How The Dual-Zone Climate Control Works

stupid question, but is the AC icon displayed when you're having this problem?

also... if it is a "feature" and not something else, just use the foot/windshield mode?

stupid question, but is the AC icon displayed when you're having this problem?

 

also... if it is a "feature" and not something else, just use the foot/windshield mode?

This is not a stupid question at all. Icon is NOT on, but AC does run. the reason I know it that if after the defogger I stitch to the regular mode, AC would stay on even though it wasn't on before.

It is hard to use windshield/legs mode because it is really cold for legs

Spider has it right, the air goes through the cold condenser, which removes the moisture and then through the heater core. Warm dry air clears the windshield the fastest.

Thanks!

So i am coming to the conclusion that it is a feature that the defogger warms the ACed air.

Fanny fact: I called 2 dealerships with that questions. One said that AC will never be on in defogging mode (the fact I know is false) another said that defogger will not warm up air - bring it in and after initial $139 for diagnostic.....

Thanks

if the AC icon isn't on when you press the front windshield defogger... try pressing it and see if the icon comes up and the air changes temp?

I tried it, icon comes on and no changes in temp to the windshield.

I was wondering, how do I match driver and passenger temperatures, so that the driver knob controls both?

You can't. Well you could, but it would involve affixing an elastic band around both knobs so that when one is turned, the other turns with it.

you could always increase the temp when using the defog/foot mode so it's not too cold, but cold enough to not heat up the car. doesn't have to be down at 65*F (or whatever your lowest setting is) for the A/C to help defog the windshield.

Well, that will defeat the purpose of me trying to cool of the car.

And I can accomplish warm air through the windshield defogger mode

My goal is to have only cold air to windshield.

+1. .. Many thanks Craig for the #1 post Operation Primer ala the automatic dual "WTF" HVAC. ..

Sduford -

Agreed. .. The Subaru MY05< '09 Legacy GT/Outback XT trim levels - POS Dual Automatic HVAC (OEM Panasonic) is a joke. Just how this grossly "failed HVAC system control" passed Subaru's operation review,, then signed off for production is a mystery. .. WTF .. For any ENG this is cause for termination.

Our "05 OB XT LTD build date was merely 45 days or so into the start of the new Legacy models 5 year production cycle. .. Say late October 2004 as a MY05 roll out. .. Having experienced decades of car ownership** where "auto" HVAC systems operated faithfully in the background without demanding attention, our new Outback XT "auto" system was going to be different; The high trim models Auto-Dual Zone-HVAC arrangement is more of a marketing ploy, rather than a fully baked modern "auto" HVAC system. .. Subaru USA corporate could/never provide operation details of their Dual/Auto arrangement. .. When push came to shove our requested meeting (Jan '05) with the Subaru Western Zone Rep, even he didn't have a clue, nor ever provided a clue as to where/how "unheated outside air vent mode" was invoked. .. The silence was deafening from Subaru USA ...

** Audi UrS6 avant, MB E320, Porsche 944S2

Excellent post Craig, hopefully it will help a few people.

 

However, it does highlight the fact that this is a poorly designed system that gets in the way rather then making your life easier as a well designed ACC system does.

 

I know, many of you disagree. What can I say, I must be whiny A-hole for wanting my car to work properly.

When push came to shove a our requested meeting (Jan '05) with the Subaru factory Rep, even he didn't have a clue, nor ever provided a clue as to where/how "unheated outside air vent mode" was invoked. .. The silence was deafening from Subaru USA ...

Set the temp dials to coldest, turn off A/C button and adjust to "fresh" from recirc.

This won't heat the incoming air as air will be diverted away from the heater core by the air mix doors inside the HVAC box.

Do you know if manual hvac can be changed to automatic? If so, what additional parts are needed?

Oh, were you serious? That would be a huge task.. The manual system has cable actuated temperature control while the automatic setup does everything electronically. There are seperate driver side and passenger side air mix actuators, I expect the ducting would be different as well. In addition to the control panel swap, there is the control unit itself, as well as some sensors that would have to be installed and wired in.. The blower motors are even controlled a little differently.

The major problem with the system is that it is scaled wrong. A range of 65 to 85 (in reality 66 to 84) does not allow enough adjustment on the bottom end. A more proper range might be 60 to 80 (in reality 61 to 79). There are mornings where I only want enough warmth to take the edge off of the the air flowing in and it's jut not possible in the current setup. It seems to me that this should be fixable by tweaking the programming or system code. Anybody out there with the knowledge or resources to do this?

Mike, it may be possible to rescale the temp sensors if we knew where they were and which ones did what. They are likely thermocouples that convert a temperature into a voltage. You can always play games with sensors like that. The trick would be figuring out how the system worked and how the sensors feed into the controller, etc. If somebody can get a circuit diagram, that would be a good start.

 

Craig

Has anybody looked into this idea further? I would have thought a simple resistor in line with the interior temperature sensor would accomplish the shift? Perhaps a little trim pot?

Well after living with my LGT for almost a week, I have to agree with danbart and c_hunter. The major problem with the system is that it is scaled wrong. A range of 65 to 85 (in reality 66 to 84) does not allow enough adjustment on the bottom end. A more proper range might be 60 to 80 (in reality 61 to 79).

...

Has anybody looked into this idea further? I would have thought a simple resistor in line with the interior temperature sensor would accomplish the shift? Perhaps a little trim pot?

The problem with that idea is that you might need to lower the resistance rather than raise it. Depends on how the sensor works.

I'm guessing that the existing temperature sensor is already providing the HVAC control module with a wider range. The issue is that the CM is programmed with the current limits.

If the program is in EEPROM, there might simply be one or two bytes you could change to decrease the lower limit. That would be a better solution, since the display would still show accurate temperatures (at least, as accurate as they are currently).

We'd first need someone to dump and disassemble the code in the control module. Note that this is the unit behind the glove box, not the factory head unit (radio) or one of the JDM HVAC panels.

The problem with that idea is that you might need to lower the resistance rather than raise it. Depends on how the sensor works.

Modifying the program would be ideal, but requires someone with programming skills. Just about anyone can fiddle around with resistors. The FSM says the sensor should be about 2.7K ohms at 20C, so we know where to start.

I may end up playing with it myself some day, but unfortunately it`s not near the top of my list.

I've heard that the dual climate control has a self diagnosis feature, does anyone know how to actuate it?

SET SELF-DIAGNOSIS MODE BY OPERATING A/C CONTROL PANEL.

1) Turn the ignition switch to OFF.

2) Start the engine while holding down the

AUTO switch and FRESH/RECIRC switch.

See attached chart

I would love to be able to mess with the fan speeds (or bias) mostly to get lower speeds in auto while heating. Any way to put a potentiometer on the output to the blower to gain a variable reduction of the whole range of speeds?

5. Diagnostic Chart for Self-Diagnosis.pdf

SET SELF-DIAGNOSIS MODE BY OPERATING A/C CONTROL PANEL.

1) Turn the ignition switch to OFF.

2) Start the engine while holding down the

AUTO switch and FRESH/RECIRC switch.

 

See attached chart

 

I would love to be able to mess with the fan speeds (or bias) mostly to get lower speeds in auto while heating. Any way to put a potentiometer on the output to the blower to gain a variable reduction of the whole range of speeds?

Just make sure the POT has a huge heat sink. Those blowers draw some serious current.