Surfdougyfresh Posted August 25, 2006 Share Posted August 25, 2006 Hey all... It's summer here in Saudi Arabia and after taking an hour drive on the highway (120F/80% humidity) in my 98 outback (2.5L), I was sweating and cursing my subie as the AC would cycle between blowing very cold and very hot. I guess it would blow cold for about 20 seconds and then blow hot for 1 minute. It usually doesn't give me a tough time and blows cold almost immediately during my short 5 minute daily commute, but the long drives during the midday heat leave me sweating. The hour drive home at 9pm (100F/80% humidity) was fine. I don't believe the compressor cycled as often, but I'm sure it cycled and didn't blow hot air during the 'down' time. I should note that the engine temp gauge would rise just a bit which seemed to kill the cold air. Once the engine temp dropped a small bit, it would blow cold again. Any connection between the engine cooling and AC systems? Thanks for any advice.... Quote Link to comment Share on other sites More sharing options...
Setright Posted August 25, 2006 Share Posted August 25, 2006 The AC takes heat from the cabin and dumps just in front of the engine radiator. So running the AC will "stress" the engine cooling system. You need to make sure the AC radiator is free of dirt/bugs/rust/stones so as to allow as much air through it as possible. A lot of gunk will accumulate between the two radiators. I've found that flushing with a hose can get a lot of the stuff out. Quote Link to comment Share on other sites More sharing options...
OB99W Posted August 25, 2006 Share Posted August 25, 2006 It's summer here in Saudi Arabia and after taking an hour drive on the highway (120F/80% humidity) in my 98 outback (2.5L), I was sweating and cursing my subie as the AC would cycle between blowing very cold and very hot.[...]I should note that the engine temp gauge would rise just a bit which seemed to kill the cold air. Once the engine temp dropped a small bit, it would blow cold again. Any connection between the engine cooling and AC systems? Hmmm, those temperature/humidity levels can be stressful for both you and the car. The A/C relay (the one that controls the compressor clutch) is directly controlled by the ECU (engine computer). I'm not sure what all the parameters are for shutting down the A/C are, but you should probably do everything you can to ensure that the engine is as cool as possible. As Setright mentioned, what sort of looks like the engine radiator (it's actually the A/C condenser) is situated in front of the actual radiator; if it's obstructed in any way, it affects both the engine cooling and A/C operation. It would also be good to verify that both radiator fans are coming on when the A/C is operating. Quote Link to comment Share on other sites More sharing options...
nipper Posted August 25, 2006 Share Posted August 25, 2006 there is no humidity out there, its desert. Check the pressure in the ac system first befor you do anything else. It sounds like you may just be low on refrigerant. nipper Quote Link to comment Share on other sites More sharing options...
mwatt Posted August 25, 2006 Share Posted August 25, 2006 Hey all... It's summer here in Saudi Arabia and after taking an hour drive on the highway (120F/80% humidity) in my 98 outback (2.5L), I was sweating and cursing my subie as the AC would cycle between blowing very cold and very hot. I guess it would blow cold for about 20 seconds and then blow hot for 1 minute. It usually doesn't give me a tough time and blows cold almost immediately during my short 5 minute daily commute, but the long drives during the midday heat leave me sweating. The hour drive home at 9pm (100F/80% humidity) was fine. I don't believe the compressor cycled as often, but I'm sure it cycled and didn't blow hot air during the 'down' time. I should note that the engine temp gauge would rise just a bit which seemed to kill the cold air. Once the engine temp dropped a small bit, it would blow cold again. Any connection between the engine cooling and AC systems? Thanks for any advice.... Our '98 Outback and '99 Legacy GT do this to some extent. First, I had the A/C in both cars evacuated and recharged to be certain the right amount of refrigerant was in the A/C system. (It turns out they were charged correctly...) I've been wondering if the temperature sensor for the evaporator is malfunctioning. Obviously, its purpose is to prevent the core from freezing by cycling the compressor on and off. You can see the sensor if you remove the glove box--it's mounted on the evaporator case right behind the glove box and it has a probe that goes into the evaporator core. I've contemplated trying to bypass it to see if the A/C will blow really cold continuously---but I realize that's a bad idea because that can cause the A/C system pressures to be much too high..... Quote Link to comment Share on other sites More sharing options...
