I had my AC refrigerant professionally changed to Red Tek in my van. It worked well for 2 or 3 years but then stopped cooling. I successfully refilled with difficulty and it was fine for a further year but is no longer cooling. I am supposed to have it to 33PSI on the low side, so when I check there with the engine off I see 32PSI, when I run the engine AC on and compressor running it reads 10PSI, when the compressors cycles off it goes up to 45PSI. Do I key on the 10PSI, engine, AC & compressor on and raise that pressure to 33? Jack


2 Answers 2


@Paulster2 is spot on. The AC clutch has to be engaged in order to set the correct baseline for high-side/low-side refigerant pressure.

I will add one point here that is often overlooked.

The ideal thing to do is find out the mass of refrigerant that is required to fully charge the system, because that is really what the whole system is designed for.

Here is an example for a Mitsubishi Lancer:

lancer example

Once the mass is known, have a qualified professional fully evacuate the existing system and then proceed to fill the right mass of refrigerant.

For topping up low pressure (as is the case here), it is difficult to ascertain how much mass is present in the system without drawing all the refrigerant out. In such scenarios it is more practical to top up based on the high-side and low-side pressures, albeit with less accuracy than the mass approach.

Measuring pressures are convenient, but the following information is required to do a proper job:

  • a corresponding temperature chart/table is provided
  • the ambient temperature needs to be known

You should be keying on it when the AC compressor is cycled on. Your 10PSI is way low no matter how you look at it. From the different videos I watched, I believe this one is probably about the best I've seen. It is quick and to the point. Basically, your AC pressure when the AC compressor is running should be between 25-45psi. This is going to vary by how warm it is outside. If it's really warm when you are doing this, your pressure needs to be higher. If it is already cold outside, the pressure is going to be lower. Remember, check the pressure when the AC compressor clutch is kicked on and the system is running. When the AC compressor clutch is not engaged, it is quite normal for the pressures to read into the red area of the gauge.

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