2001 Dodge Durango Guage fluctuation on AC rail

  • V8
  • AWD
  • 130,000 MILES

I recently made a mistake, while changing out my radiator, of loosening an Air Conditioning high pressure line. Some of the freon (or whatever you call it these days) leaked out. I was able to re-tighten the fitting, and so I don't think I completely emptied the coolant.

Since, I have noticed anytime I put on the defrost or AC, I can hear the compressor kick on, make a small "hiss" for 3 seconds or so, then kick off. And then repeat in an eerie melody for as long as the AC or defrost is on.

I've purchased a re-fill can from Walmart with a guage on it. Up until now, I've been assuming that it's due to low (or extremely low) levels of coolant in the AC system. When I put the guage on (with the AC on Max as instructed), the guage fluctuates in rythm from 0 psi up to about 50-60 psi in that 3 second "hiss" time. (On the can, the needle reaches the yellow or orange. And says do not add - have someone inspect the system.)

I'm assuming this is the AC system trying to pressurize, then realizing there is no flow (due to low coolant), and shutting off the compressor.

Is my assumtion right? I would have expected to simply see low pressure, not ramping pressure that drops and re-ramps in repetative fashion. I don't want to add more coolant to the system if that's not the problem, but I don't know what else to make of the guage reading. If anyone could help, I'd appreciate it.
Do you
have the same problem?
Monday, April 6th, 2009 AT 3:50 PM

1 Reply

Is the gauge on the high or low side hose? You should be charging from the low-side port. If the system is low on refrigerant, the compressor will draw the low side down to a pressure that is too low. A low-pressure cutout switch will cause the compressor to turn off to prevent drawing the low side into a vacuum. If there was a leak in the system, drawing the low side into a vacuum could allow air to enter bringing along the humidity with it. Moisture in the system combines with refrigerant to form an acid which will destroy metal parts.

Once the compressor was switched off, high and low side pressures will equalize, and the compressor will turn back on. The hissing you're hearing is the refrigerant changing from a liquid to a vapor at the wrong point in the system. This is supposed to happen in the evaporator in the dash board where you won't hear it.

A better way of observing the state-of-charge in the system is to watch the sight glass on top of the receiver / drier for bubbles. You will not see any bubbles when the system is very low on charge or fully charged. In your case, you know it's not fully charged. The refrigerant will go in much faster if you place the can in a pail of hot water.

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Tuesday, April 7th, 2009 AT 9:43 AM

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