# [SOLVED] A/C compressor Cycling on/off



## Nutbuster

Can anyone help? In my 99 Suburban with the a/c system on, the compressor cycles on & off. Charge is ok according to the gages. The "on" time is only aboput 2 or 3 seconds. Off time is about 5 to 8 seconds.
Anyone have an idea?
Appreciate the help!


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## Midnight Tech

*Re: A/C compressor Cycling on/off*

Sounds like a bad pressure switch somewhere...it thinks the charge is low.


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## Nutbuster

*Re: A/C compressor Cycling on/off*

Yea I figured the same thing but I need to know where specifically where the switch that would cause this might be located. Do you know?


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## Midnight Tech

*Re: A/C compressor Cycling on/off*

Either on the compressor, the lines or the drier.


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## qldit

*Re: A/C compressor Cycling on/off*

Good Morning Gentlemen, I actually had a problem with a vehicle recently something similar to this and it was being caused by a thermal switch on the bottom of the engine coolant radiator. It had three switches mounted there, one for engine cooling fan, one to disable aircon and I have no idea what the other was for.

I had never seen one previously but in this particular case the electric aircon condenser radiator fan wasn't running, things are not so simple these days!!

Another case was where the expansion valve had decided to block itself.

I expect you have no "visi-glass" to be able to see if the thing is liquifying.

Does it have any cooling effect in the cab at all?

I imagine you are considering a system low gas pressure shutoff switch to the clutch.

That would normally be on the pressure side wouldn't it? I am a bit rusty on this stuff these days.

This thing wouldn't have a thermostat in the airbox or system somewhere by any chance?

If gas charge is correct you might need a system diagram for the damned thing with expected pressures and schematic.

Why are things becoming so complicated! Dual circuit cooling and all that stuff!

I am interested to see what happens to fix this problem.

Please keep advising what you find.

Cheers, qldit.


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## mack1

*Re: A/C compressor Cycling on/off*

You need to put a can of freon in it. The switch that is causing the cycling in in the low pressure line. You need enough freon in the system to keep the low pressure above the pressure switch limit when the compressor is running. 
I would suspect the pressure switch if it wouldn't cycle when the freon was low.


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## Nutbuster

*Re: A/C compressor Cycling on/off*

Update on what I've done so far.
Although I replaced the low pressure switch the compressor was still cycling. As I had stated earlier I thought the freon level was OK turns out it was low enough to make the switch do it's job. I put freon in it up to about 38 PSI or so and got the cycling to stop.
Now comes the harder part, Suburbans have two A/C systems, one for the front and one for the back passengers. Only one compressor, but two exchangers. The rear unit is blowing good cool air but the front unit is blowing hot air, like the selector switch is in the hot mode. I can't get it to blow cool air in the front of the truck no matter what position the temperature selector switch is in. although the rear unit is blowing 58 degree air.
Thoughts???


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## Midnight Tech

*Re: A/C compressor Cycling on/off*

Front orifice tube/expansion valve.


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## mack1

*Re: A/C compressor Cycling on/off*

Hi Nutbuster,
I was quite sure that the first problem was low freon pressure. I'll venture a guess for you that probably will prove to be true. There is only one expansion unit (exchanger) that services both front and back. I would suspect that the controls for the back are more involved in mixing air for delivery to the back from the exchanger than controlling another exchanger. I also suspect that both the front controls and back controls are vacuum in nature and you probably have a vacuum leak in the controls for the front part of the system. All this is guess work. I'd buy a book if it was my system and get on with it. 

Have a nice day.


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## Nutbuster

*Re: A/C compressor Cycling on/off*

Midnight Tech, Where would the front orfice tube expansion valve be located?

Mack1, I do have a drain out the rear end for the rear unit that dumps water when the rear unit is running. That is why I was under the impression I had two exchangers.

In continuing efforts, I'll keep you guys updated!
Thanks for your help so far.


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## Nutbuster

*Re: A/C compressor Cycling on/off*

Midnight- I found the orfice. Will replace, evac, recharge, and see what I have.


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## mack1

*Re: A/C compressor Cycling on/off*

Well Nutbuster
Guess Mack1 is sitting here with egg on his face.....not the first time. If you are going to evacuate the system and recharge etc. sounds like you are on top of things and Mack1 might have been wrong about the two evaporator units. Tell me if you find two of them. That second drain smells like there is two of them. Anyway, the thing I said about the vaccuum leaks still has merits. At the age of your system, the hoses deteriate and become problematic. I have a 96 s10 blazer and the hoses on it are getting crumbly. I've replaced some of them, but not all. I'll be under the dash soon I'm sure. 
Oh, and yes. Someone mentioned that you shouldn't replace the pressure regulater until necessary. I fully agree. The thing is mounted on the spider (a unit that contains all the injector units under the first level of the intake manifold). It is hard to get at and should only be replaced if necessary. 
Have a nice day


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## Nutbuster

*Re: A/C compressor Cycling on/off*

Update;
Changed the orfice tube. The location: I found it standing in front of vehicle viewing head on to the grill on the left side of the radiator there is a point where the line actually splits. The orfice was inside the tubing. I actually got to use, and needed those "hard to place" right angled needle nose pliers I've had for years and have never needed before. I also had to use an inspection mirror to look inside the tubing after I broke the fitting apart. It was 1-1/2' down into the engine compartment and facing towards the rear. What a location!
It's fun learning something new like this. Anyway, I also replaced the high pressure valve that you hook your charging / evap lines to as it was leaking.
I evac'ed the system for about 1.5 hrs. due to the length of the lines that travel to the rear exchanger, I wanted to make real sure all the air was out. I turned off the pump, closed the manifold valves, left it sit overnight. It still had about 25" hg this morning so I'm evac'ing again for awhile this morning and will charge in a little while.
I SURE HOPE THIS IS THE FIX!!!
I'll let you know.
No Mack1, you've no egg on your face. Appreciate the help!! I get up with you later and let you guys know what happened. Wish me luck!!


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## Nutbuster

*Re: A/C compressor Cycling on/off*

Fixed!


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## Nutbuster

*Re: A/C compressor Cycling on/off*

It was the orfice tube! Thanks so much! I got it blowing 48 degree air. Outside temp is 70, LP side was 37.5, HP side was 158!
Perfect!
Now I have to wash the dang thing!! hehehe! Then start on the tie rod ends!
Thanks guys! I'll be back on a different thread with a different problem soon as my truck has 270K on it!


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## mack1

*Re: A/C compressor Cycling on/off*

Good show Nutbuster.

Have a nice day.


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## Midnight Tech

*Re: A/C compressor Cycling on/off*

NB, how badly gunked up was that orifice tube?


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## Nutbuster

*Re: A/C compressor Cycling on/off*

Midnight, You couldn't see through it. It was completely clogged!


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## qldit

*Re: A/C compressor Cycling on/off*

Good Morning Gentlemen, congratulations all round, very clever fellows, I stand in awe!!

Nothing like the satisfaction of seeing something difficult functioning again.

Well done, and thankyou for the ongoing status reports.

Cheers, qldit.


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