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AC pump repeatedly kicking in and out in certain modes


gw33gp

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Recently my AC pump started cycling on and off when the engine in at idle in AC modes. I know the AC activates in many modes to keep the windows from fogging up. Initially when it kicks on the rpm drops so much that the engine feels like it is going to die. It doesn't seem to do it when driving at higher rpm or at least I don't notice it.

I replaced the AC clutch cycling switch several years ago when it would do this sometimes with the AC on. That solved the problem then. I know AC pump can cycle on and off at a slower frequency when the charge is low, but the AC is working fine.

Is there anything else I should consider before changing the AC clutch cycling switch?
 


franklin2

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It will cycle more frequent in the winter. The cycling or low pressure switch is how it controls the cooling of the system. If you put a bunch of cold air through the evap coil in the dash, that will drop the pressure of the refrigerant and make it cycle off.

Now if it cycles during a hot day in the summer, then that is usually a sign of low refrigerant.
 

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I would put a set of gauges on it and check pressures. It can be low and still cool reasonably well. But not enough refrigerant to maintain proper low side pressure.
 

gw33gp

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I have done some more diagnostics on this problem. I realized it was doing this in all modes that calls for the A/C to be on.

I checked the low-pressure side. I don't have a gauge for the high-pressure side. I checked it at 65F and the pressure on the chart indicated it should be 25-35. That is what it was cycling at. I added a little freon to see if it would help and it did not. I got it up to 27-37.

What it was doing is; when the compressor was on, the pressure would go down to 25 and the compressor would kick out. Then the pressure would slowly rise to 35 and the compressor would kick in. That cycle would continue as long as the engine and A/C were on. I am guessing the A/C clutch would kick in and out about every 10 seconds. Meaning a full cycle was about 20 seconds.

Does that mean the A/C cycling switch is working properly? Is there something else that is causing low pressure side to cycle so quickly?

I may just take it to a pro to get it fixed, I just don't know who to trust. If I was in Kansas, I would have my brother do it but that is a long drive to have an A/C system looked at.
 

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Sounds like the switch is working properly.

To me it still sounds a bit low on refrigerant. However at 65 degrees it might be normal. I would let ambient temps get closer to 90 and see if it does the same thing.
 

gw33gp

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I only know a little about how A/C works but I thought too that the switch seems to work properly. What I don't understand is why the pressure goes up and down so quickly.

I didn't have this problem before. It seemed like the A/C clutch rarely cycled. It just activated when I turned on the A/C and deactivated when I turned it off. I have had an A/C in another vehicle cycle on and off like this when low on refrigerant. If it was low, I think adding a little would make an improvement, but it didn't.

I will be in Phoenix in April. Hopefully, I can test it then to see if it works better.
 

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What I don't understand is why the pressure goes up and down so quickly.
Lack of charge capacity. When the charge is low, when the compressor turns on it sucks up what (too little) refrigerant is available, which drops the charge pressure below the pressure switch lower limit. That turns the compressor back off, which lets the charge pressure build back up enough to close the pressure switch again, turning the compressor back on.

Yes, "it's a vicious cycle".

A full charge means that there's adequate refrigerant everywhere it's supposed be, and in the state that it needs to be (gas or liquid).

Without the equipment to service the system, it'd be a good idea to just find and pay a competent AC shop to charge it. Then see how it does.
 

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It goes quickly because the A/C compressor is a efficient pump. It sucks the refrigerant out of the evap coil very quickly. You may be correct, it may be a little low on refrigerant.

Since you are limited on the equipment you have, I would wait till warmer weather or when you get to a warmer climate. Get your can of refrigerant ready and run the system for a little bit. Make sure the windows are down and the fan is on high. You want lots of warm air flowing over the evap coil.
(That reminds me, that is another thing you can check. Most systems, you can take the blower fan out of the system, and take a peek at the evap coil. Over time they can get clogged with dirt, which turns into mud since the evap coil is wet when it's cooling. This can block the airflow through the evap coil,, and that will keep it from transferring heat from the air into the evap coil and the refrigerant. If the evap coil is clogged with dirt, that will aggravate the low pressure problem.)

If you coil looks good,, if it's warm outside, you can get the A/C going and evaluate it's performance. If you are familiar with your truck, you can tell if the air is cold enough coming out of the vents. It should start hurting your hand if you hold it in front of the vent for a little while. You can then add refrigerant till it feels about right. This is a crude way of doing it, but actually works pretty well if it's high 70's-up in ambient temps.
 

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If the system is truly low... when the compressor engages it sucks the low side (which is low on refrigerant) pressure down quickly. Without the volume of refrigerant... the compressor out paces the system... causing pressure to drop below the low pressure switch OFF threshold. System clutch disengagement. Once ambient and underhood temps raise the system pressure above the low pressure switch ON threshold... system engagement. Vicious cycle continues until you shut the system down.

Now the reason why a full system cycles fast... is simply the pressure/temperature relationship of refrigerant. If it's cool... and let's call 65 degrees cool... there will be far less heat transfered into the evaporator then if it was 90 degrees. That heat transfer and resulting pressure increase in the refrigerant keeps the low side pressure above the system cut off pressure.
 

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It was like that when I found it.
If the cycling is bugging you and you just want to shut it down until you can diagnose it, you could just pull the relay or pull the connector off the low pressure switch.
 

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The refrigerant pressure changes with the ambient temperature according to the ideal gas law - PV = nRT. As stated earlier, if the ambient temp is over 90F and the a/c cycles, then it is almost certainly low on refrigerant. However, if the ambient temp is 40F the system will cycle and that is totally normal, because the total pressure in the closed system is lower due to the temperature being lower, so the low side will get pulled down low enough to trip the low pressure cycling switch, which is around 25 psi. You didn't say what the ambient temp is when you are having this cycling, but my guess is that it is quite a bit lower than 90F, which would mean everything is normal.
 

gw33gp

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Thanks everyone for the information. That helps me to better understand how the system works. Actually, that is close to what I was envisioning, but now I can see it clearer.

I don't want to over fill it, so I think it is time to take it to a pro. I will check the condenser first.

I prefer not to pull the relay because it would not defog the windshield as well. What bothers me is at idle it almost kills the engine each time it cycles the A/C on. I have been holding the rpm up a little with the accelerator pedal when stopped at stop lights to keep this from happening.
 

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Thanks everyone for the information. That helps me to better understand how the system works. Actually, that is close to what I was envisioning, but now I can see it clearer.

I don't want to over fill it, so I think it is time to take it to a pro. I will check the condenser first.

I prefer not to pull the relay because it would not defog the windshield as well. What bothers me is at idle it almost kills the engine each time it cycles the A/C on. I have been holding the rpm up a little with the accelerator pedal when stopped at stop lights to keep this from happening.
Sounds like you have idle air control problems also. There should be a line from the A/C system to the computer, to tell the computer to change the idle strategy to prepare for the A/C compressor load. But the only way the computer can do this is with the idle air control valve. Possibly it and your throttle butterfly just need cleaning.
 

gw33gp

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Yes, I was concerned about that too. It does react to that increased load but not quick enough. I will look into that more.
 

gw33gp

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I finally got around to taking my Ranger to a shop to have the AC checked out. It was 1/2 lb. low on freon. It still had enough freon to keep it cool but low enough to cycle quickly. They found both the high and low pressure Schrader valves had slight leaks and replaced them. They also removed the freon, pulled vacuum then replaced the freon.

Next time I will try adding more freon before taking it in. That would save me $350. They did do a very thorough check of the AC system and now I know it has a clean bill of health.
 

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