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1997 Subaru Legacy Outback Wagon Limited 2.5L AWD automatic transmission
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Discussion Starter · #1 ·
1997 Subaru Legacy Outback Wagon Limited AWD 2.5L Automatic transmission.
This is my first time using a discussion forum so please forgive me.

I have had the car mentioned above for a a few months now and as it has entered the summer months I started using the A/C. It blowed mildly cold out of the vent so I decided to have my mechanic charge it. After the charge the air was much cooler initially and it was the following day for a short time. Suddenly the rpm started dropping slightly and bouncing back and it felt as if I was tapping the brakes. It does this about every 2 seconds while the A/C is on and stops when I turn the A/C off. When its pulsing like this, the air gets warm very quickly. I stop and pop the hood to take a look and I see that my A/C compressor clutch engaging, and disengaging which is causing the surging/pulsing while driving. I also notice a clicking sound coming from the drier and I can feel the pressure switch on the drier clicking as the compressor engages/disengages. I unplugged the pressure switch and used a wire to jump the pressure switch connector and the Compressor clutch engages immediately, as well as the condenser fans come on. The air coming from the vents is just slightly cool now and there is no surging/pulsing of the rpm and the compressor clutch stays engaged until the compressor turns off as normal and then turns back on. So I'm wondering if I have a bad pressure switch, I'm low on refrigerant, overfilled refrigerant, and why the fans only turn on intermittently when the pulsing is happening?
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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When you turn on the AC the clutch is engaged to circulate the refrigerant. At the same time the fans come on to pull air across the condenser to remove heat generated by the pressure. When the AC cycles off due to a high pressure situation, or a low pressure situation, the fans also stop unless the engine temperature is high enough that the computer keeps the fans on.

Power runs from the AC switch to the high pressure switch then the low pressure switch and lastly the magnetic clutch.

The pressure switches are there to protect the compressor and to also prevent damage. When the high side pressure gets too high the switch stops current and the clutch is disengaged so the rise in pressure stops. Too high pressure will blow out at the weakest point and this is generally a junction point where a line connects or the relief valve. The low pressure switch stops current to the clutch when the pressure drop would create ice on the evaporator.

When a switch is causing a rapid on/off of the clutch it is generally due to a restriction on the system and pressure is rising fast and gets to the point it triggers the switch. When the pressure drops the switch is able to close again. Same with the low switch only it's reversed. When the pressure drops the switch is open and when it comes back up the switch closes.


You have something clogging the condenser and it's creating a high pressure event which is shutting off the clutch. Either there's a "sealer" that's been added and it's caused the problem or it's overfilled. Maybe both. Some shops use that crap with sealer in it and it kills an AC system. It could be a failed switch but what you describe about the temp out the vents says there's more to it than a bad switch.
 

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1997 Subaru Legacy Outback Wagon Limited 2.5L AWD automatic transmission
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Discussion Starter · #3 ·
@cardoc thank you for the helpful info! I talked to my mechanic and he said that he would check the pressure for me and see what's going on. I asked him if he pulled a vacuum on the system and he said that he didnt because their was refrigerant in the system. He also said that the only reason you pull a vacuum is if moisture is present in the system. Is this true? I would think that regardless if its low then it's due to a leak. If their is a leak then air and moisture can be introduced into the system. So I'm thinking maybe its overfilled because of air pockets and moisture in the system along with the newly added refridgerant.
 

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There is no way for him to properly charge the system based on the pressure readings alone. He should have recycled the refrigerant in the system, pulled a vacuum and checked for leaks and once there is no sign of a leak under vacuum he would recharge the system with the proper amount, which is about 0.55 kg. He also needs to add in oil (preferably with a dye) when he recharges it. Once it's charged he would check for leaks with the system on and pressurized.

Apparently your "mechanic" doesn't know what he's doing, or he's lazy. Ask him if he has an ASE cert for HVAC.

Too much and the system won't circulate properly. Too little and the cooling effect is lost.

If it's charged properly and the high side pressure rises quickly as I describe in the video then there is a restriction in the system. Generally with high pressure increases to the point of opening the pressure switch, and the fans are working, it is in the condenser.
 

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1997 Subaru Legacy Outback Wagon Limited 2.5L AWD automatic transmission
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Discussion Starter · #5 ·
Awesome I really appreciate your help. I'll talk with my mechanic when I get him to check out the car and I'll report back.
 

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1997 Subaru Legacy Outback Wagon Limited 2.5L AWD automatic transmission
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Discussion Starter · #6 ·
@cardoc I took my car to my mechanic and he put his gauges on it and it was overfilled. So he pulled a vacuum on the a/c and then refilled it with 16 oz and the low side was already reading really high. The air was barely blowing cool but the compressor clutch wasn't engaging/disengaging and the pressure switch wasn't clicking on and off. He told me he thought the expansion valve was bad. Here are the gauge readings after he pulled a vacuum and then put in 16oz.
 

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The system holds .6 kg or 1.3 lbs (about 21 oz) avg. It can go to 1.4 lb max. That pressure is good. Tell him to add more than 1 lb.

If the expansion valve was bad the low side pressure would drop hard and fast. He's wrong. That pressure is indicative of a low charge
 

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And if a pressure switch was bad, the compressor wouldn't start at all because the power runs through the pressure switches first, then the clutch. The switches are there to shut off the clutch in too low or too high pressure.
 

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One more thing - The compressor doesn't have to cycle on/off. It can run infinitely as long as the pressure is consistent. It only cycles off when extreme low or extreme high pressure opens a switch. On newer cars a evaporator temp sensor is used to keep the evaporator from freezing.
 

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1997 Subaru Legacy Outback Wagon Limited 2.5L AWD automatic transmission
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Discussion Starter · #11 ·
@cardo would the air blow at least a little cold with 16oz in the system? When he vacuumed the system he let out a ton of oil, dye, and moisture. He said he just used refridgerant, so I'm guessing a previous owner had it charged and then when I had my mechanic charge it, it was way overfilled. So I'm wondering why it wouldn't blow at least a little cold with 16oz on a freshly vacuumed system. I'm not an hvac guy so I probably sound dumb haha.
 

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It'll probably blow cold with 1 lb at higher rpms. Otherwise, at idle it'll be warm.
 

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1997 Subaru Legacy Outback Wagon Limited 2.5L AWD automatic transmission
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Discussion Starter · #13 ·
@cardoc After driving my car to work today the A/C blew pretty cold. I called my mechanic and he said he put 21oz in the system so I was mistaken. So the a/c feels great on these 90° plus days here. So I guess the morale of the story on my problem is it should have been put under vacuum and then charged. But now when I try to change to defrost or any other setting, nothing happens. Its stuck on the dash vents no matter what i do. I tried different combinations of recirculate, turning the a/c on and off, or turning the car on and off. Is that a blend door mechanical problem, or is it an electrical problem with my hvac control unit?
 

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I believe your car has cable operated temp door and electric motors for the mode doors. It's on the left side of the HVAC box up above the accelerator pedal area. You can check the controls at the control head easier than getting to the motor.

The 10 pin connector on the back of the control unit has a black and a yellow/black wire next to each other, pins 1 & 2. The current to the motor alternates depending on which way the dial is set. If you have current when a volt meter is connected between these two wires and working the vent control, it's the motor.

If it's the motor, it takes 45 minutes to 1.5 hrs to replace depending on your patience level. 😁
 
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