I have used dry nitrogen to pressure test the A/C system in my vehicles when doing service work. However, I don't do it often enough to remember all the details.

On my Dodge truck, if I pressurize the system to 70 psi, even just the high psi side, the low psi side will equalize to to the high pressure psi. Is this normal or a fault of the compressor reed valve or some other?

Thanks,

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Brian Toscano

BTW - my Dodge uses a Sanden compressor. I am not sure of the innards. Does compressor style matter in my question?

Thanks,

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Brian Toscano

Yes, without the system running it should equalize. The idea of using dry nitrogen is that you put in about two oz. of R22, then pressurize with dry nitrogen. You can pressurize to well over 100PSI so that the R22 will be forced out of any leak present.

R22 is much more detectable by an electronic leak detector. The EPA allows up to 2 oz. of R22 to be released to the atmosphere under these conditions. Their thinking is that it is better to have two oz. of R22 in the atmosphere than pounds of 134 or 12 due to the leak.

If you don't use the R22 with the nitrogen and have an electronic leak detector, then the nitrogen is a useless step.

Good luck,

Yes I agree if you have a leak, it might be hard to find with just nitrogen. Last summer I used it to verify my A/C system didn't leak after I reassembled it (damaged from a deer accident) & before charging with R-12. Since it held pressure for a long time, I concluded it was leak free.

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Brian Toscano

You could use nitrogen as a cheep and easy way to verify if it will hold pressure, and if it leaks you use something not as cheep and easy to actually locate the leak.

It's normal for the system to equalize. The gas simply makes its way through the expansion device - orifice or TXV - through the evaporator, and to the low side service port. You cannot determine anything about the condition of the compressor; nitrogen could be leaking back through the reed valves or not, you cannot tell the path it is taking to reach the low side.

When charging an empty system, I generally liquid charge through the high side port into the vacuum with the engine off. The system fills really fast that way. When the low side starts to equalize, I'll stop charging, switch on the a/c, and evaluate the pressures. The charge can be topped off by vapor charging through the low side port.

- JimY