AC Intermittent - Especially over 85 degrees outside

[ParetosFew]

2015 Legacy Limited, 2.5

 

Ironically, only when it is extremely hot it out and most necessary the AC goes intermittent. It will run for a few minutes then blow warm air, then cold again. It does this many times, but I've noticed on longer trips it eventually just runs consistently after 20 minutes or so.

 

I checked the refrigerant with one of the can-top gauges and it was in the lower end of the green range - added a bit to bring it to center of green range but made no difference.

 

My searching showed several forum posts about the compressor clutch sticking, but my clutch looks different than a traditional compressor clutch I'm used to seeing. I think this may be because this car has a variable displacement compressor? That's a bit beyond my knowledge, but read something to that effect on one of the posts.

 

I suspect the pressure sensor but am having real trouble finding information on that. I'll attach a diagram from subaru parts with the part I'd like info on - it's the only thing on that diagram without a part number. Diagram from this page: https://parts.subaru.com/Subaru_2015_Legacy-25L-CVT-4WD-Limited/Heating-and-Air-Conditioning.html

 

Any information on testing this sensor would be helpful. I found a service manual online for a car that should be close (17 outback) and it said that sensor should read 0.5-4.9V when the ignition is on. Leads me to think that is an analog transducer but not sure if I can test that with the connector disconnected.

 

Thanks!

[aki334]

2015 Legacy Limited, 2.5

 

Ironically, only when it is extremely hot it out and most necessary the AC goes intermittent. It will run for a few minutes then blow warm air, then cold again. It does this many times, but I've noticed on longer trips it eventually just runs consistently after 20 minutes or so.

 

I checked the refrigerant with one of the can-top gauges and it was in the lower end of the green range - added a bit to bring it to center of green range but made no difference.

 

My searching showed several forum posts about the compressor clutch sticking, but my clutch looks different than a traditional compressor clutch I'm used to seeing. I think this may be because this car has a variable displacement compressor? That's a bit beyond my knowledge, but read something to that effect on one of the posts.

 

I suspect the pressure sensor but am having real trouble finding information on that. I'll attach a diagram from subaru parts with the part I'd like info on - it's the only thing on that diagram without a part number. Diagram from this page: https://parts.subaru.com/Subaru_2015_Legacy-25L-CVT-4WD-Limited/Heating-and-Air-Conditioning.html

 

Any information on testing this sensor would be helpful. I found a service manual online for a car that should be close (17 outback) and it said that sensor should read 0.5-4.9V when the ignition is on. Leads me to think that is an analog transducer but not sure if I can test that with the connector disconnected.

 

Thanks!

 

 

Hi, you will need a good scanner that can read body control module codes that start with "B" good luck.

So basically you have stored codes in the BCM but check engine light will not illuminate since it is not related to emissions and will not be stored in PCM to give you code that start with "P"

 

Keep us informed!

 

thanks - al

Edited June 23, 2021 by aki334

[bluesuby]

Check the relays. Swap the a/c relay with the cooling fans relay. If that does it replace both. Don't risk overheating because the fans quit.

[ammcinnis]

I checked the refrigerant with one of the can-top gauges and it was in the lower end of the green range - added a bit to bring it to center of green range but made no difference.

You cannot determine the proper state of refrigerant charge by pressure alone. Your system may now be overcharged. By how much? It's virtually impossible to know.

Edited June 23, 2021 by ammcinnis

[Painless]

Parsed-out the relevant sections of the '16 manual.

Should apply to your model as well.

Look for B505 Pressure Sensor.

Unfortunately, you may need Subaru Select Monitor (SSM4 = a trip to the Dealership) to properly diagnose using their computer.

Best of luck and please post final resolution.

