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94 legacy turbo maroon
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17 Posts
hey defcon i'm new here. been on nasioc for a quite awhile but i got a new car with an automatic and have been researching it.

aaaaand this came up and looks pretty damn sweet. i could def use this in the snow. my lag time for the rear end to kick in is horrible, and the packs are slipping a bit so i get a good clucnk when the rear really kicks in.

anyway i'm pretty interested in yore little project and i think this is what i need next winter for sure. awesome job!
 

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1998 Impreza L
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11 Posts
Just installed on a 1998 L, the TCU pinout is different, 3 connectors versus your 2.. but, as logic would deem, the coloring between your TCU pins and Mine was the same.

I used a DPDT switch so it also would light a LED when it was enabled.

http://img21.imageshack.us/img21/7307/lightoff.jpg
http://img12.imageshack.us/img12/2672/lighton.jpg

Great writeup, I'll post pics of my weird TCU and the wire to use and of course the resistors/sinks i used tomorrow, need sun!


>That other switch enables the FWD Relay in my ebrake light circuit... Flip it and whenever the ebrake light in the dash lights up it essentially plugs in the FWD fuse, obviously not good to drive on, which is why its used to corner in the deep snows up here in northern MN.
 

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Big Bad BH "Slow White"
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169 Posts
^ Cool.

I am working on a version that uses a single three-position switch with relays to do 4WD-AWD-FWD + handbrake mod.

Got the schematic and parts, just need to do assembly and testing, which could be weeks away with my current schedule, unfortunately...

Hasn't got any LEDs, though...that's neat.
 

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'99 OBS
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117 Posts
Discussion Starter #64
Just installed on a 1998 L, the TCU pinout is different, 3 connectors versus your 2.. but, as logic would deem, the coloring between your TCU pins and Mine was the same........
http://img12.imageshack.us/img12/2672/lighton.jpg.....Great writeup, I'll post pics of my weird TCU and the wire to use and of course the resistors/sinks i used tomorrow, need sun!
This just came up with one of the guys that I built a kit for (limited). He's got a a 98 Legacy GT and has the three connector TCU as well.
It's good to know that the wire color appears to be a constant (so far), but if you could post pictures of your TCU and connectors, that'd be great. I'm trying to track down a decent source for Legacy and Forester service manuals in case thiser are more TCU options out there

Great install - I know that DPDT switch well. It's the only reasonable 'DT' switch that Radio Shack sells. One note - did you run the LED off of +12v, or is it fed off of the TCU wire? If you fed it from the TCU, the maximum voltage to the LED will be around 11v (vehicle in park). At any other speed, voltage will drop and the LED will dim. You'll also get varying voltage as you drive, and the LED will dim and brighten accordingly, and you'll probably get some flicker due to Subaru's low frequency PWM signal.
Certainly not a deal-breaker - in fact, it might be cool. But if it drives you nuts, now you know how to fix it..
 

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'99 OBS
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117 Posts
Discussion Starter #65
my lag time for the rear end to kick in is horrible, and the packs are slipping a bit so i get a good clucnk when the rear really kicks in.
Welcome reddragons. Glad you like the project. I feel your pain with the delayed rear lockup, but in researching it further, I don't think the 'clunk' that you're hearing is a worn clutchpack. It's a fault in Subaru's AWD mapping.

Under normal acceleration from a stop, the system works perfectly. It proactively shifts a large amount of torque to the rear wheels, and does so with no clunking. I would even go so far as to say it's a very good 'predictive' system.
Where it fails, is at speeds above 10-20 mph. When you hit the gas on slippery surfaces at those speeds, the sytem is purely 'reactive'. If the wheels spin, the TCU senses the spin and locks up the Duty C solenoid - sometimes politely, sometimes not. Lag and lockup percentage are dependent on a ratio of wheelspin to throttle position.

That is exactly the phenomenon that drove me nuts in Rallycross - it makes for a very ill-handling car. Now that I've fixed it, my results haven't gotten any better, but I attribute that to my distinct inability to drive like a sane person. Sideways may be slow, but it sure is fun!
 

