Expert: Ernest (Ernie) Kenward - 6/22/2007

Question
QUESTION: I have a 1996 Chevy Suburban 1500 5.7L 4x4 with rear heat and AC.  My AC is blowing warm air.  While attempting to trouble shoot the problem, I noticed that the AC compressor will come on for approx 3 to 4 seconds, turn off for a few seconds and come back on.  I was told that is a sign the the R134A was low.  I went to a local auto parts store and picked up a recharge kit with gauge.  While doing some online research, I read that the max on the low pressure should be 50psi.  When the compressor is on, the pressure drops below 20psi, but when the compressor goes off, the pressure is near 50psi.  Any thoughts?  Thanks for any input that you may have.

ANSWER: Once the compressor cycles off, you will probably see the pressure go higher over time.  Not to worry, however, as long as the low-side pressure rises only when the compressor is off; the pressure rises only as a result of the pressure within the system equalising once the compressor is no longer pumping refrigerant through the system.  It is the action of the compressor which causes the pressure differential within the system, along with the flow of refrigerant, and also of heat, which is the purpose of the system.  Thermodynamics is a neat subject!

By the way, check out the "Duracool" website.  You might find it desirable to consider a hydrocarbon refrigerant.

Cheers ... EGK

---------- FOLLOW-UP ----------

QUESTION: So is it normal for the compressor to cycle on and off, over and over, for just a few seconds each time?
ANSWER: Sorry, I thought you already had the answer to that question!  Yes, frequent compressor cycling is one of the hallmark symptoms of low charge.

The compressor is protected from running with low or non-existent refrigerant by a low-pressure cutoff.  This cutoff acts when the system pressure is below a predetermined minimum value, a value indicating minimum refrigerant charge adequate for operation.  The cutoff automatically resets when the pressure - and amount of charge - is adequate.

Above that minimum, the compressor clutch will engage and the compressor will operate in response to the call for cooling from the cabin thermostat, and will not cycle, at least as a function of the cutoff.  Below that minimum value, the compressor clutch will not engage and the compressor will not operate, even if there is a call for cooling from the cabin thermostat.

At the border line value, right around the pressure setpoint of the cutoff, static pressure within the system may be sufficient when the compressor is off to enable the cutoff to allow the clutch to engage when called for by the thermostat.  However, once the compressor starts, the pressure at the inlet (low) side of the compressor will drop low enough to cause the cutoff to release the clutch.  The compressor then stops ... and the ambient system pressure once again equalises around the system, raising the pressure on the low side so that the cutoff resets, and the cycle repeats.

Charge the system!

You are not the only one working on air conditioning this weekend.  I have just got the system going in my recently acquired Mercedes 300CD this weekend.  Unfortunately, cool rainy westcoast weather is in the forecast for the foreseeable future.  Oh well ...

Regards ... EGK

---------- FOLLOW-UP ----------

QUESTION: EGK, Thanks for all of your help so far.  If I may ask one more question.  When I am adding the R134, should I only be doing it while the compressor is running for a few seconds, wait while it is of, then add again while it's running?
ANSWER: This is not critical, however, the system will generally accept refrigerant more readily through its low-side port when the compressor is running, as the pressure in that section will be lower to overcome by the pressure in the supply tank at that time.

Adding refrigerant relatively slowly is the best practice, whether the compressor is cycled on or not.  You want to add the right amount of refrigerant, not too much nor too little.  Adding too much could mean having to draw some off, using a vacuum pump and a recovery tank if we are to do it properly, so it would be better to err on the side of having too little, because you can always add more!

how easy to get it right depends on what sort of monitoring instrumentation you are using.  Generally, the idea is to bring down the low-side pressure while bringing up the high-side pressure.  The low-side pressure should finish up in the range of 25 to 30 psi ideally for many systems.  At the same time, if you can monitor the temperature of the air flowing from the dash outlets (fan on highest speed, thermostat controller set to lowest possible setting temperature or to manual ON, if possible), you can add small increments of R134 while watching for the minimum achievable temperature.  The optimum amount of R134 (or other refrigerants such as Duracool HC) will be the amount that results in lowest outlet temperature, other things being equal and non-varying.  Add refrigerant in small bursts and then be patient and wait to see what value the outlet temperature stabilises at before adding the next burst; again, TAKE YOUR TIME.  Stop adding refrigerant once the stabilised temperature begins to rise.

Btw, as you add R134, the compressor will cycle on for longer periods of time, ultimately staying on without further influence from the low-pressure cutoff.  The frequent cycling is an issue only when charge is low and the system pressure is borderline around the cutoff setpoint.

Have fun ... EGK


---------- FOLLOW-UP ----------

QUESTION: Yet another question.  Thanks for helping me to understand my AC system.  While doing research, I found that for optimal operation, I want a system pressure of around 50psi.  Am I correct in assuming that pressure is the operating pressure while the compressor is in operation and not the static pressure when the compressor is not running?  Right now, I have a static pressure of approx 60psi with a operating pressure of about 25psi.  I have been running the AC for the past few days while I drive it, trying to get the system to equalize.  My question is, how high can the static pressure go before I should start to worry?
ANSWER: Sorry ... I was too busy to get on line the past couple of days ... part of the joys of still completing some work-related projects (course manual revisions) even while my vacation clock is already running!

If your low-side pressure at compressor run is in the region of 25 psig, I would say leave well enough alone!  The 25 to 30 psig range is considered to be good for R134 systems while operating.  50 would be rather high, and I don't think I would use the word "optimum" to describe operation at this pressure.  (Think about it: which BP would you think would be best?  120 over 75, or 180 over 120 ...!  Being in my mid-fifties, I do long hill-climbing walks and 5BX workouts to keep mine close to the first.  I have been lucky enough to be low side of normal most of my life, but the time comes when one has to work at it!)  

With refrigerant quantity, one can have problems with too much as with too little.  Too much refrigerant makes the compressor work harder and lessens the heat absorption as the phase change from liquid to gas is inhibited by the excessive low-side pressure.  There are, therefore, diminishing returns to adding more refrigerant than is needed.

60 psig of static pressure ties in nicely with the 25 psig operating pressure.   Don't sweat the fact that there will be static pressure in the system - it is, after all, a closed system charged with a medium which has high vapour pressure at room temperature.  The system is engineered to take this.  The only no-pressure system that existed were the so-called ice-activated systems that were used on standard-era (1920's) heavy-weight six-axle railway passenger cars; the CNR had a lot of them up through the sixties.  In those, chilled water was pumped from ice bunkers under the car through the cooling coil in the car's main supply air duct, and then returned to the bunker to melt more ice.  Enough ice was used that the bunkers were replenished virtually every 150 miles, at engine crew change points.  Kind of neat seeing one of those being filled up, while a then-new 747 climbed over head.

A good book to look for would be "HVAC heating, ventilating and air conditioning" by S Don Swenson, published by American Technical Publishers, Homewood Illinois.

It sure does pay to keep one's cool, doesn't it?

Cheers ... EGK

---------- FOLLOW-UP ----------

QUESTION: Thanks again for all of your help, she is now blowing cold air!!

Answer
Glad to hear it and to have been of help.

Thanks also to you for sending in those kudos!

This task is not unlike teaching engineering.  One finds it easy to put in time and effort when dealing with those who want to learn, do their homework and ask thoughtful questions.  The value passed along passes both ways.

Cheers and enjoy your cool ride ... Ernie Kenward