After about 45 minutes of going down the highway with
the AC working just fine, I noticed the volume of air
coming out of the vents had reduced to a trickle. It
was still blowing cold, but hardly anything is coming
out. The rear AC unit still blows as much cold air as
is should, but the front acts like a 'gate' in the
duct work got stuck shut. The fan sounds like it's
blowing as it should, and changes speeds when I flip
the switch between low, mid-low, mid-high, and hi,
just as it should. But the air just doesn't come out
of the ducts.

It stayed like that the rest of the day. The next
morning the AC was blowing normal again, but then
about 45 minutes into my trip it did the same thing
again.

I don't know anything about the ventilation ductwork
and controls in my truck, but my guess is some
electrical switch that controls a gate in the ductwork
is overheating and failing closed.

Anybody have any ideas?

It looks like your AC is a little low on charge. When an AC is low on gas it will work but will hold the coil temperature below the freezing point and will freeze the coil over and cut off the air flow.


You should be able to just top off the charge and fix it. All auto AC systems leak over time and depending on how long it takes to leak down to a point of a problem will deternime if you should do more than just top it off. Try the top off first.

No. I'm not low on freon. I just had the system totally evacuated and recharged two weeks ago even though it didn't need it.


You might be on to something though... maybe frost or moisture is plugging something up and once it thaws things work fine again.

Is it a front and rear system? If so do both front and rear fail?


If only the front fails it could be a clogged metering orifice. It is located just below the edge of passanger side battery. If after several min. of operation you see ice on the rear side of the valve, just beyound the crimp marks, there it could be clogged. If it is clogged either they did not evacuate the system and air is in there or dirt has moved to that location and clogged it. If it was a hummid day when the system was opened and not evacuated for an hour or more it may have moisture in there.


However the first thing I would do is have a set of gauges put on the system and check for charge. I say this because I have seen mechanics that charged for work they never did or had no idea of what they were doing.

Yes, it's a front and rear system. Only the front fails. The rear continues to blow as hard and as cold as it should. The front will blow cold, but not hard.


The mechanic was my brother-in-law, and one of my best friends. I stood there for 20 minutes (give or take) and shot the bull with him while the AC machine did the work. But we didn't leave the system open for an hour, that I'm sure of. So maybe you're on to something with the moisture in the system.


I'll have to see if I can find that metering oriface and take a look at it.

Should I have the AC system recharged regularly?

It has become warm here this week, and I wish the AC would produce much cooler air.

As soon as I find the time to, I'll try to clean the radiator, but the coolant might be worn or so ...

Sounds to me like the A/C Intake (just above the Passengers feet) is getting stopped up are something. Maybe the motor that controls the flapper valve there is failing after 45 minutes of use. I would look that way before tearing the system open again, cause you say it still blows cold just no air flow and the rear system also works fine.( they both use the same system and compressor) and it don't cost anything to check out and see if flapper are moving open and closed. I have seen odd things plug up that intake on my Suburban before. you can use the air recycling switch to operate the flapper without A/C. Oh course I was figuring you were using the "MAX" A/C setting cause thats when this valve opens for additional cooling. Blower motor can also cause this, saw that on my 94 caprice just last month.

If both temperature remain cold and it is only the volume of air coming from the front that is in question it is not a clogged refrigeration line. I would still first look at the charge.


One way to to check if the coil is freezing over is to park the truck once it has stopped blowing full air voulme and turn the engine off but leave the front fan running. If after some time 15 to 20 min the air volume increases and water begins to flow from the front evaporator onto the ground you are melting ice off the coils.Edited by: HowieE

HowieE: Where are the coils located?


Bowtie: The intake you speak of... is that where the AC sucks air from the cab when the recirc (max AC) is active?


Thanks guys!

The coils are right over the passanger's feet in a plastic housing that you can not get into without an act of God. However if you are freezing the coils and try the experiment I mention above youwill see the melting water coming down just behind and inside the left front tire. You should normaly see water dripping there while the AC is working if all is well. But if water drips from there 15 or 20 mins. after the truck is off and the blower volume increases that is more than likely melting ice.

The coils are right over the passanger's feet in a plastic housing that you can not get into without an act of God....


http://www.dieselplace.com/forum/smileys/HiHi.gif Good one Howie!


You know, I'll just bet that's what's happening. What's a guy supposed to do to fix it? Set a heater on the passenger floor pointed towards the coils?http://www.dieselplace.com/forum/smileys/Confused.gifEdited by: w_huisman

Bowtie: The intake you speak of... is that where the AC sucks air from the cab when the recirc (max AC) is active?


Thanks guys!





You got it man and if what happens is what howieE says than something might be stopped up.

Yup. I was just told by another fellow burb owner that he had the same thing happen to him last year. He cleaned up the coil and all was good again.


I asked him how he got the coil out, but I haven't heard back from him yet. I expect it'll be one of those "man, I hope I NEVER have to do that again" responses. http://www.dieselplace.com/forum/smileys/Disapprove.gif

If you find out how he gained access to the coil please post as I am having a similar problem with my burb.http://www.dieselplace.com/forum/smileys/Unhappy.gif

Wade, if you let it idle for a while, with a/c off, and fan on, IF it's icing up, you will see the drip underneath, without a heater blowing on it. Is your accumulator (silver canister) got frost on it? Maybe that new pressure switch you have is stuck ON. Meaning lower pressure constantly, and not cycling the compressor off. Over the course of 45 minutes, that's a BIG chunk of ice.


