A/C Cycling

I bought my 05 LGT wagon 2 weeks ago and the a/c has been cycling between kick ass and warm (which is really friggen annoying). The car has the dual zone climate control. I figured it was probably low so I recharged it with 15oz but the problem continued. I found the evaporator temp sensor connector and checked the voltage across the sensor. The compressor kicks off at 2.06 volts on my car. I went to radioshack and got a 100k ohm trimmer potentiometer and soldered it to two jumper wires with spade terminals. The jumpers connect inline between the evaporator temp sensor and the vehicle harness. The pot parallels the sensor. I adjusted the pot so that the compressor kicks off at 34 degrees. Riding down the road with a/c cranked, its blowing at 38-39 degrees and not cycling

 

The temp sensor connector is under the passanger side of the dash, above the lower panel. Remove the 3 push in fasteners and pull the carpet back a bit and the panel will pull out. The wires on my car are Light Blue and Brown-White on the harness side of the connector.

 

To lower the cut off temp and reduce cycling, lower the resistance. To increase cut off temp and prevent evaporator freeze up, increase the resistance. I started at about 10k and ran the a/c at low fan speed until the vent temp got to about 38 degrees, then increased the resistance until the voltage across the connector got to about 2V. This caused the compressor to cut off at 34 degrees. This is as close to freezing as I want to get. If it freezes up I will increase the resistance to raise the cut off, but for now I'm happy with it. I didn't check to see what resistance I ended up with, but it will likely be different with each vehicle. I don't think setting the resistance too low (i.e. 0 ohms) will do any harm, but here comes the disclaimer...

 

I am not responsible for any damage, personal injury, loss of sex drive, loss of marrige, etc. that may result from applying the information contained in this post. Do so at your own risk. Yada yada yada and so on and so forth...

 

The first pic is the lower panel with the 3 retainers to remove. The second pic is the connector with the jumper and pot in place.

 

This might be a good candidate for the Compilation of cheap and/or free Legacy Mods sticky. Total cost under $5.

 

Enjoy!

Edited June 5, 2016 by Neukam

So this is normal function, I noticed that it only cycles on hot days but when it's 70*F ambient it blows icecold non stop.

 

I wonder if there is a way to fix it while keeping the stock sensor plugged up.

This method does keep the stock sensor hooked up. It just allows you to adjust the sensor's output. It's like using a piggy back computer to rescale a MAF sensor. Same concept. The a/c will still cycle but with the pot, you can set it to cycle when YOU want it to. Mine was cycling somewhere around 45 degrees, which was very noticeable when it's hot outside but not so noticeable when its cool. Now it doesn't cycle when the fan is maxed, but will cycle as needed when the fan speed drops and the demand is low, and isn't as noticable because the evaporator doesn't warm up as much because of the low demand.

Bumping this thread back from the dead just to say that I found it very helpful. My A/C was cycling at 48F so I did what this guy did and it worked awesome. Now it's cycling at 38F. Super easy fix.

FWIW, couple years back my AC would work fine for say an hour or more ,then would blow warm air. I'd turn it off, drive for a mile or so and turn it back on and the AC was fine again. I may have felt the blower slowing too, but not really sure some two years later.

 

The fix was to replace the $6.00 AC relay in the under hood fuse box.

Yeah I did see that there were a lot of complaints about the relay but that wasn't the case here. The evap temp sensor was clearly out of wack.

I'm glad it helped. I've been running mine that way for 6 years and it's still working friggen awesome

Do you have an example of a 100k ohm trimmer potentiometer that you used?

I'm mostly finding three pin variants.

I used this one since it was the only thing available to me during covid lockdown and I'm lucky enough to live near a Microcenter.

 

https://www.microcenter.com/product/466073/nte-electronics-24mm-potentiometer-100kohm-1-4w---2-pack

 

The 3rd pin just isn't used. I only used the middle and right one.

Thank you for sharing this information! For those of us that live in a hot climate the cycle pattern of this compressor is unacceptable. What are your thoughts of the below, will this work ok? I do not know how this compressor and set up got past the rigorous engineering testing they supposedly have, must’ve been testing it in the dead of winter. I have two cars they both do the same thing.. one has 100,000 miles one has 15,000 miles -05 GT-design flaw. Extremely noticeable when not in motion. Checked all the normal things relay, freon level.. AC filter. I recall seeing Mr. Subaru on YouTube describing our AC compressor as not up to snuff.. supposedly they changed it back to the older supplier after our generation.

