Power Mode Override Switch - Update

skid542 · Post by **skid542** » Fri Feb 15, 2008 3:18 am

My trans seems pretty strong and it's kind of nice having one auto at my disposal should I bang up an ankle or something. The only thing that really drives me nuts is not having full control over gear selection. The next best thing would be keeping the TCU in power mode at which my shift points would be clearly defined by RPM.

I saw this thread a long time ago and have been keeping it around - http://bbs.legacycentral.org/viewtopic. ... ual+button.

There Vikash describes an easy way of fooling the TCU - though he does state explicity it hasn't been tried. I would interupt the TPS signal going to the TCU and feed it a one volt signal fooling the TCU into thinking you are at WOT. Vikash explains, "You can generate 1 volt easily by using a 7805 to step 12 volts down to 5 volts and connect its output through a 40k resistor and a 10k resistor to ground; the middle of the resistors will be about 1 volt."

This seems pretty simple and I am including a rough schematic of this. Vikash, please feel free to correct me if I'm wrong.

https://filebox.vt.edu/users/lskidmor/p ... %20sch.bmp

I do not have any information on the 7805 chip, what's it purpose is (outside this application), or what its pinout is. I'm hoping that some of the qualified electrical gurus around here can help me out on this.

I'm also hoping someone has a TCU pinout diagram. Admittably, I haven't searched Josh's FMS scans.

What do you think, anybody tried this since the original post or have any other advice?

Thank you.

vrg3 · Post by **vrg3** » Fri Feb 15, 2008 5:59 am

That looks about right, except that a 7805 has three pins. In addition to the input and output pin, there's a ground pin.

The 7805 is a 5-volt regulator. You can feed it power at any voltage from like 8 volts to like 30 volts, and it provides a steady 5 volts.

But, actually, I change my recommendation. Instead of bothering with a 7805, just use an LM317. The LM317 does the same thing as the 7805 but it gives you 1.25 volts. You should be able to just take its output directly.

So something like this:

Code: Select all

                       +-------+
                       | LM317 |
                       +-------+
                         | | |
+12V---------------------+ | +----------O   O------TPS
GROUND---------------------+               /
                                          O
                                          |
                           TCU -----------+

skid542 · Post by **skid542** » Fri Feb 15, 2008 2:49 pm

Oh great. Thanks Vikash. Not being fully familar with the TPS operation, I assume it goes from 5V to 1V. Further, I assume 1.25V will be enough for the TCU think I'm at WOT?

The LM317, is this a standard Radio Shack part or will I need to order it? I'm hoping to have the car running mid next week and would like to put this in while I still have wiring exposed under the dash.

Subtle · Post by **Subtle** » Fri Feb 15, 2008 8:21 pm

There is a manual push-button controller for the 4eat.

skid542 · Post by **skid542** » Fri Feb 15, 2008 8:41 pm

I know. This is a temporary situation as I will be stepping up to a nice PPG 5mt before I go crazy with the power. But until that time I just want a little bit more control over the tranny for things like auto-x and general sporty driving.

And since this is a temporary situation (the 4eat), cost is a driving factor.

skid542 · Post by **skid542** » Fri Feb 15, 2008 9:04 pm

For anybody following this thread now or in the future, I found Josh's TCU pinout scans in another BBS thread - http://bbs.legacycentral.org/viewtopic. ... tcu+pinout

It also states that it considers a fully open throttle as being 0.7V to 1.7V. This means the 1.25V from Vikash's new recommendation will work just fine.

Thank you Josh.

Post by **Legacy777** » Sun Feb 17, 2008 5:40 pm

What you're trying to do probably won't work. The TCU looks at how quickly the throttle is depressed to switch it into "power" mode. If it just sees that it's open all the time, chances are it won't go into power mode, and you'll probably get some weird things happening.

If you do try it.....let us know if and how it works.

vrg3 · Post by **vrg3** » Mon Feb 18, 2008 5:57 am

Well, the idea is that when you hit the switch it'll look to the TCU like you just slammed the throttle wide open in an instant, and then that you're not letting up. I agree that if you start the car with the switch on the TCU will just get upset.

But, yeah, do let us know.

ericem · Post by **ericem** » Mon Feb 18, 2008 6:13 am

Just wondering. I was looking at your system diagnois computer vrg3 and it showed engine load. I wonder where this sensor is? and if it has anything to do with the transmission in regards to downshifting? If so would it be possible to make this sensor read the car is under more load then it actually is? Wouldn't this make a little bit of extra throttle be more sensitive to a faster downshift? Right now you basically need to floor it you can't just easy on it, if you do it just sits there making more noise. So its like either go 25% or all out.

skid542 · Post by **skid542** » Mon Feb 18, 2008 6:17 am

Thanks for the input guys.

