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05XT, 2 DC Audio Lvl3 15", DC Audio 1.2k monoblock, DC Power 270AMP alternator, SUMO 2in. lift,
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Discussion Starter #1
Back with another question about my beloved XT. Got 158,000 on the clock now.

This summer up here in Colorado though I have noticed my AC blowing ice cold, then gets warmer, quite significantly, then back to ice cold. All in about 20s or so and so on for the remainder of the drive. This is with the temperature set to 65 on both sides, recirculation on, and auto mode on. However, I have tried changing all of these settings and it makes no difference, even with auto mode off. I will feel this temperature fluctuation even if I have it up to 75 or so for the set temp. The funny thing is, if I leave the car to idle at a light or something, it blows ice cold with no fluctuations.

I have already tried switching the relays with the radiator fans, no difference. I've read countless forums and have gotten mixed messages. Some say it is normal and this is just a poor AC system, others say it could be a bad compressor clutch or low pressure switch. I'm so lost on this haha. 馃

I wouldn't care too much except that it seems to inhibit how effectively it cools the cabin. I usually have to leave the thing on full blast for about an hour or so cool the cabin enough. Any info helps, as I may take it to get looked at soon.
 

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Check the pressure in the AC lines, one of the many threads here has specs or I can pull it from the FSM. Aside from changing the relay, new cabin air filter, and compressor o-rings, mine was very overcharged and exhibited the same symptoms (but this is the basic manual HVAC system). Got the pressures correct, AC works fine now.
 

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05XT, 2 DC Audio Lvl3 15", DC Audio 1.2k monoblock, DC Power 270AMP alternator, SUMO 2in. lift,
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Discussion Starter #3
Check the pressure in the AC lines, one of the many threads here has specs or I can pull it from the FSM. Aside from changing the relay, new cabin air filter, and compressor o-rings, mine was very overcharged and exhibited the same symptoms (but this is the basic manual HVAC system). Got the pressures correct, AC works fine now.
Thanks little_joe! I just tested the low side with an AC pressure gauge today. It was sitting at about 45 psi with outside air being around 82F when at an idle. It was blowing cold air out of the dash. However, when I revved the engine up to about 2000 rpm and then let off, the pressure dropped significantly to less than 10 psi, then went back up to above 40 psi. Could this be a problem?
 

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2005 Legacy Wagon Limited 4EAT
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It could be the evaporator temp sensor being a bit skewed and shutting the compressor off thinking it's much colder than it actually is. It's function is to prevent ice formation but sometimes the reading drifts enough where it'll shut off in the mid 40's and not kick back on until the mid 50's.

Mine was doing this and I performed this mod and I haven't had the problem since.
 

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Mine was doing this since in bought the car a few months ago. Just got around to replacing evap temp sensor and it fixed it. Luckily we see a ton of Subaru鈥檚 in our shop so this was a common issue that was resolved easily. You do not need to pull the dash to replace and the parts about $38-45.
 

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05XT, 2 DC Audio Lvl3 15", DC Audio 1.2k monoblock, DC Power 270AMP alternator, SUMO 2in. lift,
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Discussion Starter #6
Awesome thanks guys for the advice!

New update: I drove the vehicle using FreeSSM to monitor the AC on/off switch, and it does command the compressor to turn off every 10s or so, and that correlates with the 6s of warmer air I get before it comes back on and goes back to cold and so on, until I come to a light where the compressor will not shut off until I get moving again. Also, my refrigerant level was recharged by my mechanic a few months ago and it is factory spec.

With the evaporator temperature sensor though, could that still be the cause of the this issue? Because my problem does not happen when I'm at an idle. It only happens when the engine rpms are raised to around 1800 rpm or more. Because of that, I'm torn that it could be a bad compressor (its the original one), but that temp sensor does make sense as well. Also, where is the location of this sensor in the passenger side dash? That's a lot easier to fix then a compressor.
 

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Awesome thanks guys for the advice!

