Power Mode Override Switch - Update

[TheSubaruJunkie]

I do have a power light, and was thinking that should be enough to alert me. But I thought a nifty LED above the switch would be even cooler.

[93Subadan]

The power lamp won't be continuously lit when the TCU is grounded so it would be nice to have another lamp to know when you are in the "sport shift" mode.

[b3lha]

The power light on the dash should be continuously lit when the pin is grounded. And there will be a definite noticable effect on the performance. Like James said: "Holy Crap!!!!".

This is not a switch to make power mode come on more easily, It is a switch to make power mode come on permanently. If it is working properly, it should not drop out of power mode at all. This is how it works on SVXes and JDM Legacies. (and probably imprezas and foresters too).

If the light does not stay on, then check your connections are good.

[93Subadan]

Will do.

[Legacy777]

I did some looking and the manual button does ground that pin at the TCU (in it's normal setup.

Under normal conditions the manual pin has 6-10 volts when the manual button is not depressed. When it is depressed, voltage should be less then 1 volt.

The manual light in the dash is fed from the illumination control circuit and has a diode in the circuit so voltage from the TCU doesn't feed back through, and only allows voltage to feed to ground through the manual switch.

So anyway....yeah I think using the manual switch is a good idea. You can just move the pin from the spot for the manual button to the spot needed to keep the TCU in power mode.

[skid542]

Thank you for confirming that for us Josh. It is interesting though because that's how I have mine wired up and my power light definately doesn't stay on all of the time. I'll have to try directly grounding it to the chassis and see if that changes it, but I don't see why it would.

James - does your light stay on all of the time as you have it grounded directly to the chassis?

[James614]

Yup, mine stays on as long as I leave my switch on. I just now read your earlier post about it dropping out of power mode and definitely should not do that. It does shift normally if you're really light on the gas, but the power light should stay on, and it should pretty much feel like it's in power mode anytime you're at more than 1/4 throttle.

The voltage from the power mode pin is a constant 6.75 - 7.25 volts, so if the manual circuit ranges from 6-10 normally and drops to 1 volt when depressed, then I imagine you'd wanna get your power mode circuit out of there.


I've actually stopped using my power mode completely, at $3.70/gal, it just feels worse, even if I am only loosing 1mpg...... Well, lemme rephrase that, I've only stopped using it unless there's a fart-piped Honda in the other lane

[b3lha]

I just wanted to update this old thread with some new information.

Somebody sent me a memory dump of a 1990 USDM Legacy TCU. It doesn't have the facility for a power mode switch in the program. The feature was probably introduced in later years.

At present, the best way to tell whether this trick will work with your TCU is to connect it up and try it.

If the power light stays lit when you drive and you notice a perfomance difference then your TCU has the feature . If not then you are SOL.

[James614]

So on some of the earlier Legacies, it won't engage power mode if you dig into the gas?

It kind of makes sense though, since the USDM Turbo didn't come out until 1991. Power mode on an N/A mostly just makes more noise, since there isn't that sudden platue of power after 2800rpm like on the Turbo cars, it just kinda feels equally flaccid from idle to redline.

[b3lha]

So on some of the earlier Legacies, it won't engage power mode if you dig into the gas?

It does engage if you floor the gas. But there is no built in facility for an override switch.

If somebody with an earlier car desperately wanted to do this mod then the best thing to do would be to fit a later TCU or upgrade/modify the software on their existing TCU.

[duganj]

b3lha, thanks for the update about the 1990 USDM legacy.

I have that car, a 90 USDM Wagon and successfully installed the power mode button. I believe I was only successful because my 4EAT is out of a 92 (I think, TZ102ZA2AA-MH) as well as the TCU (31711 AB394).

I wanted to get the users on this threads opinions on the strange power mode behavior of my Legacy to see if there's any ideas as to whats going on.

Details of my problem are described here:
http://bbs.legacycentral.org/viewtopic. ... 907#281907

Long story short, the thing shifts at high revs, even when grannying it. Toss in the manual power mode and she won't even pop out of first gear at 5k or more.

[duganj]

Couple of more bits of info I forgot to add to the above post:

Pin 4 on my B46 connector on the TCU was already filled with a teal blue wire. I cut it and wired up the 'Power' switch from there.

Anyone else had this pin occupied on their connector already?

CEL was on for knock sensor, after cutting the wire and installing the switch it went away all together.

[Primeza]

I just finished this mod, works great. I cut the wire from pin 6 and used spade connectors to wire in a switch. So at the flick of a switch the manual button is either stock or can be used as the power override button. With the connectors I can disconnect the switch and connect the pin 6 wire and it will be stock again.

Definately a worthwhile mod. Most of the time I'm going to stay out of power mode, but it's a great option to have from time to time.

[b3lha]

Couple of more bits of info I forgot to add to the above post:

Pin 4 on my B46 connector on the TCU was already filled with a teal blue wire. I cut it and wired up the 'Power' switch from there.

Anyone else had this pin occupied on their connector already?

CEL was on for knock sensor, after cutting the wire and installing the switch it went away all together.

Are you ABSOLUTELY SURE you got the right pin? Pin 4 should be empty on USDM models.

The TCU gear shifting decision is based on Vehicle Speed and Throttle Position. There is also a RPM safety limit that will force an upshift.

If your TCU is not shifting properly, then it must be getting a bad reading from either the speed sensors or the throttle position sensor. I suggest getting someone with a Subaru Select Monitor to read the TCU values (not the ECU values).

The CEL and Knock Sensor are part of the ECU system, not the TCU system. This mod should have no effect on them. You did modify the TCU, not the ECU, I hope?