REFRIGERANT PRESSURE SENSOR (B505).pdf

Edited June 23, 2021 by Painless

[ronemus]

The symptoms you are seeing could be from over- or under-charging, or a fan failure. You can watch the fans at idle - set the AC for max cold and the fan speed to high, and open the windows; if both fans are not running after several minutes that is at least part of the problem. Determining the state of charge requires knowing the high- and low-side pressures as well as the ambient temperature; there is a nomograph in the service manual that is used to interpret the readings, and there may be one under the hood as well.

[aki334]

if you can give us little more history - did anyone play with your AC system before this problem occurred? Did you have front end collision recently or did you buy car used that had front end damage before?

 

- the higher the ambient temperature the higher your AC system pressure will be. If your fans were not working, by now your underhold would be smoking 100% in stop and go city traffic with temps above 80F. As previously suggested you will need SSM4 or good scanner that can read "B" codes. I would not suggest you going to any AC place that does not have the SSM4 or similar or at the end it could cost you more.

You can try using Subaru's diagnostic chart with (yes and no) questions to diagnose, and hopefully you will solve your problem before the step where they say connect SSM4.

 

I just checked - sorry - one of the first steps is to connect ssm4 and you basically have 38 things that can go wrong. there are codes 20-58 related to AC.

 

But I do agree if you can, check the simple things first that do not cost you much.

 

How is your ambient sensor reading comparing to the temperature forecast on your phone for your area? is it close by +/- 5F ?

 

What I would do if all the simple things do not work is pay dealer 1h diagnostic time ~$150 to connect ssm4 and tell me the code. Then come back here and tell us the code so we at last have the starting point.

Edited June 24, 2021 by aki334

[ParetosFew]

Thanks for the responses here, will definitely update with what I find.

 

You cannot determine the proper state of refrigerant charge by pressure alone. Your system may now be overcharged. By how much? It's virtually impossible to know.

 

I saw your comments mentioning this on some other AC threads found while searching, and have since ordered a gauge set to see both high and low side pressure. My next step is to use this and take temperature into account to check the charge, hopefully this arrives today.

 

Forgetting how to multiquote here, been a while since I've used this type of forum, but to answer some other things:

- While I have the gauges attached, if the pressures seem in reasonable range I will try to measure the voltage from the high end pressure sensor.

- Will try swapping relays.

- Ordered one of those $10 bluetooth ODBII readers, although I think this may not get see the codes I need for this problem I like having these for check-engine lights etc. Maybe it could help here.

- Painless - thanks for that, it will be helpful.

- Car owned by my since new, no recent accidents or anything that would effect the front end. I somewhat remember working on another vehicle and possibly "topping off" the refrigerant on this one having some left in a can (realize now not a smart move).

 

I will try these few things and check back in, may end up taking it to dealership for a SSM scan if I don't make any headway with any of these steps.

 

Edit: Doh! Should have hit m quote button, didnt' see it once I'd already hit the quote button.

Edited June 26, 2021 by ParetosFew
multi quote

[ParetosFew]

Got the gauge set, here are some updates.

 

Started based on vehicle ambient temperature sensor reading 80F. With system off, the the high and low side pressures were very close. Both were within a few PSI of each other, around 75 PSI.

 

With the system on, max cool set on AC and engine at idle, and given about 5 minutes or so of trying to run AC saw the following readings from first image - LOW 30 PSI, HIGH 20 PSI. Following the table in the instructions added refrigerant until the low side was a little at 60PSI (accidentally went a bit high, was aiming for 55).

 

After the engine warmed and after finished adding refrigerant, was reading LOW 100 PSI, HIGH 100 PSI, which seemed reasonable to me given the 80F ambient plus engine heat. Wish I'd have measured the temp near the engine. At this point, the compressor was repeatedly trying to run for several seconds then stopping. I've attached a video of this, you'll see the pressures drop down when this happens. Video: https://streamable.com/7kitvx

 

Still suspecting the high side pressure sensor, I measured the signal voltage on the center conductor of the sensor. At 100PSI was reading 3.97V, when the compressor tried to run this voltage quickly jumped up to 4.5 and climbed to 4.9 before the compressor stopped trying to run. I'm confused as to why the reading would increase despite the gauge pressure dropping. Regardless, the voltage readings seem way high for a 0-5V sensor and the pressure ranges involved. I've been having trouble finding concrete information on what the range of that sensor is from Subaru manuals, but I found a general guide for this type of compressor that says the general range should be 0-5V, 0-500PSI ( GUIDE HERE, PDF ). Makes perfect sense to me that the controller would shut off of the compressor with a reading of near 5V on that sensor since that should mean the pressure is very high.