New update: I drove the vehicle using FreeSSM to monitor the AC on/off switch, and it does command the compressor to turn off every 10s or so, and that correlates with the 6s of warmer air I get before it comes back on and goes back to cold and so on, until I come to a light where the compressor will not shut off until I get moving again. Also, my refrigerant level was recharged by my mechanic a few months ago and it is factory spec.

With the evaporator temperature sensor though, could that still be the cause of the this issue? Because my problem does not happen when I'm at an idle. It only happens when the engine rpms are raised to around 1800 rpm or more. Because of that, I'm torn that it could be a bad compressor (its the original one), but that temp sensor does make sense as well. Also, where is the location of this sensor in the passenger side dash? That's a lot easier to fix then a compressor.
That was the exact same data and conditions as mine. As soon as I got off idle it would shut off. I could idle for hours and it鈥檇 be just fine. The cheapest way to figure it out is to pull the sensor out of the HVAC box. Leave it plugged in but just hanging and drive the car around. If it works then you know the sensor is faulty, that鈥檚 what I did. The sensor itself is on the drivers side of the car. Behind black plastic cover by gas pedal. Take out the 3 Phillips in the cover and youll see a small sensor in the side held in by 1 Phillips. It plugs in right there and also has an adapter harness that runs under the box to the passenger side where it plugs into the body harness.
 

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Discussion Starter #8
That was the exact same data and conditions as mine. As soon as I got off idle it would shut off. I could idle for hours and it鈥檇 be just fine. The cheapest way to figure it out is to pull the sensor out of the HVAC box. Leave it plugged in but just hanging and drive the car around. If it works then you know the sensor is faulty, that鈥檚 what I did. The sensor itself is on the drivers side of the car. Behind black plastic cover by gas pedal. Take out the 3 Phillips in the cover and youll see a small sensor in the side held in by 1 Phillips. It plugs in right there and also has an adapter harness that runs under the box to the passenger side where it plugs into the body harness.
I seriously love this forum.

Thank you so much! Turns out this is exactly my problem. On Saturday I took out the sensor (I didn't realize the evaporator is located in the middle of the dash not the passenger side, which makes more sense now) and just let it hang. Did some driving and it blows ICE cold no matter how I'm driving it. Its crazy how much taking the sensor out affected the level of cold my ac can blow now. Now, I know the sensor is there to protect the evaporator from freezing up. So I'm guessing its not a good idea to let the sensor hang and not put a new one in?

Also, is this sensor only something I can get from a dealer?
 

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I seriously love this forum.

Thank you so much! Turns out this is exactly my problem. On Saturday I took out the sensor (I didn't realize the evaporator is located in the middle of the dash not the passenger side, which makes more sense now) and just let it hang. Did some driving and it blows ICE cold no matter how I'm driving it. Its crazy how much taking the sensor out affected the level of cold my ac can blow now. Now, I know the sensor is there to protect the evaporator from freezing up. So I'm guessing its not a good idea to let the sensor hang and not put a new one in?

Also, is this sensor only something I can get from a dealer?
They鈥檙e available from the dealer or online. Usually around $40. Part number 73540AG030. I wouldn鈥檛 leave it out for long as the evaporator can freeze up. I鈥檝e never seen that cause an issue but it鈥檚 inconvenient as your AC will stop working. I would just set the temp a little higher than normal until you replace it.
 

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The sensor itself is on the drivers side of the car. Behind black plastic cover by gas pedal. Take out the 3 Phillips in the cover and youll see a small sensor in the side held in by 1 Phillips.
I took out the sensor (I didn't realize the evaporator is located in the middle of the dash not the passenger side, which makes more sense now) and just let it hang.
Did you have to remove any other trim to gain access to the screws holding the cover?

I thought I'd have a look at my 07 to confirm access and the wiring/connection to the sensor. (Not troubleshooting; just getting more familiar . . .). Looks as if I would have to remove the black under-cover of the dash, as well as the light colored section of the center console, at the least. (The harness in the photo goes to the accelerator pedal.) Is it the same in your 2005s?

thermo access 07.JPG
 

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Did you have to remove any other trim to gain access to the screws holding the cover?