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1998 Impreza L
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11 Posts
For both LED's I bummed 12v from the cigarette lighter right there.

As for the '98 4EAT TCU:
Location of it
http://img12.imageshack.us/img12/9373/98tcu.jpg

The wire for Duty Solenoid C ("AWD Solenoid"), note the way I am holding the tcu in this picture.
http://img16.imageshack.us/img16/4690/wiresolenoidc.jpg

For the resistor pack I used the same values but different housings.
2x 50 Ohm, 2x 100 Ohm, and 2 of these heatsinks to tie it all together.


just sits under the plastic between the shifter and center console piece.
 

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If I'm not mistaken, The wire for the Solenoid C is the bottom connector right? with the TCU laying on the back end of the panel.

Like Defcon5 said, I have a 1998 Legacy and it's also has the 3 connectors instead of 2. I'm not sure what has change between the years. Hopefully it's still the same wire (white/green stripe) leading to the solenoid.
 

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94 legacy turbo maroon
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17 Posts
thanks for the welcome defcon. you see i thought it was slippy packs cuz someone told me that. i really didn't know. my last subie was a 95 impreza L awd manual, so i never had this problem. i should build this and install it before next winter for sure....
 

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'99 OBS
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117 Posts
Discussion Starter #69
Nice work on the photos StartRestart. Those are some impressive resistors! The resistor/sink assembly looks like a small satellite..:lol:

To answer limited's question; yes it looks like the bottom most plug (when mounted) contains the Duty C solenoid wire, and as luck would have it it, it matches the color of the 99 Impreza wire (wht/grn) Hooray!!

Now I just need to start a list of all the different TCUs we come across and add them to the actual DIY. Thanks for the reference photos StartRestart. If anyone else has installed one of these and has a different TCU than I picture, please let me know what model and year, and I'll do my best to come up with some specific documentation for each variation.
 

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'99 OBS
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117 Posts
Discussion Starter #70
thanks for the welcome defcon. you see i thought it was slippy packs cuz someone told me that. i really didn't know. my last subie was a 95 impreza L awd manual, so i never had this problem. i should build this and install it before next winter for sure....
No problem - I thought the exact same thing. I was pretty much prepared to buy a new transfer clutch and install it. This would have been a very messy, involved installation, and could easily cost several hundred dollars - even on the cheap.

After I did a little research and plugged my meter into the car, I was able to narrow down the general problem. That's when the idea for the switch was born

You can build and install this switch/resistor assembly in an afternoon. You'll spend less than $40 in parts and tools, and you'll have the power to make the clunking go away whenever you want.

When you decide to do your install, I'm always available via PM if you get stumped.:cool:
 

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94 legacy turbo maroon
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17 Posts
^ ha thanks. i have quite a bit of experience digging around and finding wires and sodering and what not. i just wonder if the wire for my turbo 94 is the same color as na's. also where you described the box being mine looks all black instead of that metal color.
 

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Alright! sounds good.

I took a couple of pictures of the TCU, and i noticed there were a few wires that seems to be missing between the Legacy and Impreza models. Although you can't see all the wires, but I can't seem to spot out the White/green wire. I'll confirm this by this weekend.








 

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I took a careful look today and found no White wire with green stripe. I'm not sure if Subaru change the wire color for the '98 Legacy.
 

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'99 OBS
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117 Posts
Discussion Starter #75
i just wonder if the wire for my turbo 94 is the same color as na's.
Yes - it's very likely that the wire in any OEM turbo setup will be different. In fact, I recall it being different colored and possibly a different impedance (resistance).
My current setup, is aimed at the N/A cars, but I can do a little looking to see what the differences are. My main problem is that I can't get my hands on any turbo cars to confirm. If any one reading this is interested in doing one of the turbo cars, PM me and I can send you some of the documentation that I do have on the subject - that way I don't clutter up the thread with partial (possibly faulty) information.

