Diagnosing AC problem

greg334 · Jun 17, 2010

1 - Have the system checked to see what level the freon is at. A lot of the times it is the level of freon that causes the issue. Without going into a lot of details, the system leaks, all systems leak and because of that, you may have been alright until now but just enough leaked out that affects you.

2 - sometimes there is something that is called "black death" usually it is the compressor failing. This will cause the orifice tube to get clogged. This means that the compressor and tube needs to be replaced and the entire system flushed.

3 - Sometimes crap will get into the tube but as the truck/van/car moves around, the crap moves a bit and sometimes it works and sometimes it doesn't. This has happened when idiot mechanics take the system down and then forget to put on a new dryer and pull the system down with a vacuum.

4 - Because I'm tired and can't type, the other issue is if the system was worked on and the idiot mechanic didn't replace the dryer and pull the system down before replacing the freon, could also be a problem.

RETIDEPXE · Jun 18, 2010

Could be some idiot, no wait, I did this the other day.....anyway, put too much freon in the system causing compressor to cycle on for only short periods of time. Expelled a little into the "recovery bottle", yea right, and walla, cold air again.

I lost some freon and traced leak to an alum tube brazed onto the side of the condensor, bought a pkg of small hose clamps for 2.99, cut a small peice out of a radiator hose to clamp around the leak (actually wife's idea), two 12 oz cans freon for 6.87 each, and a freon installer thingy for 16.99 at Wally world, total repair, $33.72 plus tax. This was a week ago and still no signs of any further leak yet.......keeping my fingers crossed......cheap I know, but hey, it works, or at least untill the tube blows apart or something else. 429,000 miles on the M2 and first sign of A/C problem, can't complain.

Retidepxe

chefdennis · Jun 18, 2010

Ok the AC compressor is driven by a belt, that is driven off the crankshaft pulley. The hard Accel spins the crank faster (higher RPM's) you stomped on the accelerator, causing a sudden hard increase in RPMS which spins the compressor faster which builds a sudden increase in the high side pressure.....that inr crease in pressure "Could have" blown crap that had accumulated in the orfice tube away enough to now let the freon flow as it is suppose to.

Its been a while since i worked on AC systems and i wasn't really into it to begin with, but if i remember right the orfice tube is actually the restriction ( about 3 inchs long and a super fine mess screen in a funnel shape) that seperates the low pressure side from the high pressure side of the system ...so a sudden increase in pressure cause by hard accel COULD cause the air to now blow cold.

The one thing for sure is, that it ain't fixed, and nothing you are told here is going to fix it....take it to a rad /ac shop....have the system checked out....at a minimum have it evacuated and cleaned out...new dryer and orfice tube and then a qick charge of oil and a recharge, it should be using r-134-a as frigerate......

truckblue · Jun 18, 2010

I'm gonna go out on a limb and say your compressor is failing. Have you tried to charge the system? It could just need freon. Good luck.

Wolfeman68 · Jun 18, 2010

I've been doing the expedite thing for four years now, but before that I was an ASE Certified Auto Tech for 25 years. A/C repair was one of my certifications.

That said, Greg is pretty much right on with his diagnosis. You should have the CCOT (Clutch Cycling Orifice Tube) system in your Chevy. You can confirm this by following the High Pressure (Liquid) line from the condensor to the evaporator. It's the smaller of the two lines. If you have an orifice tube, you will see a small bulge with rectangular "dimples" in the line. That's where the orifice tube is located. If you see a round connector with a spring in it and a white plastic ring in front of the connector, then the orifice tube can be removed without replacing the entire line.

Three things can clog the tube. Moisture in the system that reacts to the R-134a creating acids that corrode the inside of the line and pieces flake off and get stuck in the tube. Second one is the desiccant bag ( like the small bag in some products you buy that says "do not eat") in the drier has ruptured sending its contents through the system clogging the tube. Finally, the "Black Death" Greg mentioned meaning that the compressor is failing sending metal shavings and seal material in the system clogging the tube.