May want to check vac lines?? Haven't dipped too much into these mobile a/c's too much, but I know a guy who had a wastegate line leak so bad, he said that the a/c flapper would get weird going open and shut down the road.

I think it is bad luck to read these posts.http://www.dieselplace.com/forum/smileys/HiHi.gif I think the switch is going bad on mine now. The AC quite twice in the last week. All I did to fix it was give the pressure switch a whack.

Hey Wade, maybe you don't have to take that coil out to clean it. I don't have a 'burb to look at, but I had a Cavalier one time that the front of the evaporator was a big thick carpet of dirt. No way in the world you'd ever get to that thing, but, I was able to take out the fan assembley, and reach down through the duct and clean it with brush and detergent. Might be worth a shot - if your coil is dirty.

Why are you considering to take the coils out? If the coils were clogged you would not have any more air at startup than 1/2 hour later. Have you checked to see if the coils are freezing yet? That is the type of problem that will cause decreasing air flow.

It was suggested to me last week that the low pressure switch wasn't cycling off as it should, just as you mentioned knkreb. So Friday I got a new one from the dealer ($15) and installed it. Made a one hour trip yesterday and it didn't ice over, so maybe it's fixed now. Actually the truck ran for almost an hour and a half straight and the air kept working fine.


We've got a 2.5hr trip coming up this weekend. Then I'll find out for sure.

Your replacing the low pressure switch and the AC not losing air flow is another indication that you are low on refergerant. If the LPS fails to open when your suction pressure drops below the cutoff point the coils are subject to temperatures below freezing and will cause the coils to freeze over. If you replaced the switch with a good one and the compressor clutch is cycled you will not freeze the coils, however you will also not be able to receive full capacity from the system at higher loads.


If you do not have a set of guages to test the system with do this test. On a warm day after the truck has sat in the sun for a while start the truck, open all doors, set the AC to max and run the engine at about 2000 rpm for 3 or 4 mins. Watch the compressor clutch. If it cycles off at all while doing this test you are low on gas.

I don't have a gage, and I don't think my leg is long enough to hold 2000rpm on the pedal while I watch the compressor, but I suppose I can hire the wife to help out.


The weather isn't supposed to be very warm here this week though. The hottest it's supposed to get between now and Thursday is 72F! But I'll check it out and post back. Thanks Howie.Edited by: w_huisman

You can get a gauge for the low side real cheap from an autoparts store. They work just like a tire gauge. I think the cheap ones are only around $10.

My Sierra had a similar problem where the flaps controlling the flow of air through the vents would at random close. What fixed the problem was when I replaced the HVAC control cluster, which completely burnt out on me. My friend at the local GMC dealer said that he sees it happen pretty often over the summer... the control cluster will slowly burn out, losing functions one by one (as mine did). $100 later I had a shiny new cluster, and the thing blows out the vents. On occasion it won't blow out the vents, but that will be when I start a trip, not after 45 minutes or so.


As a side note, there is terrible ventilation to that area where the climate control cluster and the radio sit. My radio (Blaupunkt unit that I installed) gets overheated on long trips (>8 hours) and the CD will stop functioning. However the aux in still works, which is what I use (music on laptop).


Since the control cluster is a pricey item, hopefully you can find another (cheaper) solution.


As far as charging of A/C systems go, I've spent the past 2 summers doing lots of work on A/C systems in various vehicles (including my Sierra 6.5). The myth that all A/C systems leak out over time is just that, a myth. A properly functioning A/C system shouldn't leak out ANY freon. We have cars come in that are 10-14 years old and still have the original charge of R-12 or R-134a in them. When vacuuming a system, the rule is that you want to suck it down for approximately 2x the time that you had the system open. However for practical purposes, 20 minutes is generally fine if you've just replaced one or two components, 45 minutes to 1 hour if you've been having problems with the system or replaced a bunch of components. Make sure you suck it down from both the low and high sides, to ensure that you get everything out. Whenever evacuating and recharging a system, put in some kind of leak detection as that will make finding leaks in the future much easier. Also, putting in some extra oil for the compressor is never a bad idea.

I doubt that the system is low on charge. With rear system still blowing out cold air, it would be the first to show signs of reduced capacity if it were low. Two reasons: farthest from compressor, (more pressure drop) and it's an Expansion Valve metering device. They are a much more accurate way of throttling the refrigerant through the evaporator. The TXV wants a steady column of liquid refrigerant coming to it all the time for proper operation. If not, you get erratic operation from it. The front system is only a fixed orifice evaporator. Thus, being closer to the compressor, (with less pressure drop), it can freeze up before the rear system will.


These compressors have a lot of capacity, so that they can bring down the cab temperature quickly in the summer time. Once the temperature is down in the cab, you don't need full capacity any longer, and the compressor begins to cycle on and off as needed to maintain a certain evaporator pressure. If the system is low on charge, the pressure switch more often than not will cycle off the compressor before the coil begins to freeze. The pressure may be low, but the superheat will be high. It can and will freeze if given enough time, but Wade's symptoms on his indicated stuck pressure switch. With switch stuck on, you have all the capacity, all the time. If the capacity of the compressor is greater than the heat load, your coil temperature drops below freezing, and bam, frozen coil.


So yes, your theory is good about freeze ups on low on charge. But, if the compressor DOESN'T cycle off, then you are not low on charge in that situation usually.Edited by: knkreb