I took a careful look today and found no White wire with green stripe. I'm not sure if Subaru change the wire color for the '98 Legacy.
Well now Fuji Heavy Industries is just messing with us! ..:mad:
I've got several years of the Legacy FSMs ready to download - I just need to reinstall my torrent client (bleh). Hopefully I can do that tonight, and I'll take a look at the wiring diagrams.
 

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2001 Impreza L
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730 Posts
I just did this mod today, but I haven't driven it yet.

The first thing I did was turn the car on, and I noticed that the AT Oil Temp light was blinking. I turned off my car, flipped the switch, and turned it back on. The oil temp light was gone.

Did I do something wrong? I'm about to take a quick drive around the block.
 

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'99 OBS
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117 Posts
Discussion Starter #77 (Edited)
I just did this mod today, but I haven't driven it yet.

The first thing I did was turn the car on, and I noticed that the AT Oil Temp light was blinking. I turned off my car, flipped the switch, and turned it back on. The oil temp light was gone.

Did I do something wrong? I'm about to take a quick drive around the block.
Hi dspec -the blinking AT light is just the TCM's way of complaining. You haven't broken anything, but it does sound like something's not quite right - let's see if we can get everything back on track:sunny:
Assuming you tested and cut the proper wire, which I'm guessing you did - since the switch is obviously affecting something. My guess is that the TCM isn't happy with your resistor bank, or the wiring to the resistors. Here are some things to double-check:

1. Switch - SPDT or DPDT (ON/ON) - Make sure you didn't get an ON/OFF/ON switch. The center 'OFF' will freak out the TCM.
2. Continuity from the switch 'out' to the resistor bank's ground connection. You did ground the resistors, right?
2a. If the continuity check fails, start at the switch and check each 'section' until you find a break in continuity.
3. Resistor bank impedance (17Ω)

Basically it sounds like you've succeeded in cutting power to the solenoid (a test drive will tell you for certain) but the signal isn't being properly routed to the resistors - which is making the TCM unhappy.

Another possibility is that your car may have one of the other TCMs that Subaru had over the years. I just built a database of all the various permutations of the years and models that I could find. I'll check into that as well.
EDIT: If this is the car in your profile - your car should match the DIY just about 100%. According to my research anyway...
My guess is that you've got a bad connection somewhere.

Feel free to post some pictures of the setup and it's components, and I'm sure we'll get you all sorted out. Also feel free to PM or email me if you'd rather.
 

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2001 Impreza L
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730 Posts
Thanks! I'll check into it.

I do have one thing for sure, and that is the resistor bank impedance. It is definitely around 17. I took numerous readings and they were all around 17.1-17.2 ohms.

My switch seems correct. It is the same one that you had pictured, except DPDT. I just used one side of the switch.

I will try to check the ground connection today. I have the ground attached to my intake manifold. I also have to note that I use the same ground for my grounding kit. Would this be affecting it?
 

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'99 OBS
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117 Posts
Discussion Starter #79
Thanks! I'll check into it.

I do have one thing for sure, and that is the resistor bank impedance. It is definitely around 17. I took numerous readings and they were all around 17.1-17.2 ohms.

My switch seems correct. It is the same one that you had pictured, except DPDT. I just used one side of the switch.

I will try to check the ground connection today. I have the ground attached to my intake manifold. I also have to note that I use the same ground for my grounding kit. Would this be affecting it?
Good man!! Those are fine numbers. My switch is actually DPDT too - a little clever photography hid the other pins ;)

I'm guessing that your ground is the issue and your continuity test will tell you for sure - the intake is subjected to a fair amount of potentially corrosive road grime, and unless it's painted or cleared, the aluminum will develop a layer of surface oxidation that could prevent a good connection. Otherwise it should be a fine ground - especially if the grounding kits recommend it.
That said, I grounded to the factory grounds located to the rear of the driver's front strut tower. It was tidier looking, and required less wire. ;)
 

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2001 2.5RS
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2,114 Posts
We recently did this mod to a customers Forester. The issue we are having is when the switch is off, the front tires are seeing a lot of wheel spin. With the wires resorted back to stock its fine. With the switch on, its fine.. Any ideas?
 
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