Did your "stomp" fix the problem? No, sorry, it may have made it worse. If some material clogging the tube dislodged like Greg suggests, that material is now going through the system and will eventually end up back at the tube. However, the tube may have ruptured from the pressure surge leaving a small hole that allows the freon to flow and cool the van, but like a slow leak in a tire, it will allow the trash to circulate through the system clogging another component.

What should you do? Take it to an A/C repair shop and have them evacuate the freon and remove the tube to see its condition and what is in it. The Tech will be able to tell what the trash is and where it came from. Normally, the first step would be to connect a gauge set to the system and check the high and low pressure. If the tube is plugged, the low pressure will draw into a vaccum, but since yours is cooling again it may not do this.

I wouldn't wait too long to get it checked. It could cost you a lot more money if you do. If I can help further, let me know.

Moot · Jun 19, 2010

guido4475 said:
It is amazing how many people ignore the preventative maintenance of keeping all of the components on the front of the vehicle clean such as the radiator, air to air, a/c condensor, etc, to maintain proper airflow through all of that, so that things work properly.

I cut a piece of ¼" galvanized hardware cloth to size and secure it to the back side of the grill with cable ties. This protects the radiator from rocks and bugs and is easy to clean.

60MPH · Jun 23, 2010

Amonger did u get the problem fixed?? I had a/c problems with my 06 express van. a/c would just start to blow hot air after idling for awhile, and the compressor would start to short cycle, the minute I began to drive it would start blowing cold air again. I ended up putting a 14" electric fan on the front of the condenser along with a manual switch "in cab" to push air into it when needed.

I have the duramax engine and with the cac and radiator up there with the condenser in that little area the condenser could not get rid of the heat fast enough "no airflow" when the van was not moving. Before installing the fan I had seen pressures as high as 460psi on the high side while the low side was just fine. That was with a freshly clean condenser, cac, & new radiator and the correct a/c charge. We'll after some temp. readings on the condenser and lines I knew I had a problem with insufficient airflow over the condenser, so that is how I corrected my a/c problem.

This is probably not the same thing that is occurring on your van, I just posted this to let other people who may have a similar problem with there van know how it maybe fixed.

Deville · Jul 16, 2010

Just my .2, in my experiance with A/C systems it could be one or a combonation of things.

Low or high freon.
Bad air circulation
the system is not recyceling causing it to freeze up.Once it warms up it works again until it freezes again. I have replaced a recycleing switch on several cars, all Fords. You may wanna check that.

AMonger · Jul 16, 2010

Hmmm...I think it has something to do with how hot it is. When this was first going on, it was HOT out, real hot. Then, after the stomp, so to speak, it started working again and has worked for weeks. Then, yesterday, it stopped again. I got it to run again when I got on the expressway, but since the next time I shut the van off, it's been out. And again, it's frikken HOT out.

Are some particular AC problems more likely to happen the hotter it gets?

aukinet · Jul 17, 2010

If you wait another couple of months to have someone put a real guage set on it and check out what its doing, it'll be cool enough outside that you won't have to worry about it 'til next summer.

AMonger · Jul 17, 2010

aukinet said:
If you wait another couple of months to have someone put a real guage set on it and check out what its doing, it'll be cool enough outside that you won't have to worry about it 'til next summer.

Ok, got it into a shop, and fortunately, it's a shop that has testimonials on the walls from customer who were getting rooked elsewhere and then came here and found the treatment more honest.

It's got plenty of freon. But the powertrain control module isn't grounding the AC compressor clutch. That happens normally under high or low pressure conditions, but this is happening constantly. So, they say, the PCM has to be replaced, at a cost of around 600-800 semolians.

60MPH · Jul 17, 2010

Before you drop that kind of money you need to ask the mechanic if he ran a ground wire directly to a ground source i.e. engine block, neg battery post etc. If he did that and the compressor came on than he's on the right path. There only a few more item's to check continuity on. The following is to be done after the system is verified that it has the correct charge and the high and low side static pressures and operating pressures are correct.

