93 4eat power mode switch

george · Post by **george** » Thu May 01, 2003 6:47 pm

I understand the pre 93 4eat had a manual switch to select the power shift mode. My 93 doesn't have this but switches automaticaly when your driving hard or if you blip the throttle. Question is,, can I install a switch to select the power mode with or without bypassing the automatic power function?

Post by **Legacy777** » Thu May 01, 2003 7:01 pm

You were informed incorrectly.

This is what each thing does.

Power Light

The little green power light serves a couple functions. I guess it's main purpose is as a check transmission light when there is problems. The light also is supposed to tell you when you are in "power mode." When you dump it, the light will come on. Essentially the transmission computer changes the shift points and shifts later to allow the engine to rev higher for more power/performance. You aren't getting any more power when the light is on. The transmission is just changing when it shifts into the next gear.

Manual Button

The manual button has some controversy as two what its function is. But I'm here to disavow any of those rumors. Firstly, the automatic transmission normally runs a power split of 90% front 10% rear. This is under normal driving conditions. Secondly, the manual button ONLY works when the shift lever indicator is in the positions "3" or "2".

Ok, so you're ready to hear what this thingy does. It is a very primitive version of traction control. When the shift lever is either 3 or 2, and the manual button is depressed, it will keep the transmission from going into first gear. By doing this, you have less torque, more evenly distributed power, and a less chance to slip and slide around.

So to recap, when the shift lever is in the 2 position the transmission stays in 2nd gear. When the shift lever is in the 3 position the transmission chooses between 2nd and 3rd gear. If the shift lever is any other position then the two above, the manual button does absolutely positively nothing.

vrg3 · Post by **vrg3** » Thu May 01, 2003 7:11 pm

I didn't think any BC/BF 4EATs had manually-selectable power mode.

You can't turn on power mode directly, since it's purely a function of the transmission control unit's behavior, but you can trick the TCU into going into power mode and staying there. Basically, you need to interrupt the throttle position sensor line going into the TCU and send the TCU a signal saying that you're at wide open throttle. It will see that change happen immediately (thinking you just rapidly pressed the pedal from where it was to WOT), so it'll go into power mode, and then it will see you still holding the pedal down, so it will stay in power mode. The TCU will also generally behave as if you were at WOT, trying to get you the best performance possible.

You have to be careful to do it right so that the engine control unit still gets the same signal; normally the ECU and TCU just both read the signal. If the ECU doesn't know what's going on with the throttle you'll get very weird behavior.

I was planning on trying this with my old '93 Turbo 4EAT but never did, so I don't know if it will really work well, but here was my plan:

One of the pins on the TCU connectors (I think it was the black wire on the middle connector) is the TPS sensing line. Cut it, and connect it to the output of an SPDT switch. One input (the one you'll usually use) should come from the existing black wire, and the other from a little circuit that outputs 1 volt, which should be a good WOT reading. 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.

Then, when you hit the switch, you should see the power light go on as the TCU starts thinking you just floored it and aren't letting up.

If you want more specific instructions, let me know and I'll dig up my old notes or write new ones.

ciper · Post by **ciper** » Thu May 01, 2003 9:43 pm

The power mode comes from the speed of transition in throttle based on the current speed.

Its easier to get power mode at low speeds. You can go from idle to WOT without hitting power mode if the change is smooth and at a normal speed. Its when you make quick stabs at the throttle that you get power mode.

george · Post by **george** » Thu May 01, 2003 11:58 pm

Thanks everyone

The reason I asked is,,, I installd a vacuum / boost guage and notice how quick the boost comes on with 'normal' ecceleration. The tranny keeps the engine speed too low it seems. It shifts up with moderate throttle (near 0 vacuum) at ~ 2400 RPM,,, but with the power shift on (blip the throttle when starting to move) and with the same throttle shifts at ~ 2800 RPM without going near boost. Has much more ecceleration without getting into the turbo.
Anyway,, I was told by the tranny repair guy out west that the pre 93's had a 2nd button on the auto shifter for selecting the 'power mode' instead of the one manual overide button (like mine) for starting in 2nd instead of 1st for slippery conditions.
Is my tranny shifting at lower RPM's than normal? Could the resistor pot on the throttle out of calibration?
I get great millage on the highway (30 ish at 70 mph) with steady speed staying out of boost. Thought the dismal city milage might improve staying out of boost a little more using more of the engine RPM without cranking the turbo on.

Post by **Legacy777** » Fri May 02, 2003 12:07 am

These will tell you how to adjust/test the TPS sensor.