 

 

 

 

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Edited May 15, 2021 by legacysti05

Hello I have what you're looking to buy!

DM your email address for more information.

 

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Hey I was just looking for this thread

Hey I was just looking for this thread

 

 

Glad you found it, i’ll let you know how the installation works out when I have a free minute.

 

Have a good weekend!

 

 

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Thank you for sharing this information! For those of us that live in a hot climate the cycle pattern of this compressor is unacceptable. What are your thoughts of the below, will this work ok? I do not know how this compressor and set up got past the rigorous engineering testing they supposedly have, must’ve been testing it in the dead of winter. I have two cars they both do the same thing.. one has 100,000 miles one has 15,000 miles -05 GT-design flaw. Extremely noticeable when not in motion. Checked all the normal things relay, freon level.. AC filter. I recall seeing Mr. Subaru on YouTube describing our AC compressor as not up to snuff.. supposedly they changed it back to the older supplier after our generation.

 

 

Those pots should work fine. I think the person responsible for testing the system is the same person that was responsible for aligning the cabin air filter to the access hole in the glovebox.

Those pots should work fine. I think the person responsible for testing the system is the same person that was responsible for aligning the cabin air filter to the access hole in the glovebox.

 

 

Neukam, you are a legend!

 

Thank you for figuring this out for the community! as apparently the engineers don’t know what they are doing.

 

Package is running late from Amazon when I get a chance between work I am eager to get this working.

 

This Amazon one has three cables do you think I should use the black and red cable? Some people in the review saying they were not wired right. I guess I’ll find out... hopefully the wire gauge is thick enough?

 

Thanks again brother, your life saver.

One of the only things I really dislike about the car is the AC.. hopefully this should fix The cycling issue.

 

This definitely should be pinned as I know everyone in hot climates would benefit from this. -At the owners own risk of modify.-

 

 

There’s only 1 million threads of complaints of the cycling issue. This is definitely a problem, back in the day the dealerships were sweeping it under the rug. Or testing it in covered garage with not direct sunlight not replicating the exact real world conditions. I wonder if this could be changed in software on the ECU as well?

 

Jason.

 

 

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Edited May 16, 2021 by legacysti05

Here’s the specs for the pots, if you see anything that’s incorrect let me know..

 

Thanks

 

 

Molence 5PCS WH148 Rotary Potentiometer, 100K ohm 3PIN 15MM Linear Taper Potentiometer with XH2.54-3P Connector Wire Cable Black Knob

 

https://www.amazon.com/dp/B08RXPJ6R3/ref=cm_sw_r_em_api_glt_fabc_DZZ8N5MYX3D5FPDS2BY5

 

 

 

 

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Edited May 16, 2021 by legacysti05

To lower the cut off temp and reduce cycling, lower the resistance. To increase cut off temp and prevent evaporator freeze up, increase the resistance. I started at about 10k and ran the a/c at low fan speed until the vent temp got to about 38 degrees, then increased the resistance until the voltage across the connector got to about 2V. This caused the compressor to cut off at 34 degrees.

 

OP, when you're calling out these temperatures, are you referencing an arbitrary vent temp measurement? Or do you have a means to reference the actual measured temperature reported by the ambient temp sensor? Just trying to understand what the actual "tuning" process looks like and what tools are required. Thanks!!

I don't have a means to read the temperature that is actually reported by the sensor, but we are essentially tuning that sensor to read whatever it needs to read to get the desired result. You can measure the air temperature at the vent if you have an accurate thermometer or you can convert the low side refrigerant pressure to the saturation temperature of you have a good set of gauges. You'll need to pay attention to the temperature at which the a/c cycles off. You then adjust the trim pot until the system cycles off at the desired temperature. You'll probably need to have the fan speed set fairly low to get the system to cycle off and on. You want the system to cycle at as low of a temp as possible without icing up. 38 degrees is a good target. You might be able to get away with 36 but i wouldn't go any lower than that.

I don't have a means to read the temperature that is actually reported by the sensor, but we are essentially tuning that sensor to read whatever it needs to read to get the desired result. You can measure the air temperature at the vent if you have an accurate thermometer or you can convert the low side refrigerant pressure to the saturation temperature of you have a good set of gauges. You'll need to pay attention to the temperature at which the a/c cycles off. You then adjust the trim pot until the system cycles off at the desired temperature. You'll probably need to have the fan speed set fairly low to get the system to cycle off and on. You want the system to cycle at as low of a temp as possible without icing up. 38 degrees is a good target. You might be able to get away with 36 but i wouldn't go any lower than that.

