A/C blows hot then cold


Cobraguy

GT Owner
The summer here in Alberta has been perfect for the last few weeks...85-90 degrees and unusually humid. I haven't been able to enjoy the gt in the heat of the day though as the A/C alternates between ice cold and smoking hot :ack. I've scanned the search results but have come up empty handed. We went on a road trip on the highway at speeds at times in the triple digits today ( no... not km/hr:facepalm:) for several hours and the A/C worked perfect... in fact I could only have the fan on at %75 or so as it was too cold. Back in the city the temp immediately started to fluctuate again. It's not a gradual change...it's as if the temp knob was being dialed all the way up and then down at 2 minute or so intervals. The temp guage has not been past 210 and there has not been any bumper to bumper slow traffic crawls, only the occasional red light so there has been adequate air flow. I noticed this behaviour once or twice last summer but it was very minor so I didn't pay much attention to it....nothing like it is now. Any members experience this in the past?
 

Kayvan

GT Owner
Jul 13, 2006
4,782
I haven't experienced this

However, I had this happen on a BMW once and the tech dEscribed it as the cooling unit will ice up when it's very humid and air flow becomes restricted not allowing cool air thru

Sorta like cooling fins turning into solid block of ice, if that makes sense

Have to modulate blue cold temp (not fan) few notches up in setting so it doesnt ice until at highway speed where airflow is stronger
 
Last edited:

Cobraguy

GT Owner
Thanks Kayvan.....I should've mentioned that was the first thing I tried. I have plenty of experience with this with a lot of the heavy equipment in my arsenal (think D-10 Cat) as these pieces are seldom going fast enough to create adequate air flow...so the rule is Fan @ Full and Temp @ half. Also, I would have to think that a gt in the Florida heat and humidity ( for example) would be virtually useless if A/C freeze-up was the issue...or not!?. We'll see if someone in the southern states has experienced this. Thanks again!
 

Sonic blue

GT Owner
Jun 1, 2007
52
British Columbia
I started up one of my GT's today and though two weeks ago it was cold as could be, today seemed as if it couldn't get up to anything near full cool. I could hear the compressor coming on and off which tells me there's pressure but not half as cold as it was 2 weeks ago? Love to hear some feedback?
 

jcthorne

GT Owner
Aug 30, 2011
792
Houston
Cobraguy, your temp readings of 210 without hard track use or stop n go traffic on 100+ deg days tells me you have a cooling deficiency. Could be temp sensor is picking up an intermittent overheat situation if there are air pockets in the system. Would cause exactly the symptom you are seeing as the AC cuts off. Just a guess, but you should not be seeing temps that high.

SonicBlue, your ac compressor cycling without full cold temps sounds like icing up the evap, or low freon pressures. Did you try a high fan position and see if there was any change? May need a set of gauges on the system to diagnose.
 

Cobraguy

GT Owner
So JC.....I am assuming you do not hit the 210 temp in Texas without track or traffic....I will have a close look at the rad behind the 05 factory rock screen...I always use an extra bugscreen on the front inlet when on the highway however the previous owner may not have and it did have over 10,000 miles when I purchased it.. BTW, the car generally runs at 190 to 200 under normal conditions...is this what I should expect ?
 

jcthorne

GT Owner
Aug 30, 2011
792
Houston
As long as I am moving, not even close. Usually sits at 180 until I sit in traffic with the AC blowing. Then it will inch up. Only time it ever goes over 200 is getting off the freeway and dropping straight into stopped traffic, but it comes back under 200 in a few minutes. This is in Houston traffic and temps at the 100deg mark.

Now, push the car on the track....yeah, it will heat up, but not in traffic. Mine is an 06 without grille or screen. May have an impact. If I was trying to diagnose your AC problem, I'd hook up my data scanner and watch cylinder head temps vs what the AC is doing and see if they are related while driving. Could be in the controls too but I would start with ruling out the easy stuff.
 

Cobraguy

GT Owner
Will do....Thanks for the advice.
 

Sonic blue

GT Owner
Jun 1, 2007
52
British Columbia
going to hook it up on gauges tomorrow, thanx!
 

Silverbullitt

GT Owner
Mar 3, 2006
1,757
Lago Vista, TX
If it is humid make sure the AC is set on recirculate. Drier air in the cabin will be running across the coil and the chance of icing will be reduced. Mine is a little slow to cool after 6 yrs, but will overcome the tx heat after about a minute, temps are usually around 180, mine is an 06 so there is no grillwork obstructing airflow.
 

roketman

GT Owner
Mark IV Lifetime
Oct 24, 2005
8,018
ma.
I too have had similar issues.I realized two fixes.One is to put in recirculate.The other is to move the temp adjustment knob to medium for just a second and than back to full cold.This seems to solve the problem.Also if you are not aware .The AC shuts off under full acceleration ,for max H.P.,than it comes back on.I believe removing the exit screens may help too.I have not had mine in for years.
 

