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Old 05-05-2010, 07:14 AM   #1
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AC Recharge


I have a 2003 Ford Expedition and the when I turned on the A/C for this first time this year it wasn't blowing cold air. Never had problems before so I figured it needed a recharge.

I bought the recharge kit and when I put the gauge on the low preasure tube it read about 20 psi. The clutch engages and disengages without any problem. I recharged the system without incident.

My question: When the vehicle is running and the A/C is set to Max Cool, the gauge will read approximately 35psi, but, when the clutch engages, the preasure drops to about 25psi. I don't want to overcharge the system but wasn't sure where the accurate reading should be taken, when the clutch is engaged or isn't?

With what I have done, the the air doesn't seem too cold. Just wondering if more should be added.

My father, a mechanic, consistently gives me the same old line "well, it could be a number of things" so that much I know. I am just trying to start with the easiest one.

Thanks in advance.

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Old 05-05-2010, 08:25 AM   #2
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Where your numbers taken at idle or 2500 RPM. What is the discharge (high) side reading?

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Old 05-05-2010, 08:51 AM   #3
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The numbers were at idle. I don't have a gauge for the high side.

I did notice that when driving, when the clutch engages it almost sounds like a gurgling coming from the vents.
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Old 05-05-2010, 10:05 AM   #4
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The reading should be taken with the compressor running, HOWEVER, what the low side pressure should be depends on the outside air temperature!

A pressure chart correlated to outside air temperatures would be in a factory service manual set of books which you can order from your dealer or helminc.com

The set of books costs around $150.00 (Get the right "tools" for the job which includes the specifications for pressures for your specific vehicle.)
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Old 05-05-2010, 10:28 AM   #5
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The reading should be taken with the compressor running, HOWEVER, what the low side pressure should be depends on the outside air temperature!

I had the ambient air temperature and correlating pressure on the can of refill so I went by that.

My question is whether the reading should be when the compressor is running or when it isn't. When it isn't the pressure is approximately 10 psi higher and I don't want to overfill the system. That would mean the the pressure in the system would be approximately 45-50psi when the compressor is not running. Is that acceptable?

Thanks for the replies
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Old 05-05-2010, 11:50 AM   #6
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Disclaimer, you need to know what your high side readings are. Think of your AC pressure readings as your blood pressure. You need both numbers to get meaningful results. Overcharging can cause the system to rupture.

Put on your gauge, start engine, turn on AC, allow reading to stabilize (record) turn off engine allow to stabilize (record) also record the ambient air temp. Post back your results.

With the system stabilized and not running, your low side will equal your high side (in psi) and should be approx ambient temp (F) for a charged system.
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Last edited by 47_47; 05-05-2010 at 11:52 AM.
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Old 05-05-2010, 01:57 PM   #7
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4747, you should just come to my garage and have a look, you're right down the road!

Anyway, thanks for the response.

I will check the readings and get back on here tomorrow.
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Old 05-06-2010, 07:07 AM   #8
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Last night I put the can of 134a into the system. My readings, after running the engine for about 5 minutes with the A/C on high, were:

Without compressor engaged - 45 psi
With compressor engaged - 32 psi

THe temperature was approximately 70 degrees so I think that I am a few psi low. The can shows it should be approximately 35 psi. The good news is that cold air began to flow from the vents.

After shutting the engine down, about 10 minutes, the reading was roughly 40psi
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Old 05-09-2010, 06:04 PM   #9
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In a perfect world everyone would have the right tools for the job. Being this isn't a perfect world you learn to make do with what you have.

A VERY general rule of thumb for automotive air conditioning; low side 25-45 (30 -35 target range for most) high side 2 to 2 1/2 times the ambient temperature. These reading are when the compressor is running and the air conditioner on high. It is normal for the low side pressure to rise once the system is no longer running since the system is trying to balance out.

You can also charge by vent temps; put your thermometer (the meat type works well) in the center vent, add refrigerant slowly only an oz at a time once you think you are getting close, give it time for the system to stabilize, once the temp no longer drops or starts to rise the system should be pretty close to full. (A slightly undercharged system will give you cooler temps)

Another ball park way to check refrigerant level, measure the inlet and outlet temp on your evaporator line. There should be about a 5 to 10 degree difference from the two.

You should make every attempt to find and repair the leak. There used to be a certain amount the EPA considered acceptable, what that is I do not recall and I am sure they now consider any leak unacceptable.

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