I bought the LM317 today and will be wiring it in tomorrow. My exhaust will be done this week - last item before starting the car back up.

So by next weekend I'll let you know how it works.

vrg3 · Post by **vrg3** » Mon Feb 18, 2008 6:22 am

ericem - Engine load is computed by the ECU; it's not a sensed value.

Lee - Cool, I'm eager to hear.

beatersubi · Post by **beatersubi** » Mon Feb 18, 2008 7:42 pm

While we're OT, vrg, what is the load gauge unit of measure on the scantool?

[/hijack]

vrg3 · Post by **vrg3** » Mon Feb 18, 2008 7:47 pm

No units. It's just a number between 0 and 255.

James614 · Post by **James614** » Wed Feb 20, 2008 3:57 am

I've been wondering about the prospect of fooling the trans into Power Mode. I'm eager to hear how it works out! I've been jonesing for a way to play with the factory shift mapping!

b3lha · Post by **b3lha** » Thu Mar 13, 2008 11:32 pm

It may be easier than you think.

I presume the Legacy TCU is similar to the SVX TCU?

The JDM SVX has a switch to enable power mode, it simply grounds pin A4 of the TCU. On a USDM car, this pin is left unconnected. We recently discovered that grounding the pin on a USDM car enables power mode just like on the JDM car.

Read the SVX World Network for more info. Somebody please try it on a Legacy.

http://www.subaru-svx.net/forum/showthread.php?t=39133

This one is for Vikash:
http://www.subaru-svx.net/forum/showpos ... tcount=619

ericem · Post by **ericem** » Thu Mar 13, 2008 11:41 pm

If it's always in power mode won't it always have the torque converter locked, and be running 50/50? So it will have some torque bind possibly.

b3lha · Post by **b3lha** » Thu Mar 13, 2008 11:45 pm

ericem wrote:If it's always in power mode won't it always have the torque converter locked, and be running 50/50? So it will have some torque bind possibly.

We think that the torque converter doesn't lock in power mode.

The front:rear torque distribution is completely independant of the shift mode.

James614 · Post by **James614** » Fri Mar 14, 2008 12:22 am

That looks way to easy for it to work on a car that I own, I'm gonna try it tomorrow, if I can find out how to get my shift console off

b3lha · Post by **b3lha** » Fri Mar 14, 2008 12:43 am

http://www.main.experiencetherave.com/s ... _page1.jpg

On the legacy wiring diagram it's B46 pin 4. The 20 pin connector.

skid542 · Post by **skid542** » Fri Mar 14, 2008 1:17 am

Hmm.... that's pretty interesting.

Unfortunately due to all the HG issues I've been having I haven't had a chance to install my 'switch', though I have the car wiring ready and also the chip wired and ready to splice into the car. Hopefully mid-next week I'll be able to give it a show.

Thanks for the other option though. I'm real curious to see how this works out.

James - you don't have to get the shift console off to get to the TCU. The TCU is located under the dash on the drivers side up beside the steering column. If you remove that panel infront of your knees you'll have pretty easy access to it. It does help if you take off the nut that holds it up in place so you get more slack in the wires to work with.

Let us know if you try it.

Oh, and regards to torque bind. I don't think it'll be an issue as I've taken tight corners from stop signs in power mode with no problems.

vrg3 · Post by **vrg3** » Fri Mar 14, 2008 5:06 pm

Neat stuff, b3lha!

James614 · Post by **James614** » Sat Mar 15, 2008 1:38 am

Didn't get to try it as I planned.... I got blind sided with a trip to DC at work today, and a trip to Virginia/North Carolina over the weekend, and so I'm stuck with the company's F-150 until Monday

b3lha · Post by **b3lha** » Thu Mar 20, 2008 12:32 am

Did anybody try it yet?

The SVX guys love this mod. I am dying to find out if it works on the Legacy auto too. I would have tried it myself, but my Legacy has the 10 speed manual box.

n2x4 · Post by **n2x4** » Thu Mar 20, 2008 1:26 pm

b3lha wrote: my Legacy has the 10 speed manual box.

Say wha?

b3lha · Post by **b3lha** » Thu Mar 20, 2008 2:25 pm

n2x4 wrote:
b3lha wrote: my Legacy has the 10 speed manual box.
Say wha?

Dual Ratio: 1 to 5 normal ratio, Then you pull a little lever and you have 1 to 5 low ratio for towing and off-road use. Like a mountain bike with two cogs at the front and five at the back.

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Power Mode Override Switch - Update

Power Mode Override Switch - Update