Imperial Homepage -> Repair -> Air Conditioning -> Lukewarm Air
Question from James:
When I was out last night it was around 10 degrees C outside (I don't know what this is in F, sorry) so I set the AutoTemp to 80F. The air it put out was mildly lukewarm and it took a long time to heat the car up. Now, in winter when its dead cold outside it will blast out hot air. Is this lukewarm air when its only mildly cold in the car normal?
Reply from Dick:
To convert C to F, multiply by 9, divide by 5, then add 32 to the result, so it was 50 F.
If you set the control to 80F, the car should have put out copious quantities of calories until the inside temp equaled 80, with the fan blowing really hard from the floor vents. Then it should have shut down to a whisper of air, just enough to maintain the temperature at 80 inside. You don't say what car we're talking about, but unless it's an SATC car (81-83), this is what should have happened. (For completeness, the SATC cars do the same except for the fact that you have to operate the blower speed control with your own little finners.)
I suspect you have a sticking water valve or an ATC control that is out of adjustment.
Did you try setting the temp higher to see if the heat would come on with more authority?
Follow-up from James:
The car is a 76 NYB. Its always done this for as long as I've owned the car. If you crank the temp to the end of the scale it will kick out warmer air. Does that help with the diagnosis? The system works really well in really hot or really cold weather, but can be frustrating when its only a little cool outside.
It appears your system is in good shape except for the calibration of the in car temp sensor. I have not personally worked on the newer systems, (my newest ATC car is a 68), and I think the control design changed slightly in the early 70's. See if Kerry Pinkerton or one of the other guys with experience with the newer cars can offer some advice.
Your best bet may be to just live with it! My sun sensor on the 68 is overly sensitive, so that when I am driving into the sun (about 80 % of the time out here in the desert!), I have to put my wallet over the sensor to avoid my wife's complaints that "it's too cold in here".
In addition to Dick Benjamin's detailed answer, when "Autotemp" is in operation, is it possible that the air conditioning is working in conjunction with the heater, making the heater output much cooler. Maybe when it's very cold outside, the A/C won't come if the temp is below a certain degree? I know this is the case with some newer a/c systems.
Bob is right, the AC won't come on if the evaporator temp is below 32F, but the control system is supposed to take all of that into account, and keep the passengers comfy in all conditions. They do that well when everything is hunky-dory.
I found this to be the case also on a 90 5Th AVE. I just set it hotter or colder as needed. These systems seem to get confused when they are in the "gray" area.
On the 74 through 78 ATC systems, the A/C compressor runs all the time in every position except OFF and VENT. This is done to dry the incoming air in the winter so as to provide better heat and a more effective defrosting action on the glass.
My ATC system works very much like what James described. I've always prided myself in having an ATC that works perfectly so assuming it is, it is a design flaw in that the ATC temperature sensor does not always get an accurate indication of the temperature inside the car.
On a recent trip to the West coast, I was so glad to have an operative AC system. What I did notice though was that in certain circumstances, the ATC would "think" the car's interior was cooler than it really was. I have a digital thermometer mounted very close to the ATC sensor and it consistently read above 80 while I had the ATC set at 65. Even in this situation, on MAX AC, the system would only blow on a lower speed and provide less than ideal relief from the heat. I would check the ATC sensor temperature by placing my hand over the small grill that it resides behind, and I found that the grille was in fact fairly cool. That is what made me believe that the ATC was doing exactly what it should be doing.
Ideally, it would be helpful to provide multiple sensors in the car's cabin so the system could "average" the temperature and provide a more accurate response to the driver's request.
On a cool fall morning, my system reacts much as James describes. It's like the system is designed to become less aggressive the closer it gets to the desired temperature. It may be annoying but I don't think it is not working the way it was designed to work....IMHO that is.
I've thought of connecting a small volt meter to the ATC sensor and calibrating it in degrees F so I can actually see what temperature the sensor "thinks" the car's interior is. I'd wager that it would be wrong 95% of the time. Having taken these systems apart a couple of times, I know they are fairly simple so one cannot expect them to be very smart at the same time.
I still love my ATC.
Follow-up from Dick:
There is a failure which can lead to the in-car sensor thinking it is cooler than it is, that is a leak, even a tiny one, in the air duct that passes near the sensor. If the grille over the sensor feels cool when the air temp is 80, I'll bet a buck that's the cause. I suppose that in a situation where heat is called for, this same leak could cause the same sluggish performance when the temperature difference is getting small, although for heat most of the air is routed through the floor vents, some still comes though the dash vents ( again, at least on the 68).
As for the AC compressor running all the time, that is my understanding also, excepting only that it cycles off and stays off if the evaporator temp goes below 32F, to prevent freezing up the evaporator core thus preventing adequate air flow. The ATC manual describes this function fully, at least in the 68 manual. I assume from this manual that if the outside air temperature is below 32, the compressor doesn't come on at all, but I've no way to check this, living where I do (southwestern desert). If I ever move back to Massachusetts, I'll be sure to notice! (Fat chance!).
Question from Tony (1956):
I had my compressor rebuilt, and all under the hood checked for leaks. Checks good. However, the Freon doesn't last more than 45 days, then warm air is what I get. The air conditioner was not used by the previous owner (for 12 + years!), so I believe there are other check/replacement points that must be looked at in the trunk unit. Can anyone please help with what should be done to correct the situation? Should anything be replaced back there?
I am no mechanic!!! So please tell me specifically what should be done and/or where I can find the instructions/parts, etc.
You need to take the car to an AC shop that has the right equipment to track down your leak. There are so many places the leak could be that it would be pure guesswork for me to advise you. A good leak detector should be able to spot that large a leak.
On the 5 AC front (Tony), I bought today a complete test kit with glasses, dyes, light etc for a whopping $45- a good investment. Anytime you have remote units or components-there are lots of connections- these must be located first and then each checked for leaking. sometimes oil will leave a trace.
I've also had the same problem on my '57: no visual leakage (there's a UV dye in the system) but after some weeks, warm air, rather, no cooling taking place!
I am going to try to add an "A/C stop leak". A friend of mine tried it on a car and it worked!! He put "A/C stop leak plus" from Uview (see http://www.uview.com/homepage.html) but there's a lot of other brands. It's not cheap, but I don't want to remove the evaporator cover.
This page last updated July 1, 2004. Send us your feedback, and come join the Imperial Mailing List - Online Car Club.