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'99 OBS
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117 Posts
Discussion Starter #81
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?
Hey Ryan,

You did it to g00fy's hot rod Forester right? Is his car OEM turbo? If so, there's a different set of rules - luckily, I think I know them.
I don't want to take this thread way off topic since there aren't that many OEM turbo guys here. Feel free to PM me, or when I get a chance later I'll PM you to get the info for the car.
 

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1998 Impreza L 2dr Red
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So the problem with the handbrake mod is the TCU is too slow to react to wheelspin or the solenoid is too slow to react? If I understood the thread correctly, the TCU is the one that's too slow to react, right?
 

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Big Bad BH "Slow White"
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169 Posts
So the problem with the handbrake mod is the TCU is too slow to react to wheelspin or the solenoid is too slow to react? If I understood the thread correctly, the TCU is the one that's too slow to react, right?
Yeah, because I think defcon's original implementation just reverted the car back to stock AWD when the handbrake is pulled. The thing to do is simulate inserting the FWD fuse (fully unlock the center diff) when the handbrake is pulled.

Not that hard to do--I'll post up some schematics eventually.
 

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1998 Impreza L 2dr Red
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In retrospect, I shouldn't have specified the handbrake mod.

How potentially harmful would it be to just circumvent the TCU and control Duty C yourself?

If I understand it correctly, all the TCU does is find the differencing between VSS1 and VSS2, look at the TPS, and find a good value in a table to tell Duty C to be at? Technically, couldn't you just grab a microcontroller and have it do the same thing, but do it faster? If you had that setup, you could have it control AWD, have a pass thru mode where the TCU signal goes through unaffected, and a potentiometer for custom control if you wanted. The TCU would still control gear changes and talk to the ECU, just AWD would have its own separate (ideally) faster computer.

From the microcontroller standpoint, this is doable, but will this have any harmful effects on the car?
 

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'99 OBS
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Discussion Starter #85
How potentially harmful would it be to just circumvent the TCU and control Duty C yourself?
No harm that I can foresee - If you look back a few posts, you'll find I'm working on that exact concept. The proof of concept Manual PWM controller is already built and successfully bench tested - just awaiting some time to install it.

If I understand it correctly, all the TCU does is find the differencing between VSS1 and VSS2, look at the TPS, and find a good value in a table to tell Duty C to be at? Technically, couldn't you just grab a microcontroller and have it do the same thing, but do it faster?
Absolutely - that's my next project after the manual PWM controller. ;)
The speed I was referring to that was complicating the handbrake mod was the OEM's response to the vehicle coming to a halt. The ONLY time the solenoid sees a full FWD bias is when the car is stationary. Any movement (VSS signal) defaults to roughly 60% - 70% duty cycle (~60/40 torque split). At that number, you'll still lock up the front wheels when the handbrake is pulled.
Obsolete is correct - mimicking the FWD fuse is the only way to truly 'unlock' the rear wheels. He's got a very cool solution to this - I'm sure a great new DIY is on the way...

As for overall response time, the OEM system is plenty fast to react to on-the-fly traction changes when accelerating from a stop - in fact, it's even a bit 'predictive'. The trouble spot is on low traction surfaces at ~20mph in 2nd gear. In my car, the front wheels will spin for 1.5 seconds before the system hard locks the rear wheels.
This is precisely the area that causes trouble when rallycrossing since you're pretty much always on the gas hard right around 20mph. In normal situations, the average driver isn't ever going to notice that quirk.
 

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Big Bad BH "Slow White"
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How potentially harmful would it be to just circumvent the TCU and control Duty C yourself?
Not any worse than what we're doing already.

If I understand it correctly, all the TCU does is find the differencing between VSS1 and VSS2, look at the TPS, and find a good value in a table to tell Duty C to be at?
I agree with all that.

Technically, couldn't you just grab a microcontroller and have it do the same thing, but do it faster?
I sort of doubt we could do it any faster or better than the stock system in terms of reacting to wheel spin.

The idea of the mod is to be proactive--the rear wheels are already engaged, so we don't need to wait for the computer to sense spin and do it for us. No matter what, as long as the system is reactive, there's going to be some waiting.

If you had that setup, you could have it control AWD, have a pass thru mode where the TCU signal goes through unaffected, and a potentiometer for custom control if you wanted.
Defcon has already built some parts of the "custom control" system using discrete components, and it seems possible to implement that whole deal without a microcontroller, if anyone wants turn-the-knob capability.