Cobraguy

GT Owner
Again...thanks to all for the opinions and/or advice....lots to work with now !
 

Kayvan

GT Owner
Jul 13, 2006
4,782
 

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,213
Myakka City, Florida
This happened to me once. Cruising easy on a 100 degree day. Coolant temp never reached 200. Hit some traffic lights and the A/C blew hot. Turned it off, turned it on, drove out of town and all was fine. I know this doesn't help with a diagnosis, but I'm just adding to the list of others who had the same issue.
 

DanQ

GT Owner
Aug 18, 2005
336
Lake Zurich, IL
When I first read about this problem I was thinking it was a stuck blend door. Not sure if it is, but its another theory for you
see this:
http://www.fordgtforum.com/forums/s...-what-could-be-the-issue&highlight=blend+door
or do a search on "blend door"

good luck
 

Cobraguy

GT Owner
[


QUOTE=roketman;314582]I too have had similar issues.I realized two fixes.One is to put in recirculate.The other is to move the temp adjustment knob to medium for just a second and than back to full cold.This seems to solve the problem.Also if you are not aware .The AC shuts off under full acceleration ,for max H.P.,than it comes back on.I believe removing the exit screens may help too.I have not had mine in for years.[/QUOTE]



Roketman...I simply put the A/C in recirc mode with temp knob @ 90%.....Problem solved !! (That was easy)
 

peiserg

GT Owner
Aug 15, 2010
283
Phoenix, az
had same issue; was low on freon. easy, cheap fix. worked great ever since.
 

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,213
Myakka City, Florida
Reading the FSM, there are many parameters that shut the A/C off. Most are normal (WOT, etc.). One is perhaps environmental. If the compressor housing temperature reaches 247 degrees F, the A/C switches off.

I can make a case in my mind for that. Hot speed run on a hot day, then stuck in traffic with little air moving through the engine area leading to heat soak.

Would the compressor housing reach 247? IDK. But, it is on the bottom, not far from the exhaust, and enclosed by the belly pan.
 

Gerry

Well-known member
Oct 13, 2006
51
Dearborn, Michigan
Poor A/C performance in Hot Weather may be due to the A/C high pressure cut-off switch being set too low. When the high side pressure gets above about 425 psi, it will cut out and not cut back in until 200 psi. You would need to change the switch on the compressor. 4 pin connector, black casing is stock. Scharader valve on it so you don't have to discharge the system to replace it. There is also a pressure relief valve (Dump) that dumps freon if pressure gets too high, usually around 550-650 psi.

Here are the specs:
Black casing switch - YF1Z-19D594-AA - Cooling fan ON - 310 to 340 psi; Clutch cut-out 435 to 475 psi, clutch re-engages at 220 to 280 psi.
white casing switch 8L8Z-19D594-A - cooling fan ON 285-315 clutch cut out 435-475 clutch re-engage 220-280
gray casing switch 7S4Z-19D594-AA cooling fan ON 227-257 clutch cut out 420-460 clutch re-engage 220-280

If you go with the white casing switch, you can get the high speed cooling fan on sooner. I could not find a 4 pin switch with a higher cut-out pressure.
You can verify it is cutting out due to high pressure by hooking up a set of a/c gauges and watch as pressure rises on the high side gauge, and watch when the compressor clutch kicks off. If you hook up a jumper with switch to the 2 pins for the cooling fan, you can control high speed fan to go on when you flip the switch.
When I forced the high speed fan on myself, the high side pressure dropped immediately and the a/c clutch stayed engaged. I did this on my Mercury with similar sysytem. I think we delayed the high fan so we could pass the alternator charging system performance test. My preference is to have the fan come on around 200 psi. My Mercury switch is 4 pin, very similar, and the obsolete switch I installed turns the fan on much sooner. Not sure if it will physically fit the GT, but the switching is the same. Cut-out switch is normally closed, the cooling fan swithch is normally open. I can provide that switch part number and a source if anyone wants to try it. About $45. Gray casing.

Gerry from Ford Service Engineering
 
Last edited:
  • Like
Reactions: Gerry B

fjpikul

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Jan 4, 2006
11,529
Belleville, IL
thank you Gerry. Mods, can you make this post a sticky?