SuBrat84 Posted August 25, 2006 Share Posted August 25, 2006 My 98 GT did that for a while in the heat out here in Tucson. It ended up being an air pocket in the cooling system. *yikes* Also, make sure both fans come on with the AC on. Cleaning that stuff out with a good pressure wash is a good idea too. If none of that helps... try installing a "Windshield washer" nozzle to spray onto your radiator (ac radiator) area. Just make sure you put water in that one and not soapy water! Quote Link to comment Share on other sites More sharing options...
OB99W Posted August 25, 2006 Share Posted August 25, 2006 there is no humidity out there, its desert. Depends on where; 100+ degrees F with 50-60% humidity isn't uncommon in some parts. Check the pressure in the ac system first befor you do anything else. It sounds like you may just be low on refrigerant.Sure, low refrigerant can cause the cycling, but it's worth looking at the condenser airflow since that's a low-tech/low-cost problem to deal with. Quote Link to comment Share on other sites More sharing options...
BigMattyD Posted August 25, 2006 Share Posted August 25, 2006 The a/c on my car worked fine until I tried to add a little refrigerant. (I was working on another car of mine, and wondered if adding a little to my Legacy would give it a boost.) I ran into the same problem you are having. The compressor would cycle on for a few seconds, then off for 30 or so. I evacuated a bit of the refrigerant from the low pressure port, which solved the problem. I still get the symptom (rarely) when the ambient temp exceeds 95 F. I think this is due to the computer disabling the compressor when it senses high refrigerant pressure. I am sure there is both a low pressure cutoff and a high pressure cutoff. That's why releasing a bit of coolant worked, and why high temps worsen the situation. Higher temp = higher system pressure. Usually if there is low pressure, you have no cold air, and the compressor fails to even start. Matt D P.S. This means we have at least 3 different theories: Low refrigerant pressure High refrigerant Pressure High coolant temp. I am of course voting for my own theory, because it seems to fit the data the best. Quote Link to comment Share on other sites More sharing options...
nipper Posted August 25, 2006 Share Posted August 25, 2006 The a/c on my car worked fine until I tried to add a little refrigerant. (I was working on another car of mine, and wondered if adding a little to my Legacy would give it a boost.) I ran into the same problem you are having. The compressor would cycle on for a few seconds, then off for 30 or so. I evacuated a bit of the refrigerant from the low pressure port, which solved the problem. I still get the symptom (rarely) when the ambient temp exceeds 95 F. I think this is due to the computer disabling the compressor when it senses high refrigerant pressure. I am sure there is both a low pressure cutoff and a high pressure cutoff. That's why releasing a bit of coolant worked, and why high temps worsen the situation. Higher temp = higher system pressure. Usually if there is low pressure, you have no cold air, and the compressor fails to even start. Matt D P.S. This means we have at least 3 different theories: Low refrigerant pressure High refrigerant Pressure High coolant temp. I am of course voting for my own theory, because it seems to fit the data the best. except the computer does not monitor the pressure in the ac system. The AC will cut out under full throttle. nipper Quote Link to comment Share on other sites More sharing options...
BigMattyD Posted August 26, 2006 Share Posted August 26, 2006 Perhaps I misstated my opinion. The ecm may not monitor refrigerant pressure, but the pressure is monitored by a pressure switch. The compressor will cut out if the pressure is too high or too low. I will attach some FSM sheets from the 2003 Legacy to prove my point, but I am pretty sure all models include a high pressure cutoff switch. This further bolsters my opinion. Being a "dumb" switch, it will cut out the compressor anytime the pressure rises above the set point, then as the pressure declines, allows it to come back on until it senses overpressure again, ad infinitum. The effect is a very annoying cycling of the A/c compressor on very hot days, but perfect performance in more moderate conditions. If the refrigerant were low enough to trigger the low pressure cutoff, the compressor would never run, and there would never be good cooling. I do not have verifiable information that the a/c system performance is modified by engine coolant temp or throttle position, but I guess it would be possible. If anyone has this info, please post it, as it would be helpful. In fact, I will attach a limited diagnostic chart re: compressor function, and you will not see any reference to coolant temp or ecm control. Subararily Yours, Matt D. ac pressure switch.pdf hvac diagnostics.pdf Quote Link to comment Share on other sites More sharing options...