From the microcontroller standpoint, this is doable...
I think you're right, but I don't wanna do it, 'cause in terms of reacting to wheel spin, I think the best system I could build would only be just as good as the original :( If you're a good hand with microcontrollers I'd love to have you prove me wrong :D
 

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1998 Impreza L 2dr Red
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Wow, and I actually read the thread too... you're right, it's essentially what you're doing. I kinda misunderstood the problem then. Since we can already set it to 50/50, is there still a problem with the low speed low traction situation or has it essentially been solved? More so would there be any point to having a more automatic (faster) AWD?

On the upside, I suppose technically we could have a somewhat customizable AWD system?
 

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'99 OBS
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117 Posts
Discussion Starter #90
tjdlegend said:
Has anyone successfully installed this on a 2007 2.5i??
or a older 98 legacy GT??
tjdlegend: To my knowledge, no one has done this on anything newer than a 2002 Legacy. That's not to say that it can't be done - it's just that nobody has gotten around to it. I'm working with one of the northeast tuners right now to install on a 2005 ForesterX, and the system is pretty much the same as earlier years. So if you're willing to take a couple of pictures of you car, we can see if it's the same setup. I have no reason to believe that it can't be done. PM me...

sixth-star: I have all of the pinout/connector info for your car (in fact most every 4EAT Subaru from 1990 on...) If you're interested in trying it out, just let me know via PM

Might as well use this opportunity to give a quick update.....
In my "spare time" I've been working up a new version of the switch mod. It's not a full blown variable version, but instead a full-featured version of the switch. I'm calling it the "MkII" for now, and it adds the ability to run a FWD mode as well as including the handbrake interlock so that you can bust out some handbrake turns like your favorite WRC (or STI) driver.

Thanks to The Korean for letting me use his car as the lab rat. The prototype is installed and working in his car - though we have some issues to iron out. Once all the bugs are worked out on the new MkII, and I figure out how to make it "kit-able", I hope to have some ready by the fall (just in time for fun in the snow) Should be pretty cool - think 'printed circuit boards and real connectors'!
 

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1998, subaru, impreza L black
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when trying to wire in my SMT6 PP6 what ever you want to call it into my 98L. i found out that the 98's are often considered the bastard child of subaru because they did many things only to the 98s and changed it (98 ecu single plug and every other year 3 plug and wire colors where different in some cases) I cant speak for the TCM but wanted to give you a heads up and i cant look into it because i have a 5 speed
 

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'99 OBS
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Discussion Starter #92
i found out that the 98's are often considered the bastard child of subaru because they did many things only to the 98s and changed it
subaruferrucci speaks the truth - the 98s are odd ducks. One of the posters to this thread (limited I believe) has a 98 Legacy and it was eye opening how many differences there were in the TCM wiring. Since 99 was a changeover year, it's possible that late production 98's could have 99 TCMs in them. After many hours of poring over the wiring diagrams, we figured it out - well, he hasn't gotten back to me in a while so I'm assuming he's up & running.
Another point worth noting is that the Ls seem to play by different rules as well. Sometimes they have parts that don't match up with the rest of the models.

But, that's part of the fun of working on cars right?
 

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98 Blue RS
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I was thinking of a way to make an off road application for the 4eat in my EJ22 hatch conversion. Would a DPDT switch work to...

1. (on) turn on AWD all the time for 4WD,
2. (off) normal AWD,
3. (on) turn off AWD for full FWD?

This way I could have AWD for the output to reach the ground w/o tearing up front tires, then "lock" into 4WD when wheelin'. At this point it is one of many versions to solve the low-range quest in my hatch project (must be finished before I can paint and put new struts under my RS)

I'd be interested in the resistor pack you'd make as well.

