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"Power mode" for 95-99 4EAT TCU's


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7 replies to this topic

#1 WoodsWagon

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Posted 02 November 2010 - 08:22 PM

As you may or may not be aware, early 90's SVX's came with a power mode button in Japan. US models didn't get it, probably for EPA fuel milage reasons. But the pin on the Transmission Control Unit was still there and the programming code still existed in the US TCU's. So it's popular in the 91-94 legacys to move the female pin on the 20 pin connector from postion 6 to position 4. This then makes the "manual" button on the shifter turn on the "power mode".

Power mode is the equivalent of a sport shift button on a newer car. The TCU uses a different shift map so that it holds gears longer, downshifts sooner, and won't lock the torque converter clutch. It makes the transmission much more responsive and fun to drive. There is a small gas milage penalty, around town it's only a MPG or 2, but on the highway it is more because of the torque converter not locking up.

So in 95 subaru switched to OBD-II. This required a complete redesign of the ECU (engine control unit), wiring harness, and TCU. All the pins on the TCU that used to control the "power" and "manual" lights on the dash and the input for the manual mode button have been redesignated as communications lines for the OBDII systems. The TCU case and board have also been redesigned with the plugs on the side instead of the bottom. But through this all, pin 4 on the 20 pin connector was still left unused.

I pulled a pin and pigtail of wire from a 20 pin TCU connector on a spare harness and put it in postion 4, which is the only unpopulated pin on the latching tab side of the 20 pin connector, which is the higest connector up on the TCU. The TCU is to the left of the brake pedal above the drivers feet by the way, big gold box with 3 connectors. You have to release the locking bar on the back of the connector, push the pin in until it clicks, fold the locking bar back into postion and plug the connector back in.

Power mode is activated by grounding pin 4 on the early TCU's. I grounded pin 4 on the 97 OBDII TCU and while it can no longer light up an indicator of power mode being activated you can tell by driving. At part throttle at 45mph it was holding 3rd gear with the engine up around 3k rpms. Unground pin 4, it would shift to 4th and lock the TC dropping the RPMS down in the same driving conditions. It downshifts as soon as you roll into the throttle, rather than having to stab the throttle to WOT to getting it to downshift like you normally do. Much more responsive and fun to drive.

All you need for the mod is a small switch, some wire, and a TCU pin and pigtail. Set up the switch to ground pin 4 of the 20pin TCU connector to a chassis ground and you will have sport mode on command. This should work for a 95-98 TCU, outback or legacy. I'm not sure about the 99's as they changed some things with the bellhousing pattern and ABS sensors among other things and I don't have a 99 handy to test with. I'll try out a 98 outback tomorrow.

Here's a picture borrowed from "Nomake Wan" on subaru-svx.net
Posted Image

Edited by WoodsWagon, 03 November 2010 - 02:54 AM.


#2 grossgary

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Posted 02 November 2010 - 08:52 PM

nice hit! it'll work on all EJ's within those years, impreza too.

#3 TheLoyale

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Posted 05 December 2010 - 07:55 PM

Oh wow, this is one great idea!

Thanks for posting it!

-Tom

#4 SchwarzeEwigkt

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Posted 05 December 2010 - 08:07 PM

I plan to try this myself.



...after it stops snowing.

#5 eulogious

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Posted 07 December 2010 - 05:53 AM

It does still lock the TC up, just takes a while at cruising. It's does make the car feel sportier than stock for sure, and I really don't think it effect gas mileage that much.

When I use my paddle shifter code for my TCU it uses the power shift maps, and it's alot of fun driving around in power mode all day long. Even on the freeway it will lock up the TC and give you decent mileage. I have even heard of guys just keeping it on all the time and not really noticing to much of a difference in anything other than having more fun driving around.

Good stuff :)

Thanks for the write up on this!

#6 eulogious

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Posted 07 December 2010 - 07:57 AM

Here's the "geeky" break down of the power button, and what it actually does in the code. Kinda cool if you like that sort of thing like I do :grin:

http://www.alcyone.o.../powermode.html

#7 TheLoyale

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Posted 25 December 2010 - 03:37 PM

Where is the TCU box? I see a small aluminum box under the dash on the drivers side, its held up with two 10mm nuts. Is that it?

Its been to cold for me to really look, I just want to know cause its been bugging me :)

#8 CNY_Dave

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Posted 04 January 2011 - 08:46 AM

Similar tweak you could do to some other cars- on my '03 H6 OBW the trans is more prone to downshift/hold a lower gear when ther cruise is engaged- in this situtaion you could hook a button to the TCU pin that signals 'cruise engaged' and you'd have a sport-shift button.

Dave




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