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Air Conditioner Cycling


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1999 LS400 190,000 miles.

When at idle in gear my A/C keeps cycling on and off. You can hear it click and then the rpm's drop the up then down etc. If I put it in neutral and rev it up sometimes it will be ok, sometimes not. It seems to only happen when it's really hot outside. Any ideas on what it could be and how to troubleshoot?

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1999 LS400 190,000 miles.

When at idle in gear my A/C keeps cycling on and off. You can hear it click and then the rpm's drop the up then down etc. If I put it in neutral and rev it up sometimes it will be ok, sometimes not. It seems to only happen when it's really hot outside. Any ideas on what it could be and how to troubleshoot?

SOP, high side pressure exceeds the safe limit as detected by the hi/lo pressure sensor.

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My previous 92 LS did this a couple of times when the refrigerant level was very low. It took a mechanic to help me figure out there was a serious leak in the A/C system that was causing it to lose its refrigerant very quickly.

That might not be what is causing your problem- but I have learned from experience that A/C work is best left to a professional. I think taking your car to one for evaluation would be the best course of action. Servicing an A/C system properly (so moisture won't get inside) is nearly impossible to do without special tools and equipment.

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My previous 92 LS did this a couple of times when the refrigerant level was very low. It took a mechanic to help me figure out there was a serious leak in the A/C system that was causing it to lose its refrigerant very quickly.

That might not be what is causing your problem- but I have learned from experience that A/C work is best left to a professional. I think taking your car to one for evaluation would be the best course of action. Servicing an A/C system properly (so moisture won't get inside) is nearly impossible to do without special tools and equipment.

Op said...

"..when really hot outside..."

That's when high side pressures will be at their greatest.

Op might want to check and make sure the electric condensor/radiator cooling fan(s) are fully functional.

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My previous 92 LS did this a couple of times when the refrigerant level was very low. It took a mechanic to help me figure out there was a serious leak in the A/C system that was causing it to lose its refrigerant very quickly.

That might not be what is causing your problem- but I have learned from experience that A/C work is best left to a professional. I think taking your car to one for evaluation would be the best course of action. Servicing an A/C system properly (so moisture won't get inside) is nearly impossible to do without special tools and equipment.

Op said...

"..when really hot outside..."

That's when high side pressures will be at their greatest.

Op might want to check and make sure the electric condensor/radiator cooling fan(s) are fully functional.

jaswood, I'm not sure I'm understanding exactly what you are trying to say. Your responses to this thread (and others) have been rather condescending and I was only attempting to offer another perspective that the original poster might understand more easily.

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Not getting involved in a dispute here.

From how my 1994 LS400s AC behaves the compressor does cycle on and off and this is why it has a clutch to enable and disconnect the drive to it from the drive belt, the cooling fan also cycles on and off.

This also depends on the ouside temperature and the temperature the AC is set to work at so on very hot days there will be more of this cycling and fan running.

The change in engine revs is due to the extra load on the engine when the compressor runs and is compensated for by the ECU.

Providing the AC is actually working with ice cold air coming out the vents it sounds pretty normal to me.

Most AC repair places will check your AC system free of charge,normally one check is to measure the air temperature coming out the vents, if you have any doubts.

If you think about your home refridgerator which is basically an AC system in reverse you will notice the compressor motor doesn't run all the time but also cycles on and off.

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I should have clarified. When it cycles it's happening every second. ON-OFF, ON-OFF, ON-OFF.

On a HOT day the compressed refrigerant GAS cannot be quickly cooled enough to exit the condensor as a liquid so pressure builds and builds until the compressor clutch cycles off due to the high pressure.

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You can purchase a AC gauge and re-charge kit (<$30) from auto suppliers. I opted for a good gauge and its paid off. Recently they have introduced a new coupling so if you have an older kit you may have to buy yet another piece. I have found that my 02 Nissan and my 04 Toyota both needed at least 1 to 2 cans of CR-134A. Follow the directions and attach the gauge to the low pressure line when the car is running and the AC is on. You simply open the valve and let the contents of the can flow into the system

In my 96 the low pressure line is on the driver's side near the fire wall and has a black cap clearly marked L for low. The high pressure line is a different coupling so you cannot inadvertently attach to it - at least that is what I have seen and read. The gauge will tell you if the system is low. I've read various postings and the consensus seems to be that some CR134A will escape any system over time. Obviously if this is happening very quickly then you need to consider getting an expert in to find the leak.

Cycling quickly on and off certainly sounds like something is wrong and hopefully its just a matter of low fluid. Hope this helps.

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You can purchase a AC gauge and re-charge kit (<$30) from auto suppliers. I opted for a good gauge and its paid off. Recently they have introduced a new coupling so if you have an older kit you may have to buy yet another piece. I have found that my 02 Nissan and my 04 Toyota both needed at least 1 to 2 cans of CR-134A. Follow the directions and attach the gauge to the low pressure line when the car is running and the AC is on. You simply open the valve and let the contents of the can flow into the system

In my 96 the low pressure line is on the driver's side near the fire wall and has a black cap clearly marked L for low. The high pressure line is a different coupling so you cannot inadvertently attach to it - at least that is what I have seen and read. The gauge will tell you if the system is low. I've read various postings and the consensus seems to be that some CR134A will escape any system over time. Obviously if this is happening very quickly then you need to consider getting an expert in to find the leak.

Cycling quickly on and off certainly sounds like something is wrong and hopefully its just a matter of low fluid. Hope this helps.

No, cycling quickly on and off might very well be SOP depending on overall conditions.

Besides which, what's the definition of "quickly"..?

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