TIA,
Lewis
 

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'99 OBS
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Discussion Starter #94
Lewis - the quick answer is "yes - kind of". For more info, I'm pasting in a quote from my last update in post #99.
Might as well use this opportunity to give a quick update.....
In my "spare time" I've been working up a new version of the switch mod. It's not a full blown variable version, but instead a full-featured version of the switch. I'm calling it the "MkII" for now, and it adds the ability to run a FWD mode as well as including the handbrake interlock so that you can bust out some handbrake turns like your favorite WRC (or STI) driver......
.....I hope to have some ready by the fall (just in time for fun in the snow) Should be pretty cool - think 'printed circuit boards and real connectors'!
To summarize: the new mod has three drive modes - FWD/AWD/4WD, plus a handbrake interlock, and possibly some blinky lights..
Note that the new version became fairly complicated with the additional handbrake mod and provisions for the LED indicators. A much simpler version could easily be built with a DP3T (on-off-on), the resistor bank, and some basic wiring. Come to think of it, with the DP3T you'll need at least one relay to handle the TCM circuit, maybe more - I'd have to sketch it out.

If you need more info feel free to PM me.

One quick aside: This mod has no control over the front and rear diffs, just the center. Unless you've put some "real" diffs in there, be careful offroading. Those open diffs will get you stuck in the blink of an eye.
 

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auSTin
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I got my parts for mine today at radioshack and 3 hours later I had it all hooked up. this is a sweet mod espesaully for my first rally x race coming up it will be usefull
 

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auSTin
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on an auto trany is there a similar silanoid that conrols the differential. like on this we're locking the center diff so the power from back and front is the same what about side to side. is that electrical stuff that i can trick. i was going to look through the service manual myself to check this out but estimated time of download on it was 7 hours-stupid dial up
 

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'99 OBS
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Discussion Starter #97
Nope - See post #94.
The front and rear diffs are purely mechanical. The only know solution to open diffs is little green pieces of paper (lots of 'em).
 

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'98 Legacy Outback 2.5 AT
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subaruferrucci speaks the truth - the 98s are odd ducks. One of the posters to this thread (limited I believe) has a 98 Legacy and it was eye opening how many differences there were in the TCM wiring.
Greetings, Guys!

I have European Legacy Outback 98 2.5 AT and... about 1-2 hours I cannot even find the TCM :)
It must be mounted right near the brake but... I have only bolts there.
Does someone have a clue? I have searched in left near fuses and in central console... found SRS and Cruise Control ECUs but TCM just hides from me :)
Everything else is working as it must be but...



 

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2001 Impreza L
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I have a quick question. If you remember, I had problems with the setup a while back that I still have not fixed yet. The problem was that everything was hooked up correctly, but when the switch was "ON", it would pop a AT OIL TEMP light on the cluster. It would not turn off even if I flipped the switch back to the "OFF" position. The only way to turn it off was to turn the car off and turn it back on.

Today, I thought of something that might have caused this problem.

The "ON" position for the switch was actually off, and vice versa. For example, I have a dot on my simple switch, which commonly denotes that when the switch is flipped towards the direction of the dot, it meant on. It is the opposite for me. Would this cause the problem I'm having? Thanks!
 

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'99 OBS
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Discussion Starter #100
Greetings, Guys!

I have European Legacy Outback 98 2.5 AT and... about 1-2 hours I cannot even find the TCM :)
It must be mounted right near the brake but... I have only bolts there.
Does someone have a clue? I have searched in left near fuses and in central console... found SRS and Cruise Control ECUs but TCM just hides from me :)
Everything else is working as it must be but...
You're in luck - I got laid off a few weeks ago, so I've got plenty of time to do some research. ;) Give me a day or two and I'll take a look at your pictures and the manuals to see where your TCM is. In the meantime, one place to look is on the firewall behind the brake assembly - that was a common location on the earlier (pre 99) cars

I have a quick question. If you remember, I had problems with the setup a while back that I still have not fixed yet. The problem was that everything was hooked up correctly, but when the switch was "ON", it would pop a AT OIL TEMP light on the cluster. It would not turn off even if I flipped the switch back to the "OFF" position. The only way to turn it off was to turn the car off and turn it back on.

Today, I thought of something that might have caused this problem.

The "ON" position for the switch was actually off, and vice versa. For example, I have a dot on my simple switch, which commonly denotes that when the switch is flipped towards the direction of the dot, it meant on. It is the opposite for me. Would this cause the problem I'm having? Thanks!
Yes - that could cause the behavior, but from your description what's more likely is that you've either got the COM terminal mixed up with one of your "output" pins, your circuit to the resistor bank is incomplete, or your resistor bank is of the wrong impedance. The only way your going to trip the AT light is when the TCM completely loses communication with the solenoid. You'll probably need a multimeter to do any further troubleshooting. Can you post, or PM some pictures and additional info?
 
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