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Thread: Simplified A/C concept?

  1. #1
    Senior Member Goldwing's Avatar
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    Simplified A/C concept?

    As Iím starting to peck away at my build again, the A/C question is bouncing around in my head again as I mentally spool up. Any feedback for cutting the heat exchangers out of the air handling sub frame in the WRX, and placing them side by side with a fan mounted on each one to blow through into a plenum for mixing and distribution to vents (aka, sheet metal box)? Rather than one fan control followed by a slider, two fan controls, one for hot, one for cold. Achieve a desired mix by varying each fan speed. I imagine using the factory setup for air so the background stuff (compressor control, radiator fan control) is already integrated, but make a new circuit for heat (matching control switch, resistor module, fan). Iíd want the fan control switches to match, so may have to sort through either a new switch to run the oem A/C, or looking at the factory switch to see if itís a traditional setup to control the heat with a second sourced matching fan switch. Perhaps a gravity assisted rubber flap (like a dryer exhaust) at each heat exchangerís output to prevent backflow if only one fan is running. And for any married guys, perhaps mount the heat on the passenger side, A/C on the driverís side, and install a butterfly valve to split the plenum to give her heat and you A/C, lol. Itíll be a bit before I act on anything here, but figured if we start the conversation now, more heads involved would either help sort out or simply point out the problems with the idea. I envisioned something like an add-on heat unit for a UTV, just with 2 parallel blowers and a baffle or two to encourage mixing. Without digging out the WRX blower unit, I assumed 2 new blower fans.

    It still requires having longer A/C hoses to be made, but allows for much of the factory stuff to be used like compressor, condenser, always open heat exchanger, etc.
    Rich

    818S in progress. 2007 WRX sedan donor.
    Powered up: 7-8-14, First Start: 7-20-14, Go kart: 8-19-14

  2. #2
    Senior Member Goldwing's Avatar
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    Something like this, but 2 of them blowing into the same plenum:A35A9EE3-43A5-4153-909B-95E2B673F914.jpeg
    Attached Images Attached Images
    Rich

    818S in progress. 2007 WRX sedan donor.
    Powered up: 7-8-14, First Start: 7-20-14, Go kart: 8-19-14

  3. #3
    Senior Member Goldwing's Avatar
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    Chewed on the donor A/C idea a little further. I was planning a little UTV heater to satisfy defrost requirements for the state. Then threw the idea around of two small units blowing into the same plenum, one heat, one A/C. Then, just decided out of laziness to use two units without combining the plenum, but run a heat only to the defrost and floor, and a second unit blowing A/C only through a plenum holding the donor evap panel to run the front dash vents. I wasn’t planning heat there, but figured it comes with a heat exchanger, and a second heat exchanger wouldn’t have to circulate all the time, so it’ll do both. I found nice matching switches/valves at JEGS. (Half the reason I didn’t pull the trigger at vintage was the vintage looking switches, cost bring the other.) Anyhooo, I came up with a shopping list and electrical diagram showing the changes in Subaru manual style.

    1) qty:2 UTV or similar heater unit
    2) qty:2 Blower fan speed switches/relays as needed, the unit I’m eyeing has high current switches.
    3) Matching (to switches) heater valve to cut heat to A/C unit.
    4) Condenser (looking at Porsche 911 97-04 unit behind front corner)
    5) Drier w/Pressure switch bung)
    6) Manual A/C version 3-wire Evaporator thermo switch/amp
    7) A/C Compressor switch (single throw, unless lighted, then double separate circuits)
    8) A/C conversion relay (standard relay, grounds ECM signal and compressor power circuit)
    9) Hoses/Fittings

    On the electrical side, I’m adapting the auto A/C to mimic the Manual version. Tie ins to the ECM are identical, but use different parts to accomplish grounding and signals to the ECM. I’m just going with an assumption that the ECM software is the same in both versions. Whether that’s a safe assumption is over my pay grade. Online searching suggests systems (A/C and ECM) are very separate and the ECM is only involved to know A/C is running to turn on fans and adjust idle. I welcome input before I start buying too much stuff. Hoses and blower unit will be the bigger expenses, the latter planned anyway. Otherwise, a relatively cheap experiment to add A/C to what’s already planned.
    4962C29B-30F9-4B69-B424-ADA302C9CFDD.jpg
    Rich

    818S in progress. 2007 WRX sedan donor.
    Powered up: 7-8-14, First Start: 7-20-14, Go kart: 8-19-14

  4. #4
    Senior Member Goldwing's Avatar
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    Manual A/C diagrams:
    7BD56D40-C2FB-4C51-AD95-C909D3C5CF08.png
    11FBC93F-6BC6-47E8-A275-7AD0920D7C47.png

    The evap thermo switch is needed to stand independently from the auto A/C logic board.