 

Understood. Given I dont have a set of proper gauges (just a low side pressure/charge hose) ill have to rely on vent temperatures to adjust. Is targeting a VENT temp of 38 too aggressive? Or is that the method you used as well?

 

I also have no idea what vent temp it produces as is, and I’m trying to judge how much temp delta will be on the table after properly adjusted. Did you happen to measure your initial temperature before targeting 38?

I started at about 10k and ran the a/c at low fan speed until the vent temp got to about 38 degrees, then increased the resistance until the voltage across the connector got to about 2V. This caused the compressor to cut off at 34 degrees.

 

Was also curious if you could elaborate on this point. As it seems its slightly different logic than your recent recommendation (of targeting a 38* cycle start temp).

 

In the above, you targeted 38* steady state? But then raised resistance to get the voltage across the terminals to 2v? Why? And why did this produce a cut off of around 34?

 

Thanks for all the help man. This seems like it will transform the system to operate much more like a "normal" car

I'll try to elaborate as best I can, but bear in mind that some details have been lost from memory after seven years, and I can barely remember yesterday.

 

Targeting a vent temp of 38 degrees is shouldn't be too aggressive. Taking your measurement at the vent is no problem. That's how I did it. You'll need to have the fan speed fairly low to have the A/C cycling so that you can make your adjustment. To clarify, you're not actually adjusting the steady state temperature. You're adjusting the low temperature cutoff (and the reset temperature as a side effect). I had initially set mine to cut off at 34 degrees, but found that the evaporator would freeze up sometimes when taking a long trip on a rainy day.

 

I don't remember the details about the voltages, and I seems that I didn't convey those details very well in my original post. You really don't need to worry about the voltage on that circuit, just adjust the trimmer until the compressor turns OFF at the desired cutoff temperature. All this mod does is make the controller think the evaporator temperature is warmer than it actually is so that it doesn't cycle off too early, and so that it cycles back on sooner. Reduce the resistance to reduce the cutoff temperature. Increase the resistance to increase the cutoff temperature (closer to stock).

 

I hope that helps clarify things.

I'll try to elaborate as best I can, but bear in mind that some details have been lost from memory after seven years, and I can barely remember yesterday.

 

Targeting a vent temp of 38 degrees is shouldn't be too aggressive. Taking your measurement at the vent is no problem. That's how I did it. You'll need to have the fan speed fairly low to have the A/C cycling so that you can make your adjustment. To clarify, you're not actually adjusting the steady state temperature. You're adjusting the low temperature cutoff (and the reset temperature as a side effect). I had initially set mine to cut off at 34 degrees, but found that the evaporator would freeze up sometimes when taking a long trip on a rainy day.

 

I don't remember the details about the voltages, and I seems that I didn't convey those details very well in my original post. You really don't need to worry about the voltage on that circuit, just adjust the trimmer until the compressor turns OFF at the desired cutoff temperature. All this mod does is make the controller think the evaporator temperature is warmer than it actually is so that it doesn't cycle off too early, and so that it cycles back on sooner. Reduce the resistance to reduce the cutoff temperature. Increase the resistance to increase the cutoff temperature (closer to stock).

 

I hope that helps clarify things.

 

It does. Thanks for all the help man. Also glad to hear its been working well for many years. Will make up one of these harnesses at work this afternoon and try to dabble with it this weekend.

 

Thanks!

You're welcome

Neukam,

 

We appreciate the additional information!

 

Thanks,

 

Some quick information I learned when doing it on my car, I had an aftermarket belt on and I think I had the trim a little too aggressive, It melted the plastic pulley. It may have been the belt that snapped & melted the pulley. Not sure… or just a bad bearing in the pulley locking up, original pulley from 2005.

 

Either way I replaced the belt with a genuine OEM one and steel pulley. I am adjusting the trim and I’ll let you know how it works out.

 

I was disappointed to see Subaru use a plastic pulley. When the belt snapped I don’t know if that’s what melted it. Either way the previous generation used a steel one.

 

Currently traveling soon will send a picture of it installed and the new metal pulley..

 

Got the parts from eBay from Subaru parts Direct.. part number attached..

 

As always perform this at your own risk, as mentioned earlier.

 

I was happy to see A solution where it would give me colder air conditioning as I live in Florida and it is hot as the sun[emoji3508]

 

 

 

 

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good find