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