AC Freon Pressure... 35 psi... low or high?

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AC Freon Pressure... 35 psi... low or high?

Postby FishTail » Fri Jun 18, 2010 11:47 pm

QUESTION: What's the proper, and MAXIMUM AC Freon pressure? 35 psi ?

CONCERN: I don't want to overcharge, and break something.

BACKGROUND INFO: My AC this year seems to be 95% dead. Compressor fly wheel spins and radiator fan turns on, but vent temp is only about 68 degrees, when outside is 75. Under the hood, tube from expansion valve is only slightly cooler than ambient right at firewall. Freon currently reads at 35 psi when running... is this low ?

Thanks for the help,

(James: 2004 Vibe, 100k)

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Postby FishTail » Fri Jul 02, 2010 8:03 am

Can anyone answer the above question for me ?

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Re: (FishTail)

Postby tpollauf » Fri Jul 02, 2010 9:05 am

Quote, originally posted by FishTail »
Can anyone answer the above question for me ?

We'll take a shot at it seeing how nobody else did. As far as NORMAL pressures on the SUCTION side (large hose) with the compressor actually running, then 35 is close to normal and if all other conditions are normal, then you should be cooling. Is the compressor definitely running (engine bogs down a bit, etc.)? if so, run another test. Perform this test after the car's been off overnight, so it will be cold! IMMEDIATELY after the car is started, turn on the A/C. It should be cold air almost immediately. This test will rule out any "stuck open" heater core valve which you could possibly have. With a warmed up car, if you had leakage through the heater core, you'll never cool down the cab area no matter how well the refrigeration process is working! Back to the pressures, though, you are very close to "normal".

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Re: AC Freon Pressure... 35 psi... low or high? (FishTail)

Postby djkeev » Fri Jul 02, 2010 9:55 am


Pressure really doesn't tell you level of charge in your system.

The 1.8 AC system takes 23.6 oz of R 134a

Freon is an old term for R 12, a refrigerant rarely seen today in auto's and incredibly expensive when it is found.

Pressure on the low side will vary depending upon the ambient temperature and relative humidity in which you are operating.

For example,
with a 40% level of humidity, 80 degrees, your center vent should be putting out 50 degree air, the low side pressure should be 32 Psig and the high side will be 240 psig.

if it is 100 degrees, the low pressure will be 46 and the high 319 with a center vent output of 64 degrees.

If the humidity jumps to 80 percent that 100 degree pressure will be 62 on the low side and 381 on the high.

If you don't know what you are doing and don't know how much refrigerant is in the system there really isn't a proper number for all situations across the board, it varies greatly.

Proper diagnosis requires BOTH a high gauge and a low gauge and there are pages of diagnostic procedures to follow.

Modern AC is dangerous and under extremely high pressure. People can and do get hurt by doing stupid things to them. Really, I'm not a mechanic fan but this is best left to those who know what they are doing if you don't have the gauges, the knowledge or the special tools for pulling a vacuum and fixing the system correctly. Not to mention it is illegal to vent refrigerant to the atmosphere knowingly.

So you are sitting there with a cheap chinese gauge on your 16 oz can of 134a. That one can is over half the entire capacity of the system! Add too much and the problems will be as bad as, if not worse, than having too little. You can damage many pieces, most notably the compressor. That little can empowers people to do what they should NOT be empowered to do.
Hook up the can if you must, don't start the engine and open the valve. As a general rule of thumb, Does it read 50 psi? If not, you need some work done to the system. What? only someone experienced in ac can help you find out, most often for a fee.


(o ! /o) (o)=I=(o)

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Postby Esteban » Sun Jul 04, 2010 10:24 pm

You have gotten good advice. I have seen those R134 cans with attached gage, & the gage didn't work correctly. Every time you used in, it would show the exact same pressure. Too little or too much pressure is not good. A trained a/c tech is most often needed. Having a good A/C is worth it.

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Re: (Esteban)

Postby FishTail » Thu Jul 08, 2010 10:02 pm

MY SOLUTION: ...bad gauges?

I ended up taking it to 2 shops to fix my problem. The first shop charge me $70 to pump it with dye. They said I had a pair of BIG leaks, and a few smaller ones. Assuming they were honest, THAT MEANS THE DO-IT-YOURSELF GAUGES THAT SAID MY FREON WAS FILLED, WERE WRONG! And all this time my system was empty.

That first shop wanted $500 to $1000 to fix "everything", which meant they would replace major components. The second shop I went to, said they could "repair" the defective schrader valves (replace few small parts in them) vs replace the full hi and low lines. They also placed a gasket in another leaky spot, and said there WERE NO OTHER leaks. This second shop, cost me only $200. Now, I'm cold and happy.

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