intermittent dash a/c???

Looking for insights. 

A few years ago we were coming back to NM from Ft. Worth and running the dash a/c.  All was going fine then suddenly the air stopped being cold.  So we stopped in ABQ to have it fixed.  The tech worked on it, thought the freon was low, did a leak check, found no leaks even after 250 psi test, pulled a vacuum and refilled the freon.  Worked fine until this spring, sorta.  We were going west and about 1 pm we switched it on again, worked great then suddenly, nothing again.  Thinking that we might be low on freon I had a spare canister in the storage and attempted to put some in using the small diameter port up front.  No luck, the compressor clutch would not engage.  So we switched to "house air" with the genny and continued the trip.  Once we got to our destination we located a repair shop and they went through the same routine that was used to "fix" it a few years ago and wall-lah it started working again.  They did observe that the freon was over the recommended level and after pulling another vacuum, refilled it with the 2.5 lbs that is recommended.  I watched every step of this process this time and noticed that after the freon/leak check was done that they checked voltage going to clutch, no voltage.  So they probed the various connectors and suddenly it started working.  That made me think that there was, perhaps, an intermittent electrical fault all along... an intermittent connection or a pressure sensor that was flakey.  But anyway, it was working so we left to come home. About half way, it stopped again so this time I opened the front and "jostled" the connections and, once more, it started working again. 

Sorry for the long story.  Now the quandry.  I attached a photo of what I see when I open the front access door.  There's the a/c ports, two pressure switches and what looks like to me a vacuum controlled valve.  The a/c seems to start working if I remove and re-attach the small vacuum line to the valve.  How could this influence the a/c?  Of course  I cannot recreate the failure while the access door is open... 

If anyone has any insights regarding this mystery, I'm keenly interested.

That could control the blend doors. Door gets in the wrong position and no cold air. Be sure your vacuum pump works, I've had two fail.

John Canfield said:

That could control the blend doors. Door gets in the wrong position and no cold air. Be sure your vacuum pump works, I've had two fail.

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Thanks John!

I didn't even know I had a vacuum pump... where are they located?

John Canfield said:

That could control the blend doors. Door gets in the wrong position and no cold air. Be sure your vacuum pump works, I've had two fail.

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One more bit of information; when the a/c works, the metal fitting with the blue cap gets really cold, the compressor clutch engages and the metal fitting with the red cap gets warm...

I'm not sure what drive train you have. Helped a friend last week troubleshoot an A/C problem with his Ford F53 V-10. The a/c compressor wasn't running and he was also get a service engine soon light. The Ford service manager said that it was very possible that the error code was not allowing the a/c to run to limit the engine load.

Checking the error code showed that cylinder 9 was misfiring. Clearing the code and Reseating the coil connector several times seemed to fix the problem. Also with the code cleared the a/c worked.

Don't know if any of this will relate to your problem. Just know that the a/c not working may have nothing to do with a/c. Check the OBD for error codes and clear if needed. Then go from there.

Tom

taoshum said:

One more bit of information; when the a/c works, the metal fitting with the blue cap gets really cold, the compressor clutch engages and the metal fitting with the red cap gets warm...

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That's normal. "Blue" side is suction side of the compressor and should feel very cold and condense moisture. "Red" is the high pressure side and should feel quite hot.

A diesel engine can't produce vacuum to operate the dash air blend doors (or anything for that matter) so it's necessary to create a vacuum with a pump.

The vacuum pump should be located somewhere around the front of your dash plenum box. When you turn your ignition key to the on position, you should hear the sound of a small motor (or pump) running. There will also be a ball shaped thingy that accumulates vacuum.

That valve looks like it's in the heater hose line to me. Shut off water to heater.

jubileee said:

That valve looks like it's in the heater hose line to me. Shut off water to heater.

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Agree.