Auto Air Conditioning: How to Evacuate the System
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Auto Air Conditioning: How to Evacuate the System

An auto air conditioning system should evacuated any time it is opened for servicing. the air and moisture that enters the system must be removed for the system to operate properly.

In the first two parts of this series, I covered recharging your air conditioner with refrigerant and how to trace system leaks. I also introduced you to the use of manifold gauges and electronic leak detectors. In this part of the series, you will learn about another essential tool for anyone serious about working on auto air conditioners. I might add at this point that these same tools could be used to service your home refrigerator, deep freezer, and air conditioners.

Single-Stage -Vs- Two-Stage Vacuum Pumps

The basic difference between a single-stage and a two-stage vacuum pump is the speed at which they pull a vacuum. Unless speed is a critical factor, a single-stage vacuum pump will serve you well. Robinair air conditioning tools are a status symbol among professional but tools of equal quality can be had for a fraction of the cost. A single-stage Robinair vacuum pump would cost you $300 or more while one made by U.S. General would cost you around $90. My point here is to pay for quality, do not pay for the name unless you want the status symbol.

Why Evacuate an Air Conditioning System?

When a system is opened to the environment for repairs, moisture enters the system. The moisture must be removed from the system for the system to operate properly. Moisture in the system will keep a properly charged system from producing the desired cooling effect. If connected, manifold gauges will indicate normal low-side and high-side pressures, but the system will blow warm air; the problem is moisture in the system. A system with air in the system will blow some cold air, but the manifold gauges will indicate abnormally high pressures on both the high and low sides.

Evacuate Before Recharging

It is a good idea to evacuate a system even if the system still has refrigerant in it. This extra step could save you from having to do the job twice because there was a leak in the system to begin with. If you pull a 50 micron vacuum and the system holds it, you can rest assure that there are no leaks in the system.

Compound Gauge –Vs- Micron Gauge

The low-side gauge on a set of manifold gauges is a compound gauge. A compound gauge measures a vacuum in inches of Mercury (Hg.) The problem with a compound gauge is that it is extremely difficult to distinguish between 28.9 and 29.9 inches of Hg. With a gauge calibrated in microns, it is extremely easy to tell the difference, it is the difference between 25,000 microns and 50 microns. When using microns, the lower the number the higher or stronger the vacuum. When evacuating an automotive air conditioning system, the vacuum pump used must be capable of pulling a vacuum under 500 microns. This low vacuum is necessary to be sure, that all moisture is removed from the system. Water does not boil until a vacuum of 1,500 microns is achieved.

How To Evacuate A System

  1. Connect the "Yellow" hose on the manifold gauges to the vacuum pump.
  2. Connect the "Blue" hose on the manifold gauges to the low-side service fitting.
  3. Close the high-side valve on the manifold gauges and open the low-side valve.
  4. Start the vacuum pump and let run long enough to pull approximately 30" Hg.
  5. Close the low-side valve and shut off the vacuum pump.
  6. Remove the "Yellow" hose from the pump and cap off.
  7. Reopen the low-side valve and check the compound gauge reading at interval to see if the vacuum is holding. Any loss in vacuum indicates a leak in the system.

Next time we will discuss intermittent cooling problems and their causes.

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Comments (5)

Good common sense advice! Thanks! :-)

Another helpful article!

Such useful information. TDS Thanks for your writing.


This is a nice writing !

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