 

I am going to keep researching that part to see what else I can find, please let me know if I've made any errors in my troubleshooting so far.

Edited June 27, 2021 by ParetosFew

[ammcinnis]

Started based on vehicle ambient temperature sensor reading 80F. With system off, the the high and low side pressures were very close. Both were within a few PSI of each other, around 75 PSI.

That is expected. According to standard refrigerant tables, the saturation pressure of R-134a at 80 degrees F is ~86 psig. This alone doesn't tell you anything about state of charge, just that there is some refrigerant in the system.

 

With the system on, max cool set on AC and engine at idle, and given about 5 minutes or so of trying to run AC saw the following readings from first image - LOW 30 PSI, HIGH 20 PSI.

That doesn't make sense. With the variable-displacement compressor at anything other than zero displacement, the HIGH side pressure should always be higher than the LOW side.

 

Following the table in the instructions added refrigerant until the low side was a little at 60PSI (accidentally went a bit high, was aiming for 55).

STOP! At this point your system is almost certainly grossly overcharged, and you risk permanent damage if you go any further. (Per the FSM, the capacity of the entire A/C system is only 13.3 ounces of R-134a.) Your only recourse at this point is to recover, evacuate, and recharge the system (by weight), and then resume troubleshooting.

 

The good news is that the refrigerant in your 2015 system is R-134a, which is still widely available and relatively inexpensive.

[ParetosFew]

I think at this point I will have it evacuated and recharged since I don't have the equipment to properly evacuate and recover. Once I'm sure the charge is at least correct, will continue to monitor for intermittent issues at higher temperatures.

 

I may have not given it enough time when I took that first photo. Will possibly try measuring again from cold system just to see if I can recreate that measurement.

[ammcinnis]

If you haven't already, check to see if any Diagnostic Trouble Codes (DTCs) are set; there are more than two dozen of them related to A/C. The Factory Service Manual has an entire 118-page chapter devoted exclusively to troubleshooting the HVAC system.

[ronemus]

It is likely that either your compressor or your pressure regulator is toast, or both. The AC uses a pressure regulator to keep the low side at ~30 psi. and the high pressure should climb as refrigerant is added. In your case the high and low sides are at equal pressures, so you have a major problem. Don't bother with recharging until you fix the problem.

[ammcinnis]

In your case the high and low sides are at equal pressures, so you have a major problem.

To clarify ... with the engine not running, the high and low sides should converge and stabilize at the same pressure value, since the temperatures at the condenser and the evaporator will be approximately the same. I have low confidence that the pressures reported with the engine running are accurate.

Edited June 28, 2021 by ammcinnis

[aki334]

if both sides are reading low in general that could be one of the 4 things:

 

1. low refrigerant

2. Expansion valve stuck closed

3. Orifice tube plugged

4. High side restriction

 

 

you can evacuate everything replace the components, and charge it with one regular 12 oz can from Walmart. That will bring you very close to 13 oz.

- Remember when you evacuate you will suck out the oil from the compressor also so make sure you put in the same amount of oil for ac compressors. Research which oil is used for your type of compressor.

 

Another thing what you can do is connect your multimeter to battery red to positive black to negative open the hood place the DVOM on the windshield so you can see the readings from inside the car. Start the engine and watch the voltage then turn the ac on while monitoring voltage. You should see some decrease in the voltage by at last 1 - 1.5 v then the voltage will raise up to around 14.25V If you do not see voltage drop at all that means you could have pressure related problem or electrical. Pressure related because the pressure switch will prevent ac compressor from turning if pressures are too low or high. And or electrical problems no voltage going to the clutch coil.

