Monday, December 14, 2015

Fixing a Classic Chevy Auto Air Conditioner System with a Manifold Gauge Set-Part 1

Fixing a Classic Chevy Auto Air Conditioner System with a Manifold Gauge Set.
Diagnosing an  Auto Air Conditioner System on a Classic Car.  
Classic Air Perfect Fit A/C Install.
Retrofit A/C "Hillbilly How To".
Reflections of My Experiences of Installing A/C in My Classic Tri-Five 1957 Chevy.
  -Alan Arnell

So you did everything the directions told you and you have completed your retrofit A/C project for your BelAir.  Life is great, until you notice no cold air is blowing from the vents.  First be happy that nothing is on fire or being thrown from the engine bay.  Second, there is still hope, because you have a manifold gauge set.  There are many tools out there to fix A/C systems, but even for the most seasoned auto mechanic the manifold gauge set is the first, if not, the primary weapon in the arsenal for diagnosing a bad A/C system.

As a perquisite to this blog it helps if you understand how a refrigeration cycle works.  I discussed the cycle in the previous Blog section "How an A/C Works".  Understanding the role of pressure on the different A/C parts in your system is the only way to successfully fix the A/C' dysfunction.  The gauge set measures the pressure in the system to pinpoint problems.

If you are not familiar with a manifold gauge set you may read my blog section  "Manifold Hose/Line identification and Use"  for help.  In the earlier blog, I explained what the different gauges were and explained the color coding of the lines.

You now have the basics of the manifold gauges, but what are all those numbers on the two gauges? The blue gauge is the compound gauge.  We will refer to the compound gauge as the low side pressure gauge.  What all that means is the gauge is used to measure pressure or vacuum. The numbers around the outside of the gauge shows pressure in pounds per square inch (PSI).  The numbers near the bottom shows vacuum in inches of mercury.  The small scales in the middle of the gauge list the temperature relationship of different refrigerants.  The scale designated as PSI is the one you will use to read the system’s pressures when charging and diagnosing an A/C system  The gauge reads for 0 to 120 PSI.

The red gauge on the right is the high pressure gauge.  The high pressure gauge measures, you guessed it, the high pressure side of the A/C system.  This gauge only reads pressure.  As common  sense would dictate, this gauge measures the higher pressure side of the system.  The gauge therefore reads from 0 to 500 PSI.

Static Pressure
With the A/C system off and the temperature is stable that pressure reading is called static pressure.  If the manifold gauges are hooked up to measure static pressure the low and high side should be the same or very close. Both the high and low sides have equalized within the system.

Refrigerants  have different static pressure at different temperatures.  So, that static pressure of the system changes with the up and downs of the outside the A/C system temperature. The higher the  heat the greater the pressure.  There are charts out there to find static pressure related to different temperatures. Here is a link to a 134a System Pressure Chart chart of static refrigerant pressures

Static pressure can not be used to determine if a system is fully charge.  However, a good rule of
thumb is if the static pressure is 87 PSI at 79 degrees you can assume the system has some amount of liquid refrigerant.  The system may be or full not not full.

However, if at the same temperature of 79 degrees F the gauge shows 75 PSI then you would know the system is low.  This is because the static pressure on the gauge is lower than what is listed on the chart for your outside temperature.

So, if  you can't tell if you have one ounce or 20 pounds of refrigerant in the system with this test what is the point?  This test tell you there is enough pressure to satisfy the low pressure switch and let the compressor to do its job.

Static pressure may also be used to see if a can or more likely a jug of refrigerant has water in the jug.  Bad!

Your static pressure should not be lower than 50 PSI when leak testing.

Short Cycling
Have you ever witness an A/C system that the compressor clutch keep cycling on and off?  The rapid on and off of a compressor is referred to as short cycling?  That is because most systems have a pressure cutoff switch that turns the system off at around 20 PSI.  Even with low refrigerant after shutting down the pressure will build back up to 45 PSI allowing the switch to turn the compressor back on to just turn off again at 20 PSI.

With that knowledge most static pressures needs to be around 45 PSI for the system to work

Let's say you only have 45 PSI and warm air blowing out of the A/C vents.  You look at you gauge and the suction side draws pressure, the low side gauge will drop quickly and more than likely the compressor will shut on off.  The gauge will climb back to  45 PSI as the sides equalize.

Charging Pressures
I bet you would want to know what the pressure should be when testing.  But the experts can't say for sure.  Too many variables for them to say definitely.  It would be nice if they would just say 30 on the low and 200 on the high but, they won't.  Funny, and amazingly enough, when things are all said and done the gauges will be close to 30 and 200.

Here is why:  A reading of 30 on the low side is close, because that translates into an evaporator temperature close to the freezing point of water.  Remember that the left gauge has temperature scale.  The low side all-in-all is really measuring evaporator temperature.  We want to keep the evaporator temperature usually above 32 degrees F, because that part of the system is prone to  collecting water.  Frozen water is of course bad in your system. Generally, 134a's low side is slightly lower than freezing of water at around 27 PSI.

High Side
With the old R12 systems the high side pressure was usually 1.8 to 2.1 times the outside temperature.  Therefore, on an 80 degree day with average humidity the gauge would read 144 to 168.  On hot humid days like we have in Houston the old R12 pressures could go up 100 PSI.

With 134a it is common to see high side pressures at 2.2 to 2.5 times the outside temperature.  So that would be around 176 to 200 psi on the high side with 134a.

Due to the heat load on the evaporator, humidity in the outside air, airflow across the condenser  and engine speed makes the specific range to vary.

Meaning, that on you average 80 degree F day, the high side should be around 176 to 200 PSI.

Please check back for Part 2 of
Fixing a Classic Chevy Auto Air Conditioner System with a Manifold Gauge Set


  1. Hi, that is a great guide to repair A/C. My dad used to repair his old car all by himself. But I prefr to use service of prodessional specialists, like this They are qualified and expirienced to do their job well.

    1. I agree for my daily driver cars. For my hobby car learning to do mechanical jobs is part of the fun of the hobby.


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