[b3lha]

I just finished this mod, works great. I cut the wire from pin 6 and used spade connectors to wire in a switch. So at the flick of a switch the manual button is either stock or can be used as the power override button. With the connectors I can disconnect the switch and connect the pin 6 wire and it will be stock again.

Definately a worthwhile mod. Most of the time I'm going to stay out of power mode, but it's a great option to have from time to time.

Great. Thanks for the feedback.

[wtdash]

Couple of more bits of info I forgot to add to the above post:

Pin 4 on my B46 connector on the TCU was already filled with a teal blue wire. I cut it and wired up the 'Power' switch from there.

Anyone else had this pin occupied on their connector already?

Are you ABSOLUTELY SURE you got the right pin? Pin 4 should be empty on USDM models.

Looking for input on the best way to try this w/OUT frying my TCU or worse!

I have a '91 TCU (PN: EV 31711aa822) and a '91 Turbo 4EAT, and I still have the stock '90 NA harness. The POWER mode light works normally as is....floor it and the Power light is lit.

It also has the 'teal/blue wire' in B46/PIN 4 (A4???) location.



With the key ON, there are <7volts @ the Pin4.

Do I Add a Ground to this wire? Disconnect it and Ground the pin on the TCU? Or ??

I'll try it w/out the wire connected, but don't want to modify it....yet!

The TCU I-O diagrams from Josh's site don't mention this pin, but they may be from newer models?:
http://www.main.experiencetherave.com:8 ... _page1.jpg
http://www.main.experiencetherave.com:8 ... _page2.jpg


More pics:

Other side of B46:

Plugged in:

TCU info:

[ericem]

Funny with my TCU on my 93 L I tried the mod and it never worked. I think it only works on turbo TCU's. I still need to try with my 94 SS.

[Legacy777]

Todd,

I don't think there's supposed to be a wire in that pin. You'd have to add one.

[wtdash]

Todd,

I don't think there's supposed to be a wire in that pin. You'd have to add one.

JC,
Not sure I understand?

The wire is there from the factory - I've not added/removed/touched it. And since it already has a wire in it, I don't want to mess w/it, w/out knowing what I could mess up!

Do you have a TCU wiring diagram from a '90-91?
Thanks,
Td

[Legacy777]

Todd,

I don't think there's supposed to be a wire in that pin. You'd have to add one.

JC,
Not sure I understand?

The wire is there from the factory - I've not added/removed/touched it. And since it already has a wire in it, I don't want to mess w/it, w/out knowing what I could mess up!

Do you have a TCU wiring diagram from a '90-91?
Thanks,
Td

Sorry,

Misunderstood. I had thought the whole power mode switch/button thing was just adding a wire to an unused pin of the TCU.

http://www.main.experiencetherave.com/s ... _page1.jpg
http://www.main.experiencetherave.com/s ... _page2.jpg

[wtdash]

Sorry,

Misunderstood. I had thought the whole power mode switch/button thing was just adding a wire to an unused pin of the TCU.

http://www.main.experiencetherave.com/s ... _page1.jpg
http://www.main.experiencetherave.com/s ... _page2.jpg

JC,

No need to apologize.......I just want to ensure I'm on the same page....as you well know, wiring isn't my forte!

I also thought/hoped it was just adding a wire..I even got it all ready w/a switch and wire cut from an extra TCU harness I had, only to find out somebody (the factory) beat me to it.

I looked @ the scans, but don't see anything referencing the B46/PIN4 location, so maybe those are newer? I do believe there were wiring differences in the TCU - depending on the year, based on what I've read on this thread.

Thanks,
Todd

[Legacy777]

The I-O doesn't always have all the pins.

It appears that B46.4 goes to the cruise control for the 1990 MY.

http://www.surrealmirage.com/subaru/fil ... ntrol2.jpg

http://www.surrealmirage.com/subaru/fil ... ntrol2.jpg

However, on the 1992 FSM, the AT Control page does not list a wire coming from B46.4. So if you're running a turbo TCU or newer TCU, that pin 4 wire on connector 46 shouldn't serve any specific purpose. However, not sure what that may be causing to the cruise.

Anyway, if you have the 92 FSM scans from me, you can see what I'm talking about.

[wtdash]

It appears that B46.4 goes to the cruise control for the 1990 MY.

http://www.surrealmirage.com/subaru/fil ... ntrol2.jpg

http://www.surrealmirage.com/subaru/fil ... ntrol2.jpg

Josh,
Thanks.

These are informative (and I think I'm SOL). I'll likely lose my CC if I mess w/the B46/Pin4 wire.

But I don't understand why it works now...unless my '91 Turbo TCU also uses it?

Td

[wtdash]

It Works!

I started w/my '91 Turbo TCU:
1. Disconnected B46/Pin4, connected B46/Pin4 to Ground.
-Result - No Power Light change but Cruise Control (CC) still worked.
2. Reconnected original wire to B46/Pin4 and put Ground into Pin5 (it was empty).
-Result - No difference (no surprise).

Swapped in a '93 Turbo TCU:
1. Disconnected B46/Pin4, connected B46/Pin4 to Ground
-Result - POWER Light in D, 3, 2 and Cruise Control still worked!

Notes:
-When CC is engaged, the POWER light goes out. Hit the brake, and it comes back on (My '90 didn't come w/a Cancel option on the CC lever).
- RPMS @ 70 are same as before @ about- 2750. So I think the TC/OD are working correctly, too. I did notice some different shifting while going down a hill @ about 25mph, so maybe the TCU will need to do some learning - I hope - it wasn't bad, but unusual.

I previously had tried the '93 TCU since it matches my '93 Turbo ECU, but seemed slow to shift into OD when cruising on the highway, 'til it completely warmed up. I can live w/that.

I'll update if needed.

Thanks again to LCBBS!

[Legacy777]

Glad you got it working Todd.