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AC works when driving, but fails when idling too long

Posted by mrsig (My Page) on
Fri, Jun 23, 06 at 15:06

I have a 2000 Nissan Frontier truck (w/ V6). Recently, I've noticed that the AC will no longer blow cold air if I've been idling too long (either parked, sitting in traffic, at a long traffic light, etc.) Once I start driving again, it cools back down like normal.

What causes this?


Follow-Up Postings:

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RE: AC works when driving, but fails when idling too long

Take it to an auto air specialist. Let him hook his gauges up... diagnostics are real quick to a guy that knows auto a.c. and can read gauges. Simple stuff actually.

Is it spitting out water or getting really extra cold when you're driving along at a steady state ? Then its probably just low on refrigerant, next question will be, where's the leak.


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RE: AC works when driving, but fails when idling too long

What is your cooling fan arrangement?

o 1 electric fan, fixed speed?

o 1 electric fan, variable speed?

o 2 electric fans?

o 1 belt driven fan + 1 electric fan?

o Other arrangement?

I suspect that there isn't enough air flow across the AC's condensor while at idle.

Check to see if your fan(s) are operating properly.


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RE: AC works when driving, but fails when idling too long

Thanks for the replies...

mikie - No, there isn't any water-spitting and it's not unnaturally cold when it's working...I was afraid of a refrigerant leak too...

jemdandy - I honestly don't know what the arrangement is, I guess I need to do some tinkering around to find out.

Some research into this indicates it could be a bad "compression clutch." However, I'm concerned that a mechanic will prefer to just go with a "system replacment" fix rather than a simple one that would also fix the problem. Just trying to load up on some info as best I can before I opt to take it in for a diagnosis.


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RE: AC works when driving, but fails when idling too long

Typical causes are low refridgerant, not enough air flow through the condenser...jemdandy touched on a couple causes for that(electric fan), some kind of obstruction between the radiator and the condenser such as leaves, paper, that kind of thing. My corvette doubles as a road vacuum cleaner in that regard. The expansion valve partially plugged will also cause poor cooling. I suppose the compressor clutch slipping could be a cause, but I'd think you could see that if you just watched it at idle, or you might even see burned up material around the clutch. Just a few ideas. I'd bet pretty much a routine service will take care of it. My money's on low 134a.


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RE: AC works when driving, but fails when idling too long

mrsig you wrote "Some research into this indicates it could be a bad "compression clutch." However, I'm concerned that a mechanic will prefer to just go with a "system replacment" fix rather than a simple one that would also fix the problem. Just trying to load up on some info as best I can before I opt to take it in for a diagnosis."

Why would you jump to "a mechanic will prefer to go with a system replacement"? You know when you walk in the door expecting to be taken advantage of, a tech that does do everything correctly actually has no chance of being successful in the end, because your going to find something wrong no matter what.

Your research "up to this point" is faulty. When you say "Some research into this indicates it could be a bad "compression clutch." You don't take into account that the system starts cooling again as soon as you start moving. A faulty clutch will randomly work, and not work whether you are moving or not. Besides, all you have to do is look at the clutch on the front of the "compressor" and see if it's applied, because if it is it will be spinning with the pulley.

Let me show you just how wrong it can be to start with an assumption, and make a conclusion without having all of the facts. "Assumption" I think you have a bad thermostat, and you didn't have any heat last winter. "Conclusion" So you put a piece of cardboard in between the radiator and the condensor so that you got some heat. Only you forgot about it and now that it's warm and it's still in there affecting the airflow through the condensor and reducing the AC systems cooling at idle. Now did you really do this? Only you know. But what you did do from my POV is slap me in the face by accusing mechanics of wanting to replace the whole system, for a performance complaint.

The best part of what I wrote there is some of it is very likely true!. You very likely have an airflow problem, because the fact that the system cools on the road is an indication that it is able to pick up heat, but it's not able to get rid of it at slower speeds. Now an undercharged system can give similar symptoms, but alas, so can an overcharged system. It takes a trained mechanic to know the difference! Too bad you don't know one, or trust one to figure it out for you.

BTW, we never replace the whole system, or to put it another way I HAVE NEVER replaced an entire system on a car, or truck, or whatever. We troubleshoot a reported problem, and if authorized, we repair/replace as required to restore the system to operating specs. If you need a new compressor clutch, then thats exactly what you get. No more, no less. If the system is undercharged, then we test for leaks, and if none are found, we add UV dye, and charge the system up. The UV dye will show up in the future where the leak is at, and we can then repair the system. If there is an airflow problem, such as the cardboard between the condensor and radiator (and yes I have seen that, more than once) we remove the "blockage", ensure the system works correctly, and send the car down the road. If the system is overcharged, we remove and recycle the excess refrigerant for use in someone elses car.

If the system has had a sealer installed, or blended refrigerants so that it is not pure R134, or the older R12, then once I confirm that to be the case, I charge for my diagnostics to that point, and park the car outside because I am done with it. Sealants can damage my $4500.00 AC recovery/recycling/recharging machine. Blended refrigerants will contaminate the refrigerant that I have to charge up other vehicles. (That could cost me hundreds of dollars in losses). So basically if someone is in a position to get ripped off today, it's not you by fooling around with something you have no training on, (AC) It's the shop (me) when you fail to fix your AC and you have done things that potentially put my equipment at the risk of damage. Damage to my equipment, when I'm trying to solve a problem for you affects my ability to get a return on the investement and earn a living with the equipment that I have invested thousands of dollars in.