The 3 items he needs to check continuity on are both the high side switch "on Compressor" and the low side switch "on the accumulator" there might also be a thermostatic switch near the evaporator to. He needs to check these as someone is switching the ac knob in the van unless he has a computer hooked to it where he can command it on himself. If he is a good mechanic than he would of already done this!! If he fights with you than YOU BETTER GO SOMEWHERE ELSE those 3 switch's cost penny's compared to a pcm. And any one of them can cause a intermittent grounding problem. Those 3 switch's are ran in series with the pcm and bcm and they will give the pcm a incorrect value, which the pcm will read and react to which will cause problems.

If those 3 switches work properly and he checked the wires on the compressor and switch's to make sure the are not corroded or chaffed then he might be going in the right direction.

Also he needs to check the a/c relay for corrosion and continuity and the socket it plugs into in the fuse box.

The pcm and the compressor are the last 2 items that I would consider replacing on a a/c system. They are the most expensive and if they don't fix the problem then you are out big bucks!

Make sure he checks all the ground wires on the van it is odd that the pcm would only complete the compressor ground sometimes and not all the time when commanded to do so. The pcm is a electronic device it is either going to make the connection all of the time or none of the time. There are no moving parts to wear out. The only way they usually go out is that they burnout the tiny switch "for lack of a better word" or they get corroded because of water.

One other thing to check would be the compressor clutch it self they do burn out, and they can be a on going intermittent problem until they finally go out for good.

p.s. I can't reiterate any better that the PCM would be the LAST thing I would replace. I don't want to see you drop that much money on something that may or may not fix it correctly.

ALSO ASK IF YOU CAN RETURN THE PCM BEFORE HE ACTUALLY INSTALLS IT. BECAUSE ALOT OF SHOPS WILL TELL YOU ONCE IT IS INSTALLED THEY WILL NOT RETURN IT BECAUSE IT IS CONSIDERED USED SINCE THEY HAVE TO PROGRAM IT. THEY WILL TELL YOU ONCE IT'S PROGRAMMED TO YOUR VAN IT CAN'T BE PROGRAMMED TO ANY OTHER VEHICLE. AND IF IT DON'T FIX THE PROBLEM YOU ARE OUT 800.00 BUCKS WITH NO RECOURSE. DEPENDING ON THE YEAR OF YOUR VAN YOU MAY JUST HAVE A PILL AND THEY CAN POP IT IN AND THEY WON'T ACTUALLY FLASH THE PCM. THEY CAN RETURN THESE BUT IF YOU HAVE A MODEL THAT HAS TO BE FLASHED THEN THEY WILL NOT LET YOU RETURN IT. IT IS A SCAM BECAUSE THEY CAN SEND THEM OFF TO HAVE THEM REFLASHED AND RESOLD. BUT ALOT OF SHOPS AND DEALERS JUST WON'T DO IT. CROOKS!!!!

AMonger · Jul 17, 2010

60MPH said:
Before you drop that kind of money you need to ask the mechanic if he ran a ground wire directly to a ground source i.e. engine block, neg battery post etc. If he did that and the compressor came on than he's on the right path. There only a few more item's to check continuity on. The following is to be done after the system is verified that it has the correct charge and the high and low side static pressures and operating pressures are correct.

He did say that they were able to jump the system from every other point. That the same thing?

60MPH · Jul 17, 2010

I don't know what he jumped maybe the lowside switch?? Did he say the compressor started when he jumped the other switch's??
Cause if it started when he jumped them he was just completing a circuit. which would tell me the pcm is working as it is grounding out and completing a circuit. Has he already put the pcm in??

AMonger · Jul 17, 2010

60MPH said:
I don't know what he jumped maybe the lowside switch?? Did he say the compressor started when he jumped the other switch's??
Cause if it started when he jumped them he was just completing a circuit. which would tell me the pcm is working as it is grounding out and completing a circuit. Has he already put the pcm in??

He was pointing to a diagram as he was telling me this, said they were able to jump it from every other part of the system.

He's going to call for parts and prices on Monday. I have to get approval from my owner before spending that much money.

Wolfeman68 · Jul 18, 2010

AMonger said:
He was pointing to a diagram as he was telling me this, said they were able to jump it from every other part of the system.