nipper Posted August 26, 2006 Share Posted August 26, 2006 Perhaps I misstated my opinion. The ecm may not monitor refrigerant pressure, but the pressure is monitored by a pressure switch. The compressor will cut out if the pressure is too high or too low. I will attach some FSM sheets from the 2003 Legacy to prove my point, but I am pretty sure all models include a high pressure cutoff switch. This further bolsters my opinion. Being a "dumb" switch, it will cut out the compressor anytime the pressure rises above the set point, then as the pressure declines, allows it to come back on until it senses overpressure again, ad infinitum. The effect is a very annoying cycling of the A/c compressor on very hot days, but perfect performance in more moderate conditions. If the refrigerant were low enough to trigger the low pressure cutoff, the compressor would never run, and there would never be good cooling. I do not have verifiable information that the a/c system performance is modified by engine coolant temp or throttle position, but I guess it would be possible. If anyone has this info, please post it, as it would be helpful. In fact, I will attach a limited diagnostic chart re: compressor function, and you will not see any reference to coolant temp or ecm control. Subararily Yours, Matt D. AC cuts out when the temp gauge gets 3/4 of the way up to take the heat load off the cooling system. Ac cuts out at full throttle so that all the HP gets to the rear wheels. This has been a common thing since the 1980's. The ECU is connected to the AC relay. The reson why isa so that the ECU can cut out the AC in full throttle or when the car starts to run hot. This is common to almost all modern cars. ALso in limp home mode AC on some cars ac wont operate either, and i think subaru may be one of them. Fortunitly no one gets there car that bad to get in limp mode. The diagnosis given is done with the car at idle, standing still. Not everything is in a diagnostics book, as i see nothing is mentioned about properly reading gauges. Thats the very first diagnostics that should be done with any AC system. There just are some things that you need to take us on our word on, as we seem to have over 1000 or more years plus combined exp. :-p. nipper Quote Link to comment Share on other sites More sharing options...
Surfdougyfresh Posted August 26, 2006 Author Share Posted August 26, 2006 Thanks for all the replies guys. I had the refridgerant checked a few weeks ago and it was fine so I was pretty sure it wasn't that. After writing initially and searching a bit, I took a look at the radiators. There was a napkin on the bottom, so I removed that and cleaned both the radiator and condensor with a garden hose. Both fans were working. With my digital meat thermometer in tow, we were off to Bahrain for a 'legal' beer. it was 120F again when we left at 1030am. I think the humidity had let up a bit, but their is certainly no 'dry heat' here in August and September. On the highway the temp would cycle between 51F and 60F...very nice! Even idling at customs for 20 mins., the temp wouldn't go any higher than 70F. I'll have to give a bit more attention to the front of the car when washing from now on. Thanks again! Quote Link to comment Share on other sites More sharing options...
Martinjmpr Posted August 26, 2006 Share Posted August 26, 2006 there is no humidity out there, its desert. nipper You'd be surprised. I was in Kuwait for almost all of 2004 and there were some days in August and September when the humidity approached 90%. The air was so thick we almost choked on it. Quote Link to comment Share on other sites More sharing options...
BigMattyD Posted August 28, 2006 Share Posted August 28, 2006 I don't deny that the ecu may control the a/c based on any number of factors, I am just looking for some comprehensive info regarding its function. I have some FSM's, so I will take some time to look for the info when I get some free time. I would like to better understand the system as a whole. I have found that reading the FSM and technical documentation is very enlightening, both with my cars, and with my motorcycle. In fact, my Suzuki GS 450 GA is an automatic transmission motorcycle with a torque converter, and the service manual has a whole section detailing the function and theory behind its operation. Extremely helpful information. matt d Quote Link to comment Share on other sites More sharing options...
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