My a/c compressor is cycling off and on at the wrong pressure. It's cycling on at about 50-55 psi, which as far as I can tell is about right. But it's cycling off at closer to 5-10 psi, which is too low. And it's cycling quickly, meaning it's on for about 5 seconds, and off for only about 5 seconds. It's not on long enough to really cool anything down, but it does start to cool down as it kicks off.

I've checked the freon pressure (obviously from that above), and it's fine. I've also replaced the cycle control switch that I understand is supposed to regulate the cycle time based on pressure (which is in part based on freon temprerature).

Any suggestions? I'd drive the car a lot more if it weren't for this little problem. But the high temp/humidity levels are keeping me in my other car (not a bad thing in itself). Never the less I'd like to be in the Vette a little more.

Cheers!
W

 

Sounds like you do more AC then me however whenever I have had that problem it was due to the Freon level being low.

 

Your cycling off because your pressure is to low. You did not say what your high side was reading at what temps. Just by what you show looks like you are low on freon.

 

Faulty Low Pressure switch - assuming your gages are accurate, but if it's pulling down to 5/10 psi, it's sounds like a plugged up orifice. If R12, switch threshold is 25 psi; for R134, it's 22.5 psi. They're cheap enough so I'd just replace it, and then get the high and low sides. Have you done any work on it and if so, what did you do?

 

If the system is low........then when the compressor kicks on, it pulls the gas to a pressure that that is too low and the low pressure switch kicks off the system.

Now Freon circulates around the closed loop system and the low side starts to see a rise in pressure...the low pressure switch allows the compressor to kick on again

the whole cycle starts all over again.

Just fill your system and see what happens.

 

I wouldn't just juice it up - the switch is the freeze protection device and pressure will vary with outside air temp. At 70 or above, it won't matter; but drop to 50 and run the defrost and it's going to make a big ice cube out of the Evaporator when it doesn't cycle off until 5/10 psi. Also, if it's plugged up, adding gas can blow it up if the high side cutoff doesn't cut it off.

 

^^When the compressor kicks on and you can see the pressure drop below the level....and then it kicks off.......wouldn't it make sense that there is not enough freon?

I know you know a lot about A/Cs......

but when My system is low, the A/C cycles just as the OP describes....when there is enough gas for proper circulation, the Low side needle stops pinging high then low and starts to give a constant reading in the green section of the dial.

I can add another can and get the needle to the higher part of the green section at times.

The only time the A/C compressor cycles in 5 second intervals is when my slow leak has allowed enough gas to slip out and the system is just on the cusp of not having enough to register with the low pressure switch.

Even in times like that, when I don't have time to stop in and get a can of freon.....if I drive for a while and the whole system warms up with the engine temps, the cycling stops as the gas has expanded enough to keep the low pressure switch from cutting the power to the compressor....

 

Close, but not exact. Pressure will vary according to temp as you have experienced. System is designed to maintain a range of pressures, both high and low, but it can't overcome the effects of the outside air temp or even maintain consistent pressures due to variation in compressor speed. To do what it can, it uses one or more cooling fans (at low speed) and the Low Pressure switch to cut it off before it makes ice cubes (or runs completely out of gas) and monitors the high side to make sure the compressor is making pressure, but not so much that it blows a hole in the hood. Throw in a variable that it wasn't designed for - like a plugged up line - or run without enough gas to carry the oil that lubes the compressor and bad things happen. GM designed these later years to last (well at least through the warranty) by using the ECM to monitor both parameters - high & low - for deviations from normal. Normal is outlined in the pressure/temp charts for whatever gas it's using which is universal for all mobile, fixed orifice systems. GM's ditched this and uses a variable stroke compressor (C5 and most builds except some trucks) to overcome some of the systems' weaknesses and the hybrids are using a constant speed (like your home unit) electrical unit. Of course, neither of the latter matters, though they can still plug up, leak gas and suffer from lack of lube, so most of the OEM's still monitor the high side to keep them together while it's on their dime.

 

As mentioned above, it sounds like a clogged orifice tube. If it were my car, I would evacuate the system, change orifice tube and O-rings while it's down, accumulator/drier, vacuum system and recharge with specified amount of refrigerant and oil.

 

And if you find a lot of metal in the orifice, you may want to consider a new compressor, though if this is a Nippo ('88 through '96), they seem to hold up pretty well even after shedding a few shards of aluminum.

 

Master SunCr, what do you recommend as a flushing agent to remove contaminants from an A/C system? I've heard some people use brake parts cleaner. Is there a product made specifically to do this?

 

Brake Parts Cleaner works for me, but I only flush the lines. Unless there's a hole in the orifice, the only thing in the Evaporator (which you should have a shop leak check with an electronic detector before you start) and beyond is an oil film and the Condensor is impossible to flush, so I've replaced it when rebuilding. You can get a flush agent and probably a kit at www.ackits.com, but I've never found it necessary as a DIY. Anyway, hold a clean cloth over one end of a line and spray Brake Cleaner until it runs clear then follow with a blast of compressed, filtered air. Compressors should be flushed with whatever oil you're going to use. Dump out what's in it and add 4 ozs back. Turn the shaft a dozen or so times in one direction, then the other, drain and repeat (good way to make use of the quart you have to buy). Leave 4 ozs in it and put the balance of the oil charge (8 ozs for Vettes and most of the GM's I've owned) in a new Accumulator. That ensures a good supply when you crank it up.