TR: Installing FWD/AWD/4wd switch on 2002 Subaru 4EAT auto trans

[ill-advised strategy]

So, I have this 2002 subaru. Aside from the other Subaru problems we all know well...my subaru is an automatic, which means it mostly drives the fronts until it senses a need to engage the rears by way of a hydraulic clutch. My 1996 subaru had this same transmission at 200something miles and the clutch/differentiation was shot...it bound and shook on tight turns on pavement but it handled so much better in snow.

I drive on snow a lot in Upper Michigan, and I'm the kind of guy who slides turns and rips donuts when nobody's looking. I'm a professional driver and ripping around in my POV before or after having to mind my Ps and Qs at work is a small joy of my life. This fucking car won't allow that, and what's worse is when you try and slide her around it gets the fronts spinning you into an understeer then it slams that clutch closed and pops the rears into play a lot harder than I would prefer. Hard on the running gear, handles funny...the whole thing sucks. It's just not a drivers car in that particular config. Since I've had the subaru problems already and the body work is shit now, the car is not worth a tiny fraction of what I have into it, I have to live with it for awhile so I'm motivated to solve this handling problem.

Anyway--that hydraulic clutch is controlled by one solenoid, which is controlled by one wire. Zero voltage to the solenoid produces a locked clutch and full 4wd. Full voltage produces front wheel drive. There is a normally-open fuse location under the hood to allow for a driver to install a fuse and run the car in front wheel drive in the case of a flat tire or mechanical problems with the rearend. So for 4wd you could just install a simple switch, but that would make the transmission control module think there's an open circuit on the transfer solenoid and may cause the tcm to go into limp mode. In order to cut power to the solenoid but still keep the tcm happy, you have to divert power through 10-17 ohms of resistance (what the tcm expects from the transfer solenoid). So, following some of the directions available elsewhere on the internets, I have set my car up to be selectable between FWD/AWD/4wd. Here's how to do it

PARTS LIST WITH LINKS
Note: the main source of info I followed for this built his resistor unit out of cheap looking shit. It seemed to me that these resistors would get HOT and need to be durable. So I came up with using a thermoelectric cooling unit and aluminum-housed wirewound resistors of much greater wattage rating than I expect to encounter in this application. This could probably be done cheaper and entirely without the cooling unit, but it's bomber this way.


Peltier thermoelectric cooling module:
http://www.adafruit.com/products/133...FZSCaQodDDcAXA

(2) 30 ohm, 50 watt wirewound aluminum-housed resistors
http://www.newark.com/vishay-dale/rh...50w/dp/01F9926

Bosch-style automotive 5 pin relay. You can get these at auto parts stores.
http://www.qualitymobilevideo.com/50...FQgtaQod6BEAkg

Fuse-tap, add-a-circuit kit (mini-fuse style)
http://www.suntekstore.com/goods.php...tm_medium=paid

DPDT On-off-On style rocker switch
http://www.delcity.net/catalogdetails?item=7700044

6mm LED indicator light.
http://www.ebay.com/itm/6mm-12V-LED-...algo=undefined

Male and female quick-connectors (menard's and parts stores)
http://www.allelectronics.com/make-a...le-blue/1.html
http://www.allelectronics.com/make-a...le-blue/1.html


Roll of thinner-gauge automotive wire. 16-18 is plenty for this. Nothing in this assembly is using much power.

Factory Service Manual
http://jdmfsm.info/Auto/Japan/Subaru...0Outback/2002/

PREWORK:
Once you have all your parts, you can bolt the two resistors to the cooling unit. This is not as easy as it looks. I had to countersink the holes in the resistor bodies with a dremel tool in order to have enough thread/screw length to screw them in. Build a little wiring harness to wire the resistors in parallel (two 30-ohm units in parallel makes 15 ohms of resistance...perfect) . Decide on where the switch, relay, the indicator light, and the cooling/resistor unit are going. There isn't a lot of room on this car to do this really clean. I ended up removing the CD part of my stereo and the ash tray I never use and putting a blank plate in and using that now-open space for the resistor/cooling box. I removed the switch for my heated wipers (that don't do shit) and used that hole for my rocker switch. Relay went near the interior fuse panel.

INSTALL PROCEDURE:
1) Download the factory service manual. It's enormous and took a long time on a very fast connection here.
2) search to find the description of the wiring harness(es) going into the transmission control module (tcm). This took me a long time to actually locate. It’s in a section called wiring....pages WI-01 WI-20 etc. You’re trying to identify the wire going to the “AWD Solenoid”…sometimes this is referred to as the “transfer solenoid” or occasionally as the “duty C solenoid”. Get the position on the connector and the wire color. Hopefully yours is right where the book says it is. Mine was the colors the book said, but a few spots off from the location. Testing to be sure you have the right wire is critical!
3) Locate your tcm above where your left foot goes in the footwell. If you think you can work with the wiring harness where it is…go to it. I had to remove 2 harnesses from the module and remove the module itself.
4) Compare the connectors/harness with what you saw in the manual. If you have an idea what wire is the transfer solenoid wire you can start testing to make sure.
5) Testing: with the harness plugged into the tcm and the key on, set your multimeter to 20v. Ground the negative multimeter wire and probe into the suspected wire in the connector. With the shifter in 1st and the gas pedal to the floor it should read low, like less than 1 volt or around that. When you let off the gas (key on, not running) you should see the voltage move. Now go under the hood to the underhood fuse panel and insert a spare fuse (any amps is fine) into the FWD fuse. Probe again and you should see high voltage +10…
6) Once you’re SURE you have the wire you can cut it and very carefully connect some connectors to begin laying out the wiring in the diagram here.




From here it’s just wiring and testing. If you are very careful to wire it properly and produce good grounds this will work.

We had some wet slippery snow after I did this and I am STOKED! Biggest surprise was in really slick conditions at highway speed...because the tcm pretty much always goes 90/10 front bias once you're past like 40mph....and engaging full 4 was like night and day. Donuts are precise now. It's sweet.

Space reserved for a few photos.

[assman]

Interesting project. It's funny that in this day and age changing the characteristics of a car involve electronic manipulation. Just a couple of decades ago it involved twisting wrenches.

[sb]

Pretty fly for a white guy.

[Tippster]

Nice rat-work on your car, Yeti!