 

When you evacuate freon and when you start to charge it with the can. Ac will have to be turned ON on cold and fans blowing at high speed. But since it is out of freon Pressure switch will stop Ac compressor from running and you will not be able to charge. So you will have to connect compressor straight to battery voltage to start sucking from the can. If you are not sure about this leave it to pros.

Edited June 30, 2021 by aki334

[ammcinnis]

If you are not sure about this leave it to pros.

The Gen 6 Legacy A/C is not a Clutch-Cycling Orifice Tube (CCOT) type system. Thus:

 

• There is no clutch on the compressor. It's a variable-displacement design.
  
• There is no orifice tube. The system employs a thermostatic expansion valve.
  

Edited June 30, 2021 by ammcinnis

[ParetosFew]

I setup an appointment at the dealership, but thought I'd try to retry my measurements. Well, you were right ammcinnis and it only took another few dumb moves on my part to prove it out. I setup the gauges again since I was suspecting my original measurements of being inaccurate as well. Saw pretty much the same thing I measured previously (100PSI on low side while compressor not running, dropped down to about 55 when compressor tried to run but no change visible on high side which was at around 100 PSI the entire time). Being stuck in the mindset of that sensor not reading correctly, I forced a voltage of a little over 3V using AA batteries to simulate what should be an OK pressure. Admittedly, this was not a very safe thing to do. This did cause the compressor to run for about 30 seconds until what I'm assuming was the safety blow-off valve let out a blast of refrigerant.

 

Of course after this I was now highly suspect of my 100PSI high side reading since clearly there was a lot of pressure there. I'm embarrassed to admit this, but I did not have the high side valve on the gauge quick-disconnect opened enough. Not quite sure how I got that wrong, but I swore it was moving and seemed to be reading prior but this was probably the same issue on the readings from my previous post. Once that was open, I saw the high side pressure max out at 500 PSI every time the compressor tried to run for a few seconds. This matches up with the voltage reading on the high pressure transducer, and now makes me very confident it is in fact just grossly overcharged.

 

Another evidence of it being overcharged is this started out intermittent, and it is now not working at all since I added more refrigerant. I still have the appointment for July 6th, I will let them evacuate and charge properly and I'm pretty sure that will get it working again. I will keep updated what I found out then.

 

Thanks for the suggestions and help, and as far as your question about the DTCs I'm not sure how to read them. I did connect the $10 bluetooth ODBII reader with Torque app, confirmed I was connected and checked for fault codes. Nothing was reported, although I'm not sure that would get enough detail.

[Brisvegas]

2015 Legacy Limited, 2.5 Ironically, only when it is extremely hot it out and most necessary the AC goes intermittent. It will run for a few minutes then blow warm air, then cold again. It does this many times, but I've noticed on longer trips it eventually just runs consistently after 20 minutes or so.

 

It sounds like that with the help of ammcinnis and others you have the problem covered ... but coincidentally I just watched the following Youtube clip ...

 

[ParetosFew]

To follow up on this, it was in fact just over charged. Dealership evacuated and recharged by weight and it is now running great. Anyone looking at this later, make sure your high side gauge is reading correctly as this would have confirmed what ammcinnis was telling me early on.

[medsark]

When I owned a Subaru, I often experienced problems with the cooling system. I don't know what the reason was for it constantly breaking down on me. The air conditioning is a great thing that helps create a comfortable air temperature. You only realize how handy it is when it breaks down. I have experienced heating cooling system breakdowns both in my car and in my home. If it's easy to fix in the car, you might have trouble in the house. Breakdowns can occur for various reasons, but what they all have in common is that only an experienced technician can fix them. It is essential to choose such devices wisely to avoid unnecessary costs. Edited December 14, 2021 by medsark