I thought I'd have a look at my 07 to confirm access and the wiring/connection to the sensor. (Not troubleshooting; just getting more familiar . . .). Looks as if I would have to remove the black under-cover of the dash, as well as the light colored section of the center console, at the least. (The harness in the photo goes to the accelerator pedal.) Is it the same in your 2005s?

View attachment 489890
Did you have to remove any other trim to gain access to the screws holding the cover?

I thought I'd have a look at my 07 to confirm access and the wiring/connection to the sensor. (Not troubleshooting; just getting more familiar . . .). Looks as if I would have to remove the black under-cover of the dash, as well as the light colored section of the center console, at the least. (The harness in the photo goes to the accelerator pedal.) Is it the same in your 2005s?

View attachment 489890
I just unclipped the lower lighter piece at that plastic clip. Then unclip the harness from the black plastic panel. Remove 3 Phillips screws in black panel and wiggle it out. Then the sensor is easily accessible.
 

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I just unclipped the lower lighter piece at that plastic clip. Then unclip the harness from the black plastic panel. Remove 3 Phillips screws in black panel and wiggle it out. Then the sensor is easily accessible.
Thanks. I'll have another go at it. I could se only two screws, one visible in the photo at 5:00 o'clock of "pedal harness", and one in a recess under the white tag on the pedal harness. Are those two of the three? Where is the third?

Also, Subaru Parts Online from Subaru of America lists two sensors for my 07 2.5i. 73540AG030 ($50), and 73540AG010 ($120). Any idea how they are different (other than price!)?
 

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Thanks. I'll have another go at it. I could se only two screws, one visible in the photo at 5:00 o'clock of "pedal harness", and one in a recess under the white tag on the pedal harness. Are those two of the three? Where is the third?

Also, Subaru Parts Online from Subaru of America lists two sensors for my 07 2.5i. 73540AG030 ($50), and 73540AG010 ($120). Any idea how they are different (other than price!)?
The 07 may be slightly different. I鈥檇 call the dealer with your vin and they can give you a for sure number.
 

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The 07 may be slightly different. I鈥檇 call the dealer with your vin and they can give you a for sure number.
Thanks.

That photo was very helpful. The third screw would be under the light colored trim piece. I'll see if it can be pulled away far enough to remove the screw, and then the cover. Want to see how the sensor is fitted inside, and how it's connected to the harness. On line pictures and drawings that I could find aren't complete or detailed enough.

Thanks
 

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Thanks.

That photo was very helpful. The third screw would be under the light colored trim piece. I'll see if it can be pulled away far enough to remove the screw, and then the cover. Want to see how the sensor is fitted inside, and how it's connected to the harness. On line pictures and drawings that I could find aren't complete or detailed enough.

Thanks
Yes you鈥檙e correct. Once you pull that out of the way slightly you can sneak it out. The sensor is held in with one Phillips then it just slides out. It plugs in about 3鈥 from there. The harness then runs under the hvac box and over to passenger side where it connects again to the harness. You don鈥檛 need to worry about that connection though.
 

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Yes you鈥檙e correct. Once you pull that out of the way slightly you can sneak it out. The sensor is held in with one Phillips then it just slides out. It plugs in about 3鈥 from there. The harness then runs under the hvac box and over to passenger side where it connects again to the harness. You don鈥檛 need to worry about that connection though.
Thanks again.

I haven't found any photos of the 73540AG030, but ones for the 73540AG010 show a box at what I believe is the opposite end of the harness under the hvac box.

Here's the parts catalog diagram of the 73540AG030
489936


And here's a photo of the far end (the "?" above) from a search for 73540AG010 [source, with more photos: 2006 Subaru Legacy Thermistor Heater Sensor Module Part # 73540AG010 | eBay]
489943


I wonder if the box contains some electronics that amplifies/modifies the sensor signal and sends it on to the ECM. The cost difference between the . . .AG030 (lower) and the . . . AG010 (higher) might be that the former is just the sensor (to the first connector) whereas the latter might be the whole assembly including the harness under the hvac box and this light-colored box at the other end.

When you ordered the 73540AG030, what did you receive?
 
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