Diagnose AC not cooling problems
How to diagnose AC not cooling
There can be many causes of an AC not cooling condition: low refrigerant, refrigerant overcharge, faulty compressor clutch, faulty compressor, faulty compressor clutch relay, improper compressor clutch air gap, clogged or inoperative orifice tube or expansion valve or faulty pressure switches.
Diagnosing an AC not cooling problem should be an orderly process, not one where you automatically add refrigerant or start replacing parts. I’ll give you the step by step process. But first, you’re going to need some special tools. Oh stop whining! There’s no magic bullet here. If you want to fix your own car you’ve got to have the right tools.
AC diagnostic tools required to conduct tests
Meter and thermocouple or, if you already own a meter that can accept a “K” style probe
Temp probe alone
Using the worksheet to diagnose AC not cooling problem
You’ll be conducting three separate tests where you’ll measure temperatures. Record the result on this worksheet so you can make a full assessment of the system when you’re done. Click on the image below to download a PDF version of the worksheet.
Step 1 Check compressor clutch operation — Diagnose AC not cooling
Start the engine and turn the AC to the MAX position. Pop the hood and examine the compressor clutch to see if the clutch disc is spinning. If so, you can proceed. If not, see this post to determine why the compressor clutch isn’t spinning.
If the compressor clutch is spinning and you have an orifice tube system, continue with the following tests. To determine whether your vehicle uses an orifice tube or expansion valve setup, see this post
Step 2 Conduct a condenser sub cooling temperature drop test — Diagnose AC not cooling
Start engine and run at 1,500 – 1,700 rpm
Turn AC to MAX and highest blower speed
Open all doors and run the system for about 5 minutes to stabilize the system before taking the readings.
Then close the doors and windows and set the blower to low speed. With the engine running and AC on, set blower motor to low speed.
Use the temperature probe to measure the LINE-IN temperature as close to the condenser as possible. Then measure the LINE-OUT temperature as close to the condenser as possible. Record the findings and calculate the difference.
A properly operating system should drop the temperature of the refrigerant by at least 20°F but no more than 50°F.
AC Condenser isn’t cooling by at least 20°F
1) Check for bent fins or debris in fins that are prevent proper airflow across condenser. If you find bent fins or debris, clean/repair and retest.
2) Check for proper electric radiator fan operation. In a two fan system, both fans should be running at high speed when the AC is on. Many systems have multiple speed radiator fans. Make sure the fans are running at high speed.
3) Check the condition of the radiator fan shrouds and seals. These are far more important than you think. They direct the airflow through the condenser and radiator. A missing seal can greatly affect AC cooling.
4) If the fins are clean and in good shape and seals and shrouds are good, but condenser isn’t cooling by at least 20°F, the cause may be:
• Internal contamination
• Too much refrigerant oil
• Refrigerant overcharge
Do not try any corrections until your finish the other tests
AC Condenser is cooling above 20°F and below 50°F but AC still not cooling
Move on to the System performance check
AC Condenser is cooling 50°F or more
Check for low refrigerant charge
Check for contamination
Condenser may have internal restrictions
Step 3 Conduct a system performance test— Diagnose AC not cooling
1) Measure ambient air temperature one foot in front of car’s front grille. Record the temperature on the worksheet.
2) Start engine and run at 1,500 – 1,700 rpm
3) Turn AC to MAX and highest blower speed
4) Set the system to recirculate
5) Open all doors and run the system for about 5 minutes to stabilize the system before taking the readings.
6) Then close the doors and windows. With the engine running and AC on, set AC to MAX and blower motor to on high speed.
7) Insert the AC thermometer into the center duct and wait until the dial reads the lowest temperature. Record the results on the worksheet
If the AC system is operating properly, you should see the temperature drop at least 30°F from ambient temperature measured in front of the grille.
AC system performance is less than 30°F
Check blend door for full closing (not passing heated air across evaporator coil)
Check the heater valve is fully closed (in vehicles with a heater valve located in the heater hose, make sure it’s not circulating hot coolant into the heater core)
Check the recirculation door to make sure it’s not pulling in outside air
Check for debris blockage at evaporator
Check evaporator fins for scale or bacterial buildup
Check refrigerant charge
Move on to evaporator superheat test
Step 4 Conduct an evaporator superheat test on an orifice tube system— Diagnose AC not cooling
1) Start engine and run at 1,500 – 1,700 rpm
2) Turn AC to MAX and highest blower speed
3) Set the system to recirculate
4) Open all doors and run the system for about 5 minutes to stabilize the system before taking the readings.
5) Then close the doors and windows. With the engine running and AC on, set AC to MAX and blower motor to on high speed.
6) Measure the temperature at the evaporator inlet as close as possible to the evaporator.
7) Measure the temperature at the evaporator outlet as close as possible to the evaporator.
For orifice tube systems, there should be NO difference in temperature between the inlet and outlet temperatures, but the maximum acceptable range is +5°F to -5° colder or warmer than the inlet temperature. Read this note on expansion valve systems.* For more information on superheat, see this post
AC evaporator outlet tube temperature is more than 5° colder than inlet.
The system is overcharged
The orifice tube O-rings are leaking liquid refrigerant past the orifice
The orifice tube is the wrong size for the system.
AC evaporator outlet tube temperature is more than 5° warmer than inlet.
The AC system is allowing superheat
Check for low refrigerant charge
Check for orifice tube restrictions—dirty filter screen, wrong orifice tube size.
Check for excessive oil in system
“The refrigerant charge level on a TXV system cannot be accurately determined by measuring evaporator superheat as it can on a CCOT system. By design, the TXV tries to maintain the appropriate level of superheating in the evaporator and increases or decreases refrigerant flow to match the heat load on the system. As the refrigerant charge level drops in a system due to a leak or from normal refrigerant loss as the system ages, the TXV will increase refrigerant flow to maintain evaporator superheat within specification. A TXV system will maintain a normal superheat value even when the system charge level has dropped significantly. For this reason, evaporator superheat cannot be used as a reliable method of confirming the refrigerant charge level on a TXV system.” 4 Seasons
Posted on by Rick Muscoplat
- ac evaporator outlet tube
- ac evaporator outlet tube temperature
- ac to max
- close the doors and windows
- compressor clutch
- diagnose ac
- doors and run the system
- engine and run at 1,500
- engine running and ac
- evaporator outlet tube temperature
- max and highest blower speed
- open all doors and run
- orifice tube
- orifice tube or expansion valve
- run at 1,500 1,700
- run at 1,500 1,700 rpm
- running and ac on set
- start engine and run