He's going to call for parts and prices on Monday. I have to get approval from my owner before spending that much money.

I doubt that the PCM is at fault here. If this is a well equipped shop they should have a diagnostic scanner. The scanner will have a display for the A/C clutch that says "off" and "on". The PCM only reads that the ground circuit is completed and sends power to the compressor clutch turning the display on the scanner from "off" to "on". Most systems get their ground from the switch in the control head in the dash. His grounding at every part of the system is just completing the circuit. He needs to determine where the system ground originates and do continuity tests before the PCM is replaced.

aukinet · Jul 18, 2010

I agree that the PCM would be the last item on my list. All grounds should get checked.
I generally start with the compressor. That allows me determine if the power side (relay, 3 fuses) or ground side(PCM etc.) are the culprit. The order in which things go are like this: AC gets turned on and 12v goes from HVAC fuse 10 through control head, through high pressure switch and on to the PCM. THis is called AC request. PCM then supplies ground to the clutch relay and the ground side of clutch. Clutch relay switches on and supplies 12v to the clutch. If the AC low pressure switch is open(low pressure) the clutch won't be grounded. if the AC high pressure switch is open the PCM won't get it request signal from the control head.
Sooooo, I would replace fuse10,22,32.Ive seen a number of intermittant fuses. I would jumper the high pressure switch(located on the back of the compressor or somewhere else in the discharge side of the system). If it runs replace the switch. If not,probe the light green wire on the switch for 12v. If none, suspect the control head.
Pull the plug on the compressor clutch and with system ON you should see 12v on the Dark Green wire. You should have ground on the black wire. If you don't have power, suspect the relay or relay control circuit. If you don't have ground, jumper the low pressure switch and see if you get it. If you do then replace the switch. If you don't, suspect PCM or wiring to PCM.
One thing you MUST do is very carefully inspect the plugs on the clutch and the switches. It is very easy to bend the contacts down in those plugs. Also check the wiring to the compressor clutch and look for chafing or damage or corrosion. Any bulges in the insulation are suspect as I've seen Chevys with corroded wires in the middle of a run. No explanation except defect in manufacturing.
Most vehicles I work on when you floor the gas the compressor will shut off to give you a little more power on full throttle. The PCM controls this via the sensors in the electronic throttle. Could there be a glitch in there? Good Luck!

AMonger · Jul 23, 2010

Well, this shop appears to be run by an honest man. He ordered the new PCM, had to have it overnighted from Wisconsin, otherwise it would have been three business days. That was $45.

So I sit there for a while and he tells me they're almost done with my van, which was strange because it wasn't supposed to be done so soon. He comes back and tells me it wasn't the PCM after all. When they started to take apart whatever needed to be taken apart to install the PCM, they found that if they bypassed the compressor clutch relay, it would operate. So they wired it back up with the clutch relay bypassed to get me AC until they could figure out the real solution. I suppose a lot of shops would have installed the PCM, bypassed the clutch relay and told me it was all fixed.

It seems that the company through whom they get their schematics has old information from GM, or maybe GM hasn't updated the info they put out. Apparently, the system changed with the 2007s, making the latest information out there out of date. They have to contact their company and have them get new info from GM. But he says it looks like GM is saying that they no longer supply clutch relays separate from the entire compressor.

So because of whatever fault exists in the clutch relay, the computer doesn't recognize the freon in the system, of which there is plenty. Because of that, no AC.

So until they can get the new information and figure a new repair solution, I have AC. He says it should run this way indefinitely.

Total charge so far: 45 semolians.

greg334 · Jul 23, 2010

So if it isn't the ECM, then it is the clutch relay, and they replaced the relay?

There isn't much between the relay and the ECM, the high pressure switch and if there isn't a signal from the ECM to the clutch relay, then it is not the ECM but something in between. I don't think it has changed enough to prevent someone with good troubleshooting skills to locate the problem.

There isn't much to the a/c electrical system, the high pressure switch, the pressure cycle switch and the clutch control relay.