# AC compressor cycles on and off



## mpd1966

I have a 1998 ford escort with 140k under its belt. Here is the senario: My ac did not work so i decided to try to fix it. I replaced a relay with one i bought at radioshack and the ac finally started working. The problem is that the compressor (i think it's the compressor) cycles on and off. on for about 4 sec, off for 10. It is pretty noticable, expecially at highway speeds. the charge is full at 45 psi. what could be the problem? any help would be greatly apreciated


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## Wrench97

Did you have High side and Low side gauges on it if so what were the readings when it was on?
Cycling can be several things Low charge will cause the Low side to drop below 10psi and open the low side switch, Too much refrigerant or a blockage will cause the high side to get too high and the high side switch will open. Bottom you need both gauges to properly diagnose it . If you see frosted spot on one of the A/C lines there may be a blockage.


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## mpd1966

I used the gauge that was on a freon canaster. It was on the low side. I had the ac to the coldest setting and fan on high and got the 45 psi reading. I didn't see any frost, but i also didn't look at it for long either.


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## Wrench97

Was it empty or low, How many if any, cans did you put in?


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## mpd1966

My freon level was full, so i did not have to add any. I just used the gauge off a freon can I borowed from advanced auto parts. It looks like a pretty cheap gauge, so i don't know how reliable the reading was. It read 45 psi (max recomended psi on the can). I was suprised to see that It kept a full charge for about 5 years


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## Wrench97

Yea well that's a little misleading the only correct to "Charge" the system is to evacuate it and reinstall by weight all the Gauge/Pressure methods are only approximations, however I doubt this has much to do with your problem, If it's cycling it's hard to tell without Hi side/ Low side gauge readings where to look I.E. if the high side spikes quickly and the clutch disengages there is a blockage between the compressor and up to and including the expansion valve. The faster it spikes usually means look closer to the compressor, If the Low side drops then you look between the expansion valve and the compressor. This is kind of oversimplified but I hope it points you in the right direction.


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## Raylo

Like Wrench said it is nowhere near as simple as a pressure reading. The only time they are really relevant is with the system at steady state with good airflow over the condenser. And in a properly operating system the low side pressure will have to get a lot lower than 45 psi to provide adequate cooling. When you first start the a/c both high and low pressures will be equal based on ambient then will separate as the compressor, condenser and expansion device go to work. It takes a while for all this to reach steady state. Engine RPM (which changes compressor speed) is a contributing variable, too.

If your car has what's known as a CCOT (cycling clutch orifice tube) system the clutch is supposed to cycle off depending on cooling demand, usually around 21-23 psi depending on car make. If it is cycling off when the vent temps are warmer than they should be it *probably* means that your charge is low, which probably means you have a leak somewhere.


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## mpd1966

Thanks for your help so far, I'll try to get both pressure readings as soon as i can find a gauge that will test the high side. If i can't fix this problem will it hurt my engine or fuel economy? also, there is no frost whatsoever on the lines, only condensation.


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## Wrench97

Does it blow cold/cool air?


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## mpd1966

can my charge really be low if my ac blows pretty cold air? raylo, you said it had to get a lot lower than 45 psi for proper cooling, I thought that 45 psi was in the correct psi range. the cycling is most predominant when the car is at highway speeds (car already cool), so its hard to say if it cycles before it gets cold


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## mpd1966

ya, after a few minutes its really cold in there


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## Wrench97

I think you may just be be a little low on freon.


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## Raylo

That gage you are using is just a rough check and really means next to nothing without accounting for all the other parameters. The compressor is supposed to cycle off when the cabin temp cools. This happens when the heat load gets lower and the compressor is able to pull the low side pressure down to the setpoint which is usually between 21 and 23 psi. Then it kicks back on at about 40 psi which happens in a couple of seconds. The whole purpose of this is to keep the system from freezing up the condensation on the evaporator and accumulator. If you had the gage hooked up while you were driving you'd see it work.

Bottom line is that if the air is cold enough there may be absolutely nothing wrong with your a/c as it is. If it is a little too warm you may need to top off the charge.


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## A#1

what does the viewer window in the receiver/dryer look like if it has one...if the R/C has the view window and the fluid flowing looks foamy instead of clear...means a slight leak and air in the line...a cycling compressor is usually a sign of low freon levels


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## Wrench97

A#1 said:


> what does the viewer window in the receiver/dryer look like if it has one...if the R/C has the view window and the fluid flowing looks foamy instead of clear...means a slight leak and air in the line...a cycling compressor is usually a sign of low freon levels


U.S. cars haven't had sight glasses since the late70's early 80's


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## A#1

wrench97 said:


> U.S. cars haven't had sight glasses since the late70's early 80's


lol...your probably right...that's why i like my old 1969 'Cuda Hardtop...two A.C. belts...and one waterpump/fan belt...still runs like new...don't drive it much...maybe a few times a year in the summer...


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## drgerry

A#1 said:


> lol...your probably right...that's why i like my old 1969 'Cuda Hardtop...two A.C. belts...and one waterpump/fan belt...still runs like new...don't drive it much...maybe a few times a year in the summer...


CUDA!?!?!?!? I had a '70, "Baracuda", 383 w/440 shaved heads, Muncie "rock-crusher" 4-speed w/Hurst shifter, Holly double pumper; N50-15" on rear (that's how they numbered tires back then); dual pipes with glass-packs

OH! . . .those were the days!!!!!!!


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## Basementgeek

This almost 5 year old thread is closed.

BG


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