A/C intermittent operation

Kawboy · 07-29-2009, 06:13 PM

This problem has popped up twice recently. I drove 90 miles on the freeway in 90 degree weather and the A/C worked fine. Would freeze us out if left on high Fan. Got into heavy traffic and it started to work intermittently. After the first time I checked the charge and it was fine. (43 psi on the low side while running) Today it did the same thing. would intermittently cool and then not. I do not have availibility to a wiring diagram so I do not know what kind of safety switches, pressure switches etc are installed in the systm other than the switch on the high pressure line. Anybody got a clue????

leolkfrm · 07-29-2009, 08:03 PM

what year etc truck? vac controls, or electric controls could be bad

bradrhale · 07-30-2009, 09:01 AM

vehicle year and make?? pressure could be getting too high if you are not getting adequate heat dissapation out of the condensing unit.

Kawboy · 07-30-2009, 09:46 AM

Oops, sorry, It's a 1999 F250 Super Duty with 7.3 Turbo Diesel. I seem to think that it is a vacuum problem. Becuse it quits cooling when speeds are changed. I did find out yesterday that when running at 50 MPH or lower the A/C does not cut out.

bradrhale · 07-30-2009, 10:31 AM

lower or higher than 50?? you mentioned a moment ago that it did it in stop and go traffic but not at highway speeds. very possible that you might have a blend door problem. as for vacuum related, i dont know what ford uses for those controls. dodge and chevrolet uses electronic controls. look into that.

BernieB · 07-30-2009, 10:48 AM

Just a couple of thoughts...

I know some vehicles have a compressor cutout that will disengage the compressor clutch when the engine is under load, like when accelerating. Don't know if yours does or not.

Is it the compressor cutting out or just the fan?

Are you keeping it on "max cool", or whatever the Ford equivalent is? The max cool setting isn't supposed to be used constantly, it's there just for quickly cooling down a hot vehicle and can cause the evaporator to ice up solid... which would have the effect of blocking airflow through the evaporator. No airflow, no cooling, and probably a musty smell when the ice melts.

Kawboy · 07-30-2009, 11:00 AM

It seems to run fine right at 50 MPH. Once it starts acting up in traffic it continues to act up even at highway speed. I don't think the condensor coils are not dissipating enough heat otherwise why would it run fine during the 90 mile run at speeds of around 60 MPH. I drove it this morning to the store and back in moderate traffic and it ran fine, (about 10 miles round trip). It only acts up after running at highway speeds and then hitting traffic. BTW, the temperatures both times it did it was varied from 75 degrees the first time to 100 degrees the second time. So I don't believe that external temperatures have any bearing. I noticed that there is an electrically driven vacuum pump near the right wheel well, could this be a problem? The pump also had one "T" fitting that didn't have a hose hooked to it.

bradrhale · 07-30-2009, 11:04 AM

i have a similar problem at the moment with a nissan. shop said that a hose had a leak in it and leaked all the freon out. funny, the weekend before, the unit was full and still had the same problems. under their advice, not mine, the bought a new line and had it put on. the unit ran fine but now seems like it is starting to develop the same problems all over again. when i find out what the story is, ill let you know.

Kawboy · 07-30-2009, 11:15 AM

BernieB,

The fan does not cut out, and it moves plenty of air when not cooling. I am well aware of the max cooling position that recircultes the cabs interior air. It quits cooling even on external air use. And yeah like you, I believe that it has a cutout when under heavy load. But after driving in traffic and even after getting back to a steady highway speed of 60-70 MPH it continues to be erratic. The first time it did it I pulled over, shut the engine down for 5 minutes, restarted the engine, and it ran fine after that. Intermitttents are an S.O.B. to troubleshoot.

bradrhale,

By mixture door problem do you mean the door that switches from Floor outlet to, vent oulet, to defrost outlet, etc?

BernieB · 07-30-2009, 11:23 AM

Okay, well that narrows it down some. I'm still thinking it could be the evap icing up, but it would be nice to know if the compressor clutch was engaging or not when the problem happens.

Two ideas, both oddball...

Rig up a test light to the compressor clutch connection that you can see while driving. That'll tell you if the clutch is getting power or not when it happens.

Next time it happens, roll down all the windows and turn the heat on high for a couple of minutes, then try the a/c again. If the evap is icing up that blast of heat should be enough to melt some of the ice and let airflow through the evap again, restoring the a/c.

Oh, and even if the clutch is getting power it could still be a bad clutch, but by knowing you have power to it you narrow it down some more.

07-29-2009, 06:13 PM	#1
Kawboy Full Member Join Date: Dec 2006 Posts: 39	A/C intermittent operation This problem has popped up twice recently. I drove 90 miles on the freeway in 90 degree weather and the A/C worked fine. Would freeze us out if left on high Fan. Got into heavy traffic and it started to work intermittently. After the first time I checked the charge and it was fine. (43 psi on the low side while running) Today it did the same thing. would intermittently cool and then not. I do not have availibility to a wiring diagram so I do not know what kind of safety switches, pressure switches etc are installed in the systm other than the switch on the high pressure line. Anybody got a clue????

