Loading... Please wait...

Diagnosing an A/C System

Posted by

The weather is getting hot again so let the A/C System failures begin. This is the time of year when we see countless drivers come in and ask us to “add a bit of Freon” to their truck. We can add freon, but most likely you have a different problem. Below is a list* of common A/C system issues to look for.

Cool But Not Cold

The first thing to remember is that you’re A/C system is not intended to make you cold. Every A/C system is different but you will see only a 25 -45 degree drop in a properly functioning truck a/c system.

If the air conditioning is set to max cool and fans on high, but is only blowing moderately cool air.

  1. Check to see that the cooling fans on the condenser or radiator are running when the air conditioning is on.
  2. Look for any restrictions like leaves, bugs or dirt that would keep air from passing over the condenser.
  3. Check the cabin air filter to ensure that it is not clogged.

You may need to check the pressures in the system using a Manifold Gauge Set. If you are not properly trained in using a Manifold Gauge Set please take your truck to a Certified A/C Mechanic.


When diagnosing an air conditioning issue, it's easiest to begin at the compressor.

  1. With the engine running turn the A/C on max cool, set fans high, and make sure that the clutch is engaging on the compressor.

Note: this is not the pulley, but the center piece that engages the pulley to the compressor shaft. If the clutch is engaging and disengaging rapidly, the freon may be low.

  1. If the clutch is not engaging, use a voltmeter to check for voltage getting to the compressor.
  2. If there is voltage - the clutch may be bad.
  3. If there is no voltage - a cycling switch may be bad, a fuse may be blown and the system may not have enough refrigerant pressure to trip the low pressure cutoff switch that cycles the compressor.


Leaks are the most common problem associated with the air conditioning system. If you are adding freon on a regular basis you have a leak.

If pressure is low, then there may be a leak.

  1. Check the Schrader Valves. (Very common problem and a cheap fix)
  1. Using a UV A/C Leak Detection Kit is the easiest way to find a leak.
    • Follow the instructions on the packaging to introduce UV dye to the system and UV light to find the leak.
  2. Check around all fittings to verify that they are all secure.
  3. Check hose manifolds on the compressor.
  4. Check the front seal and the o-rings sealing the pressureswitches on the back of some compressors.
  5. Check where the hoses are crimped onto the fittings.
  6. Check for pin holes in the condenser.
  7. Check where the evaporator drains condensation with UV light. Sometimes oil or dye can be seen.

Note: The dye check will not work if the system is too low and the compressor is not cycling.

*This list was barrowed in part from Auto Zone.

Sign up for our newsletter

View Cart Go To Checkout