A/C High-Side Fluctuates Rapidly

John Ingalls

Former Automive Service Department Manager
Aug 2, 2016
The purpose of this article is to answer previous questions on this topic and provide additional information on this subject. This article also includes frequently asked questions (FAQ's) from similar forums.

What would cause an A/C high-side to fluctuate rapidly at idle?

Several situations may cause this issue based on the year, make and model of the vehicle and this is common for some A/C systems. The compressor is the main component of all automotive air conditioning systems. The most common compressors used in the automotive world are rotary piston, scroll and variable displacement that function differently with the same end result. You must first identify the type of compressor used on your vehicle to determine the probable cause of this issue.

Some Ford Factory Manuals will actually include a statement similar to this:

Note: Vehicles equipped with a fixed scroll air conditioning compressor can experience rapid high side fluctuations during cold startups and when the refrigerant system has been recently recharged. This is a common characteristic of this type of compressor and not an indication that the A/C compressor is defective.

This condition is normally identified by rapid fluctuation of the high-side gauge of approximately 10 psi and the A/C will continue to function properly. You may also actually see the high side discharge hose pulsate or vibrate.

The size and the configuration of the A/C compressor may also be a contributing factor for fluctuations. Compressors can vary from a two to six cylinder configurations. The more cylinders working together results in less fluctuation. It is considered normal for a two cylinder compressor to be a little less stable.

Larger fluctuations of 50 psi or more are a sign of of possible defective components or an under or over charged refrigerant system. In this situation the system will most likely not be producing cold air. An A/C compressor with internal failure will normally affect the low side gauge reading as well. Refrigerant will bleed back to the low side though worn or defective reed valves or pistons raising the low side or cause it to fluctuate.

What should the low-side and high-side pressures be?

The proper pressures are determined by several different factors and will vary accordingly. Normal pressures range from 25 to 30 psi for the low side and 175 to 200 psi on the high side. The main factors that affect the A/C system pressures are the ambient air temperature and the RPM of the engine. The pressure rises proportionally when the outside temperature increases or the engine is accelerated.

An common example of calculating the high side pressure requirement is 2.2 to 2.5 times the ambient air temperature. If you don't have a calculator available twice the air temperature plus 50 will give you a ballpark figure. This means that if it is approximately 80 degrees outside and the engine is at idle the high-side pressure should range between 176 and 200 psi. When the engine accelerates the high-side pressure will increase. On most systems the high side should not normally exceed 250 psi unless the ambient air temperature exceeds 100 degrees. The low-side on most A/C systems should not normally exceed 35 psi. Of course, all basic rules are subject to exceptions based on the specific year make and model of the vehicle involved. These are just generic examples to briefly explain the concept.

What does higher than normal pressures on both sides mean?

The two most common reasons for this condition is insufficient condenser airflow or an overcharged refrigerant system.

Several conditions and components can affect the amount of airflow through the condenser coils. The condenser fan clutch could be malfunctioning because of a mechanical problem, an electrical circuit issue or a defective electrical component. The condenser fan may still be operating, but not properly adjusting to temperature and load variations. The most common cause for lack of airflow is dirty, obstructed or damaged condenser coils. A Thorough visual inspection can normally point you in the right direction.

Overcharging occurs when too much refrigerant is added or when the system is charged with air or refrigerant still in the system.


What if the low-side pressure is high and the high-side pressure is low?

This is a common indication that the A/C compressor is weak or defective. When a compressor is functioning properly is raises the high-side pressure and lowers the low- side pressure. Normally, if a compressor is weak or defective other issues within the system may be responsible for this occurring. A complete system evaluation should be performed to prevent damaging the new compressor.

What if the low-side pressure is low and the high-side pressure is high?

A low-side pressure reading is a text book example of a restriction within the refrigerant system. Several defective or clogged components can be responsible for this condition. The most common cause is a clogged orifice tube. The high-side pressure is evaluated due to the fact that the compressor is attempting to correct the issue.

What if the low-side pressure is low and the high-side pressure is low?

The most common reason for this situation is a low refrigerant level based on the A/C compressor that is incorporated into the system. If the system contains a variable displacement compressor that is malfunctioning both sides will read low as well. If a variable displacement compressor is in the system it is normally recommended to evacuate the system and measure the contains to avoid over charging the refrigerant system and damaging additional components.

Note: This article is just an information source to help you understand the operation of an automotive air conditioning system. You should consult with the specific technical manuals for your vehicle. I would recommend for this issue and any other automotive malfunctions or trouble codes that you start by reviewing the tech bulletins on your vehicle for known issues.

Mobile Dan

Mar 1, 2002
So much good info here...Title of thread should at least hint that many (more common!) symptoms are discussed.