in the red hose. There is also a valve to allow for on or off connection of the red hose to the yellow (center) hose.
A blue hose that is to be hooked to the low pressure side of the system. The blue hose is on the left side of the gauge set. Just above where the blue hose attaches to the gauge set there is a blue gauge that reads the pressure in the blue hose. There is also a valve to allow for on or off connection of the blue hose to the yellow (center) hose.
A yellow hose that is used to connect the gauge set to the vacuum pump or refrigerant.

Things To Keep In Mind When Using a Gauge Set

Now days, all three of the service gauge hoses have quick disconnect fittings at their attachment ends. This keeps from venting unneeded refrigerant into the atmosphere. In the old days we would bleed the hoses after hook up to a charged system. Also we would bleed the yellow hose after hookup to refrigerant. Bottom line, never let air get into the air conditioning system!
Never Run the compressor with the gauge set high side service valve open! This could let the high pressure of the high side blow up the can of refrigerant!.
When I pull a Vacuum, I open both valves and draw down (evacuate) both sides of the system.
I prefer to use 12 oz. cans of refrigerant rather than a big tank.
When I service an air conditioner, I pressurize both sides with gas first (can turned so vapor refrigerant comes out). Then I shut off the high side service valve before starting the vehicle!
I charge the system through the suction side (low pressure side) of the system with the compressor running. The can will get cold as the pressure in it drops. To speed up the process warm it and shake it. I some times set the can in a bucket of warm water.
You can also charge a system up on the high side while the compressor is not running with liquid refrigerant. To do this you would use a charging station or by putting the can in hot water.

Dynamic Checkout (Engine Running at about 2000 RPM) *

* We must have refrigerant in the system, compressor running, and the refrigerant flowing before we can make the dynamic checkout!
I run the blower on low blower when first checking the A/C system and servicing it. This puts the system under low load and helps it draw in the refrigerant. I then check it at other blower speeds.

Sufficient Refrigerant for system to run.

If there is not enough refrigerant, the low pressure cut out or the clutch cycling switch may prevent the system from coming on.

Does Compressor Run

The compressor should run. Your can see this by looking at the clutch hub (front plate). The hub is the item on the front of the compressor that is connected to the shaft. If the compressor does not run, check to see if the clutch is getting power and that it has a ground. If it is not getting power I usually disconnect existing wires and apply power to see if it will function.
Note: Do not run the compressor if the condenser fan is not coming on!

Is Compressor Noisy?

If the compressor is too noisy, it may be a waste of time and money to go much farther.

Sufficient Compression

The compressor should be building head pressure. Your high side gauge should be building pressure. You can feel the line coming out of the compressor (discharge line) to see if it is getting hot (system moving heat).

Test Gauge Readings

Pressure Temperature Relation-ship of Refrigerant
The first thing you must understand, is that for each temperature of a container of refrigerant (at rest) there is a unique pressure. You can see pressure numbers on the out side rings of your gauge set and the temperature numbers on the inner rings of your gauge set. There is also a Pressure Temperature relationship chart (PT-table) on this web site.
See PT-table.
There are two spots in an air conditioning system that match the Pressure Temperature Chart, some where in the center of the condenser and some where in the center of the evaporator. So the gauge set pressure readings give you an indication of the temperature of the center of the condenser and the center of the evaporator. This is a general rule and may not always be true if the system is not working properly or is overheating.

Air Conditioning System Operation

I will not try to fully describe how an air conditioning system works here but you must know a few things:

Heat is taken in by the evaporator, where refrigerant is changing state from a liquid to a vapor (evaporating).
Heat is given off at the condenser where the refrigerant is changing state from a gas to a liquid (condensing).
If you take in more heat than you are getting rid of, you will have high discharge pressure.
If you have a restriction in an air conditioning system anywhere past the condenser both gauge reading will go low. This is because the refrigerant will dam up before the restrictions and there will not be enough refrigerant flow to make pressure. In fact if there is enough room in the first part of the condenser to store all of the refrigerant, the last part could have a restriction and still have the gauge readings go low.
High Side (Discharge Side): 150PSI to 300PSI (maybe 340PSI with R-134a).
The high side pressure will give you an indication of the average temperature of the condenser. I think that any pressure over 250PSI hinders air conditioning operation and at any pressure over 300PSI (maybe 340PSI with R-134a) the system should be shut off.
If you have a vehicle with high head pressure you can spray cold water on the condenser to keep the pressure down.
Low Side (Suction Side): 20PSI to 40PSI. The low side pressure will give you an indication of the average temperature of the evaporator. See PT-table.

Evaporator Outlet Cold

The outlet line of the evaporator will give you an indication of how the A/C system is cooling. It should feel like there is ice in it.

Is the A/C system cycling?

Every A/C system has to have some method to keep the evaporator from freezing up. I some times disconnect the blower motor in order to test for evaporator freeze up and/or cycling.
There could be a:

Thermostatic switch, that cycles the clutch when the evaporator temperature gets down to its' setting (usually just below freezing).
Pressure cycling switch that usually mounts on the accumulator and cycles the compressor when the suction pressure gets down to 25 PSI.
Variable displacement compressor, like the GM V5 that changes its displacement in order to control evaporator temperature.
Suction throttling valve, which controls the suction of the compressor to the evaporator in order to control the evaporator temperature.

Any Wrong Cold Spots

It is important to know what temperatures different points of the A/C system should be.

The discharge line (line from compressor to top of condenser) should be hot (heat of compression).
The condenser should cool the refrigerant from a high-pressure gas and condense it to a high-pressure liquid.
The liquid line (line from bottom of the condenser to drier if present) should be warm.
Second liquid line if present (line from the drier to the expansion valve located at the bottom of the evaporator) should be warm. If the drier is plugged this line will be cool.
The evaporator should be cold because of the refrigerant evaporating in it.
The suction line (line from the top of the evaporator to the compressor) should be cold. If the refrigerant is not flowing properly or if the suction pressure is not getting low enough, there will not be sufficient cooling.

Fan Clutch Normal

The fan clutch should engage before the engine gets up to 220°F and/or before the A/C system get up to 300PSI (maybe 340PSI with R-134a). A standard cooling system fan clutch should spin at 33% of water pump speed (RPM) when not engaged and 75% of water pump RPM when engaged. A heavy duty cooling system fan clutch should spin at 33% of water pump speed (RPM) when not engaged and 90% of water pump RPM when engaged.

Condenser Fan Operates

Some vehicles have an electric cooling system fan instead of an engine driven fan clutch. Front wheel drive vehicles with the engine sitting side ways do not have an engine drive cooling system fan.

Blower Runs At All Speeds

It is important that you check the controls and that the air coming out of the vents is proper.

Modes & Air Handling Ok

Check and make sure that the control panel operates all of the air delivery modes. I usually make sure that I can hear the temperature door hit in both directions, by sliding the temperature control lever back and forth and listening for the noisy of the door hitting full positions.

Heater not Fighting A/C

Some systems flow all the air through the evaporator core then they flow all of this same air through the heater core. These systems require that the heater control valve work 100%!!

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Patrick Parish
70 North West St.
Dover, DE 19904
DATE: Jan. 21, 1997