Automotive A/C Diagnostic Checkout Procedures

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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):
  1. 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.
  2. 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.
  3. 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

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:
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:
Any Wrong Cold Spots
It is important to know what temperatures different points of the A/C system should be.
Fan Clutch Normal
The fan clutch should engage before the engine gets up to 220F 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

Copyright © 1997