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Tandorvir 7/22/2018

1989 Honda Civic DX 4 Cyl 1.50L

Body & Interior

A/C refrigerant pressure drops to 0 when turned on

Air conditioning hasn't worked since I got the car, finally decided to recharge. After recharging, I detected no change (after several days). I found that the refrigerant pressure was normal until the AC was turned on, at which point it dropped to 0. When turned off, it returned to normal. The "off" pressure hasn't changed over several days, so it doesn't seem like a leak. Any ideas on what the problem could be and what might help?

1 Answer


Jimm 7/25/2018

R134A pressures are as follows:
Low side- at idle and ambient temp between 70-90 degrees F, good pressure will be between 25-35 psi.
High side-same conditions, good pressure will be between 175-185 psi

The high-side line is smaller and heads toward the condensor after leaving the compressor. The low-side line is larger and runs from the firewall to the compressor.

The A/C system is designed to be charged with a certain amount of refrigerant measured in weight.

While the system will operate within a predictable range of pressures, pressures should really only be used to diagnose malfunctions within the system. Always start with a known proper charge. You don't charge by pressure. You charge by weight. You diagnose by pressures.

If your vehicle is cooling to a comfortable level (usually between 36-42 degrees is the best you'll get) at pressures on high side of between 175-190 +-5 and low side pressures between 30-35 +-5 at ambient temperatures between 75-95 there is no need to put more refrigerant into the system.


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