    The A/C conversion relay was to provide grounding. I might be able switch polarity on the UTV heaters so the common tab on the switch ties to ground, but I was feeling a bit skiddish since the switches handle the high current. Don’t want to fry anything. So using a relay to provide the needed ground to ECU and the compressor power circuit. Compressor power would pass from the fuse 17 tap, through the pressure switch, through the A/C clutch relay, then the evap thermo switch, then the A/C switch I install to the ECM and conversion relay to ground. I can run the blower without A/C due to the A/C compressor switch.
    Last edited by Goldwing; 11-06-2019 at 09:39 PM.
    Rich

    818S in progress. 2007 WRX sedan donor.
    Powered up: 7-8-14, First Start: 7-20-14, Go kart: 8-19-14

  5. #5
    Senior Member Goldwing's Avatar
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    Rich

    818S in progress. 2007 WRX sedan donor.
    Powered up: 7-8-14, First Start: 7-20-14, Go kart: 8-19-14

  6. #6

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    Reviewing your plans I think you have a good roadmap. On some vehicles, but don't know the Subaru, when the engine is asked above some torque level, the ECU turns off the A/C compressor. In the Subaru with automatic, the transmission computer (TCM) sends a load signal to the ECM, used for shift time power cuts AFAIK, and probably... a combo of the TCM's load signal and the throttle position/RPM are used within the ECM to determine whether to cut the compressor for best accelleration. In a manual trans ECM I'd bet it uses just throttle position and maybe RPM. So you do want to keep the ECM connections - which you plan unless I missed something.

    Fitting the heat exchanger(s) can take some fiddling and thinking with the available space and plumbing routing. You definitely need dimensions and either get one or mock one in cardboard then go see where it/they fit and how you will plumb in. Seems simple but I'd hate to buy two of them then tackle that step with the $ already spent.

    I wanted to put A/C but got lazy about dealing with the compressor vs my custom intake. Be my proxy and do it !
    Art Quillen

  7. #7
    Senior Member Goldwing's Avatar
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    Definitely working on it. Yep, planning to keep/reroute ECM connections using the manual A/C control setup. The fans are an easy fit, designing a plenum to also accommodate the much taller evaporator is the bigger challenge. Started looking at the wiring on the car today and already ordered the initial parts to test fit stuff. If that works/fits I’ll order a few more parts letting each stage let me know if I’m on the wrong track.

    I noticed the FB32 circuit isn’t switched, so I adjusted the AC blower relay so the field just goes to ground and powers up with IGN, since I’m not using the Subaru heat/AC switches due to having two units. I’ll run the A/C unit off that relay, as well as AWIC pump (it’s been waiting for a power source), and maybe the planned seat heaters. I don’t like hot air in my face, so I won’t have the A/C blower on high with the seat heat if it would become too much for that circuit, unless I find the rear defrost circuit and power seat heat there. I’ll run a separate switched via relay circuit to power the heat only fan or tap into the yellow or green switched wires and set a fuse along the tap. So a little adjustment after realizing the planned circuit wasn’t switched.
    Rich

    818S in progress. 2007 WRX sedan donor.
    Powered up: 7-8-14, First Start: 7-20-14, Go kart: 8-19-14

  8. #8
    Senior Member Goldwing's Avatar
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    I ordered two of these jegs units on sale:
    84E274E2-FCAF-4BEA-A990-FE5303A9DCEF.jpg
    They are 4.5” tall so fit on the top side of the dash frame. They come with oem looking switches and I’ll throw in a heat valve to have heat available in the A/C unit too.
    6EB74801-570C-4F48-97F4-1D8AD63E0057.jpeg
    The plenum will theoretically look like this outline. I’ll remove the front plate and vents before sticking it in there.
    A926DA01-4FDB-462B-9C66-949B20E6F065.jpeg
    I’m hoping this will fit centered above the dash frame facing rearward with the blower sitting above the front firewall allowing the evap panel section to sit in the triangular hollow between the firewall and dash frame rail.
    Rich

    818S in progress. 2007 WRX sedan donor.
    Powered up: 7-8-14, First Start: 7-20-14, Go kart: 8-19-14

  9. #9
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    I have NO idea what you’re doing but I’m interested to see the end result. Not sure how much more simple this is opposed to a vintage air setup but I’m watching!

  10. #10
    Senior Member Goldwing's Avatar
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    Perhaps “budget plan” is a better description? Just trying to use the Subaru parts and wiring on hand to keep things on a budget and forgo the Subaru big enough to heat a small house furnace unit.
    Donor compressor
    Donor wiring (modified to manual A/C equivalent)
    Donor evaporator (but swapping in manual A/C thermometer)
    Donor pressure switch
    Aftermarket condenser, drier, and hoses fitted to an aftermarket heater unit.

    Prepped the vents for 2” hose I’m planning to use today: DA955C30-48D4-4BC3-A51A-A772C4B036BB.jpg
    They’re off an early 2000 Audi TT. Used drain adapters for 2” PVC.
    Rich

    818S in progress. 2007 WRX sedan donor.
    Powered up: 7-8-14, First Start: 7-20-14, Go kart: 8-19-14

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