The last page is the actual adjusting info

http://www.main.experiencetherave.com:8 ... sting1.jpg
http://www.main.experiencetherave.com:8 ... sting2.jpg
http://www.main.experiencetherave.com:8 ... sting3.jpg

ciper · Post by **ciper** » Fri May 02, 2003 12:25 am

Im curious why you care about holding low rpm if you arent using 90%+ throttle. So what if the transmission shifts early at low throttle, thats the point!

george · Post by **george** » Fri May 02, 2003 2:03 am

Good question for sure, thats the problem with guages I spend too much time watching things. Under 'normal' acceleration in traffic it seems the turbo comes on because the RPM's are forced so low. I had a non turbo 82 wagon and it got the best millage and it drove the best shifting at 3K. If I shifted it around 2K it would bog. Should I ignore the guage and just drive? Just don't like 200 miles per tank in the city when my 2 ton 240 hp Honda Odyssey uses less gas.

Post by **Legacy777** » Fri May 02, 2003 1:52 pm

just drive.......subarus are not very fuel efficient IMO.....compared to other car manufacturers. Add a turbo in the mix...fuel mileage will suck even more.

Part of it is the design of the engine, the other part is the fact the factory sets the fuel maps rich to protect the motor.......the reason they do this is because of cheap/incomplete engineering. On the turbo legacy....they didn't give it an intercooler....therefore they have maps on the really rich side to quench the hot air and prevent detonation. On the WRX, sitting in traffic intercooler temps can reach insane temps, factory maps dump tons of fuel at it.

These are just some of the reasons in my mind why subarus, especially turbos get such crappy mileage.

jmaziarz · Post by **jmaziarz** » Tue Apr 05, 2005 6:00 pm

Legacy777 wrote: Power Light

The little green power light serves a couple functions. I guess it's main purpose is as a check transmission light when there is problems. The light also is supposed to tell you when you are in "power mode." When you dump it, the light will come on. Essentially the transmission computer changes the shift points and shifts later to allow the engine to rev higher for more power/performance. You aren't getting any more power when the light is on. The transmission is just changing when it shifts into the next gear.

In the 6 years I have owned my FWD 93' Legacy L, I have *never* seen this light come on while driving (it does light up when the car is started though). Is this for turbos only or might I have a problem?

Post by **Legacy777** » Tue Apr 05, 2005 6:18 pm

jmaziarz wrote:In the 6 years I have owned my FWD 93' Legacy L, I have *never* seen this light come on while driving (it does light up when the car is started though). Is this for turbos only or might I have a problem?

Do you consider yourself an agressive driver? If not, that's probably the reason it hasn't come on.

Next time you're out driving, and cruising, just dump the gas pedal to the floor quickly, and leave it there. It'll come on

You can do it from a dead stop too. The power light comes on based on accelerator depression speed & travel.

tris91ricer · Post by **tris91ricer** » Tue Apr 05, 2005 6:28 pm

I've seen it flash, but never come on and stay on unless I was having some sort of a problem.. I'm not too concerned, though.. it does what it does and I don't mind.

jmaziarz · Post by **jmaziarz** » Tue Apr 05, 2005 7:48 pm

Legacy777 wrote: Do you consider yourself an agressive driver?

I try not to be.

Yesterday I tried to get this light to come on by coming to a dead stop at the end of a not-very-busy highway on ramp. I turned off my headlights, HVAC, radio (not that they would really make any difference) and floored it. I did not let up on the gas until I was @ 90MPH. I would have thought the light would come on then but it didn't.

Post by **Legacy777** » Tue Apr 05, 2005 9:20 pm

how fast did you floor it? It really has nothing to do with speed of the car, it has to do with throttle position, and acceleration of throttle opening.

jmaziarz · Post by **jmaziarz** » Tue Apr 05, 2005 9:48 pm

Legacy777 wrote:how fast did you floor it? It really has nothing to do with speed of the car, it has to do with throttle position, and acceleration of throttle opening.

Fast enough to chirp the tires.

dzx · Post by **dzx** » Tue Apr 05, 2005 11:01 pm

That's strange, mine is a manual and my power light still comes on when in floor it.

jmaziarz · Post by **jmaziarz** » Wed Apr 06, 2005 2:16 am

Well here is the deal. After work today I tried again (actually 3 times) and I got the power light to come on 2 out of those 3 times. One good side effect is that I think I burned any carbon deposits that might have been in the engine out for sure!

Chef · Post by **Chef** » Wed Apr 13, 2005 2:59 am

My power light never seemed to come on even when I stomped on it (except briefly at start-up). But today i noticed it came on a couple of times when hitting the gas hard. Not sure why this happens.

Project_Legacy · Post by **Project_Legacy** » Fri Apr 15, 2005 4:37 pm

if u hit WOT it is supposed to come on... mine does anyway.

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93 4eat power mode switch

93 4eat power mode switch