Diagnosing AC problem

AMonger · Jun 15, 2010

My AC works intermittently. Generally, it works while idling, but when I drive above a slow, city-traffic speed, it shuts down. Sometimes it'll start again after 45 minutes or so of highway driving. Then, when I park, sometimes it'll quit and it may take me an hour to get it working again. I can hear the compressor kicking in, and sometimes I get about 3 seconds of cold air before it quits.

I've asked a couple people, and one said it sounds like a blend door problem, and the other said it sounds like a vacuum leak. So I don't know which to believe.

Any ideas?

greg334 · Jun 15, 2010

What kind of vehicle?

stamp11127 · Jun 15, 2010

Make it simple first, check that the a/c system is working correctly and not cutting out due to high pressure. Should be easy to see the clutch in operation. If the system pressurizes and quickly cuts out its time for the gauges. The rednecks down here just jump the pressure switch to get'er runnin. Risky to say the least depending on how high the pressure is. That is one fast way to find the weakest link in the system.
If the system is operating properly then a problem with the blend door is possible but you should be able to get a/c in the cab, just not where you want it.
Check the operation of the blend door without the a/c on so that you can hear it change, or if your lucky maybe a hiss from a leak. Easy to check if you have a Mighty Vac, draw a vacuum on the source line and see if it bleeds down.

davekc · Jun 15, 2010

Could be several issues. See if the compressor is cycling properly. If so, use the sight glass if so equipped to see if freon is running through it. If not, feel the hoses on the dryer. One side should be hot, and the other cold. That will address the air/blender, pressure or expansion valve issues.
Lastly, when blowing cool, inspect the clutch and engine fan. When the AC stops working, is the fan still running?
The fan must be running to pull air through the condensor. Even more so if moving slow. What controls the fan depends on the type of vehicle. Could be electric or air operated.

guido4475 · Jun 15, 2010

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.Take the raidator out, and wash everything thoughly.It will surprise you what comes out of those fins.And how much better things work. I do mine once a year on everything I own.

greg334 · Jun 15, 2010

I agree Guido, but it sounds like an expansion valve/orifice tube issue which means a good flushing of the system and replacing of the valve/tube.

I asked about what vehicle because GM products didn't use vacuum for their blend doors but a servo.

davekc · Jun 15, 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.Take the raidator out, and wash everything thoughly.It will surprise you what comes out of those fins.And how much better things work. I do mine once a year on everything I own.

Totally agree and it is so easy to check. Just run water from the garden hose through the rad and condensor and see if it is passing through.
If not, get a can of Gunk or for the budget minded, Easy Off oven cleaner and presto. Number one cause of overheats and cooked fans on gensets as well.
Good post.

AMonger · Jun 15, 2010

greg334 said:
What kind of vehicle?

Chevy Express 3500.

greg334 · Jun 15, 2010

Then the orifice tube is getting crap in it. I had the same problem but only replaced the tube to get it to work and never got to flush the system out before selling the van.

The blend door is servo operated and don't fail often. I would try this. On a cold (cool) morning, start the van up and turn the controls to cold without running the air. If you get cool air (same as the outside), move the control to hot and see if it is hot (warm air) and then back to cold. If there is something wrong, you will get warm air.

AMonger · Jun 15, 2010

Ok, I don't really know what all the parts on the AC unit do. Is the clutch similar to a transmission clutch? One that could slip? Because when I'm at idle, if I rev the engine a bit, I can hear and feel the AC unit kick in and I'll get some cold air. Sometimes it keeps producing cold air, and sometimes it doesn't. Would that be clutch slippage?

OntarioVanMan · Jun 15, 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.Take the raidator out, and wash everything thoughly.It will surprise you what comes out of those fins.And how much better things work. I do mine once a year on everything I own.

No kidding...guilty here!! after collecting 5 years worth of every bug and bee and bird crap...finally had it cleaned out between the AC condensor/turbo cooler and rad....tech wore a mask and safety glasses....almost a shovelful of garbage..van run a few degrees cooler with just that being done..

greg334 · Jun 15, 2010

The clutch is an electro-magnet. It is the thing that goes "snap" when the compressor kicks in. Unlikely they slip, seen them burn up but not slip.

You know I should have asked this;

when was the last time the system was checked to see if there is enough freon in it?

AMonger · Jun 15, 2010

greg334 said:
You know I should have asked this;

when was the last time the system was checked to see if there is enough freon in it?

I'm hearing and feeling that snap when it kicks in, but sometimes I have to rev the engine to get it.

We've never had it checked. This van is new-to-us, a 2007. But short of a major leak, it should be fine. The AC was ice cold until just recently. And low freon wouldn't be temporarily remedied by revving the engine, would it? Nor would there be a difference between driving and sitting, would there?

chefdennis · Jun 15, 2010

When you rev the engine, the ac compressor runs faster, building up pressure moving the freon. i'd bet its low on freon or as greg said the orfice tube in the high pressure line is clogged...take it to a AC/Rad repair shop, its not a big deal...oh and has guido said, clean out the rad and ac condenser....

AMonger · Jun 15, 2010

chefdennis said:
When you rev the engine, the ac compressor runs faster, building up pressure moving the freon. i'd bet its low on freon or as greg said the orfice tube in the high pressure line is clogged...take it to a AC/Rad repair shop, its not a big deal...oh and has guido said, clean out the rad and ac condenser....

Ok, but then what accounts for the fact that I get no cool air on the road at highway speeds (usually, but sometimes I do).

chefdennis · Jun 15, 2010

After a while of running at higher RPM's the pressures equalize and then the low freon level come to the fore as the issue....

stamp11127 · Jun 15, 2010

I haven't run across a competent auto a/c mechanic in years. Every person that I know has problems about every two years on average from the "experts" repairs, so I do the work myself.

Check out this site for info on auto a/c:

Troubleshooting with Gauges FAQ

Would be worthwhile to have a recharge hose with gauge like below on hand for moments like these. Got this one from Walmart years ago, just checked and they don't carry the exact same one but have one similar to it. Should you get one, make sure the can is replaceable.

Like my drywall backdrop??? Guess what the "boss" has me doing today!!!!
I'll be "Muddin Beech" tonight!!!

AMonger · Jun 17, 2010

Here's another twist I can't figure out. Yesterday, I had to stomp on the accelerator while merging. After that, the AC worked fine and has worked fine ever since. I don't know what to make of that. Any ideas why that would be?