07-29-2009, 08:03 PM	#2
leolkfrm Senior Member Join Date: Mar 2008 Location: N.Y. Posts: 4,926	what year etc truck? vac controls, or electric controls could be bad __________________ 4 wheels move the body 2 wheels move the soul B.W.A.I

07-30-2009, 09:46 AM	#4
Kawboy Full Member Join Date: Dec 2006 Posts: 39	A/C intermittent problem Oops, sorry, It's a 1999 F250 Super Duty with 7.3 Turbo Diesel. I seem to think that it is a vacuum problem. Becuse it quits cooling when speeds are changed. I did find out yesterday that when running at 50 MPH or lower the A/C does not cut out.

07-30-2009, 10:48 AM	#6
BernieB Senior Member Join Date: Jul 2009 Location: Fredericksburg, VA Posts: 3,876	Just a couple of thoughts... I know some vehicles have a compressor cutout that will disengage the compressor clutch when the engine is under load, like when accelerating. Don't know if yours does or not. Is it the compressor cutting out or just the fan? Are you keeping it on "max cool", or whatever the Ford equivalent is? The max cool setting isn't supposed to be used constantly, it's there just for quickly cooling down a hot vehicle and can cause the evaporator to ice up solid... which would have the effect of blocking airflow through the evaporator. No airflow, no cooling, and probably a musty smell when the ice melts. __________________ If the guys who design 'em had to take one home, use it every day for 2 years, and do all the maint and repairs themselves, cars would have a button on the dash to change the oil, filters & plugs.

07-30-2009, 11:00 AM	#7
Kawboy Full Member Join Date: Dec 2006 Posts: 39	A/c It seems to run fine right at 50 MPH. Once it starts acting up in traffic it continues to act up even at highway speed. I don't think the condensor coils are not dissipating enough heat otherwise why would it run fine during the 90 mile run at speeds of around 60 MPH. I drove it this morning to the store and back in moderate traffic and it ran fine, (about 10 miles round trip). It only acts up after running at highway speeds and then hitting traffic. BTW, the temperatures both times it did it was varied from 75 degrees the first time to 100 degrees the second time. So I don't believe that external temperatures have any bearing. I noticed that there is an electrically driven vacuum pump near the right wheel well, could this be a problem? The pump also had one "T" fitting that didn't have a hose hooked to it.

07-30-2009, 09:01 AM	#3
bradrhale Senior Moderator Join Date: Apr 2006 Location: Texas Posts: 9,528	vehicle year and make?? pressure could be getting too high if you are not getting adequate heat dissapation out of the condensing unit.

07-30-2009, 10:31 AM	#5
bradrhale Senior Moderator Join Date: Apr 2006 Location: Texas Posts: 9,528	lower or higher than 50?? you mentioned a moment ago that it did it in stop and go traffic but not at highway speeds. very possible that you might have a blend door problem. as for vacuum related, i dont know what ford uses for those controls. dodge and chevrolet uses electronic controls. look into that.

07-30-2009, 11:04 AM	#8
bradrhale Senior Moderator Join Date: Apr 2006 Location: Texas Posts: 9,528	i have a similar problem at the moment with a nissan. shop said that a hose had a leak in it and leaked all the freon out. funny, the weekend before, the unit was full and still had the same problems. under their advice, not mine, the bought a new line and had it put on. the unit ran fine but now seems like it is starting to develop the same problems all over again. when i find out what the story is, ill let you know.

07-30-2009, 11:15 AM	#9
Kawboy Full Member Join Date: Dec 2006 Posts: 39	A/c BernieB, The fan does not cut out, and it moves plenty of air when not cooling. I am well aware of the max cooling position that recircultes the cabs interior air. It quits cooling even on external air use. And yeah like you, I believe that it has a cutout when under heavy load. But after driving in traffic and even after getting back to a steady highway speed of 60-70 MPH it continues to be erratic. The first time it did it I pulled over, shut the engine down for 5 minutes, restarted the engine, and it ran fine after that. Intermitttents are an S.O.B. to troubleshoot. bradrhale, By mixture door problem do you mean the door that switches from Floor outlet to, vent oulet, to defrost outlet, etc?

07-30-2009, 11:23 AM	#10
BernieB Senior Member Join Date: Jul 2009 Location: Fredericksburg, VA Posts: 3,876	Okay, well that narrows it down some. I'm still thinking it could be the evap icing up, but it would be nice to know if the compressor clutch was engaging or not when the problem happens. Two ideas, both oddball... Rig up a test light to the compressor clutch connection that you can see while driving. That'll tell you if the clutch is getting power or not when it happens. Next time it happens, roll down all the windows and turn the heat on high for a couple of minutes, then try the a/c again. If the evap is icing up that blast of heat should be enough to melt some of the ice and let airflow through the evap again, restoring the a/c. Oh, and even if the clutch is getting power it could still be a bad clutch, but by knowing you have power to it you narrow it down some more. __________________ If the guys who design 'em had to take one home, use it every day for 2 years, and do all the maint and repairs themselves, cars would have a button on the dash to change the oil, filters & plugs. Last edited by BernieB; 07-30-2009 at 11:26 AM. Reason: Added data

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