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RE: AC works when driving, but fails when idling too long

Gary - Thanks, I had considered low refrigerant too, but was hoping for another cause since low refrig would likely mean a leak in the sealed system.

JohnG - I didn't mean to offend, but by "some research" I just meant a few searches on the internet for my problem that, in a few cases, turned up the compression clutch as a possible problem. I did say "some" research though, which should indicate that it wasn't a "thorough/complete" research.

FWIW, my experience with A/C going "bad" has resulted in both times with the mechanics wanting to replace, if not the whole system (bad choice of words on my part), a significant part of the system (compressor)...without looking elsewhere for a proper diagnosis. It was a shame that shopping around for different mechanics changed the source of the problem, the repair solution, and the significant $$ of work performed.

Anyway, thanks for the detailed response.


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RE: AC works when driving, but fails when idling too long

Head for your local Vo-Tech and buy the air conditioning text book. Good reading and you'll likely learn something.

First time in my life that I needed a new auto ac compressor it was like $300 to have the compressor replaced.
..VoTech here I come,,, that's what I did... bought the vo-tech text book,, big sucker... gauge test set, vacuum pump, refrigeration thermometer,,, and chunky tank of refrigerant ffrom a sister business and headed to the local auto/air parts supplier - compressor, receiver dryer, prob something else ... in the end cost me the same as paying somebody to do it. But I learned & had some new shade tree skills and tools for friends/family to bug me about.


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RE: AC works when driving, but fails when idling too long

mrsig wrote "JohnG - I didn't mean to offend, but by "some research" I just meant a few searches on the internet for my problem that, in a few cases, turned up the compression clutch as a possible problem."

The internet is a wonderfull thing in the way it brings information to our fingertips. But it also can bring poor or incomplete information. Plus decent information can be missapplied by someone that is not properlly trained to use it, internet based or not.

Mrsig wrote "I did say "some" research though, which should indicate that it wasn't a "thorough/complete" research."

Now if an auto tech approached his/her job that way would you be happy with the effort to that point?
mrsig wrote. "FWIW, my experience with A/C going "bad" has resulted in both times with the mechanics wanting to replace, if not the whole system (bad choice of words on my part), a significant part of the system (compressor)...without looking elsewhere for a proper diagnosis."

Daignostics is usually the hardest part of auto repair, and especially something like refrigeration (AC). Here in Pa. our AC season is really about 4 months long, with it really hitting hard in June and July, then the rest of the year we may not use that equipment, training, or skills at all. That does not make a missdiagnosis any less wrong, it's just the way it is, and few people would do better in their own jobs with a situation like that. Now there are people in the field that still excel with those repairs even under those circumstances.

mrsig wrote. "It was a shame that shopping around for different mechanics changed the source of the problem, the repair solution, and the significant $$ of work performed."

Ok so two times you had AC trouble, and the first diagnosis was wrong. So you took it somewhere else and they got it right? My first question is, why are you "shopping around" if you found someone that is capable of diagnosing your AC trouble correctly? (This ties in so well with the question vivian asked about capitolism) People find a shop/tech that can do the work correctly the first time but abandon them for the first discount that comes down the street. Then they only go back to the good shop when the discounter can't do the work, or at least do it correctly.

Which BTW we do have to qualify, your statement about two AC repairs is incomplete. If the AC problem was the compressor was leaking, and someone else "fixed" it by putting in sealer, then the debate is not over in reguards to the problem being fixed or not correctly. While with limited knowledge a consumer may be happier with the sealer for their solution the correct repair really would have been to replace the compressor, and the tech/shop that was making that recommendation was in fact the one that was right. You see the answer isn't always the fewest dollars from the consumers pocket "today" is the correct one. Adding sealer makes the system unserviceable in the future, period. I have a test kit to identify sealers added to the system and if I detect a sealer, the car gets parked outside and there is nothing I can or will do with it at all. It may have saved you the day it was put in, but it only delayed the real cost, which is more than you want to pay.


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RE: AC works when driving, but fails when idling too long

Mrsig wrote "I did say "some" research though, which should indicate that it wasn't a "thorough/complete" research."
JohnG wrote: Now if an auto tech approached his/her job that way would you be happy with the effort to that point?

No, of course not. The difference is I dont claim to be an "auto tech," or in a better analogy, an "expert internet searcher." Im not paid to do that, Im just trying to find information as best I can. The internet, including this forum, are tools I used for that purpose.

mrsig wrote: "It was a shame that shopping around for different mechanics changed the source of the problem, the repair solution, and the significant $$ of work performed."

JohnG wrote: Ok so two times you had AC trouble, and the first diagnosis was wrong. So you took it somewhere else and they got it right? My first question is, why are you "shopping around" if you found someone that is capable of diagnosing your AC trouble correctly?

Well, if you must know, both of those instances were when I lived in another locale, another state to be precise. Id rather not drive 500+ miles just to have my vehicle inspected, thanks. Further, the vehicles serviced were done so at model-specific locations. I guess a Pontiac service *would* be happy to *attempt* to service my Nissan, but they wouldnt be the first place Id think to call since Id imagine much of their equipment would be model-specific. Could be wrong there.

Also thanks for the additional discussion regarding "capitalism," I wasnt really looking for a discourse in that regard, but Im glad I gave you the opportunity to expand on your thoughts.

Thanks again for the info.


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