4EAT Diff lock switch and Handbrake mod - "Torquemada Lite"

[defcon5]

Originally Posted by Area1320

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.

[renyo]

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?

[obsolete]

Originally Posted by renyo

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.

[renyo]

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?

[defcon5]

Originally Posted by renyo

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.

Originally Posted by renyo

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.

[obsolete]

Originally Posted by renyo

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.

Originally Posted by renyo

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.

Originally Posted by renyo

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.

Originally Posted by renyo

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.

Originally Posted by renyo

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

[renyo]

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?

[tjdlegend]

Has anyone successfully installed this on a 2007 2.5i??

[sixth-star]

Originally Posted by tjdlegend

Has anyone successfully installed this on a 2007 2.5i??

or a older 98 legacy GT??

[defcon5]

Originally Posted by tjdlegend

Has anyone successfully installed this on a 2007 2.5i??

Originally Posted by sixth-star

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'!