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how does TCU determine AWD or FWD?


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34 replies to this topic

#1 grossgary

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Posted 09 April 2008 - 11:55 PM

how does the ECU or TCU determine if the vehicle is FWD or AWD? the wiring diagrams/FSM's don't seem to shed light on this.

i swapped a FWD trans and TCU into a 1996 AWD Legacy EJ25 and it runs great. i believe it still "thinks" it's an AWD as the trans light does the blink on start up thing.

wiring diagram doesn't really show much of anything here. Duty C wiring and FWD switch...both of which aren't working so it shouldn't "see" those anyway.

i see the ECU and TCU are talking through some wiring but the FSM doesn't say what those wires are for.

i can live with the blinking since there are no issues...but if there's a simple identifier pin that i can deal with to make it all right that would be nice.

#2 Legacy777

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Posted 10 April 2008 - 07:56 AM

Have you checked the TCU for codes? Usually if the trans light blinks, that means there's a code stored.

#3 grossgary

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Posted 10 April 2008 - 08:47 AM

no i haven't checked the codes - i've never had luck getting the codes to come out. actually i've never seen the procedure for doing it on an EJ vehicle, but never got it to work on the first generation of 4EAT's in the EA/ER stuff. if there's a way i'll do it.

i'm just assuming it's because of the FWD trans...but it's "looking" for an AWD.

i'll try and pull the codes to be sure.

#4 Gloyale

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Posted 10 April 2008 - 10:49 AM

but never got it to work on the first generation of 4EAT's in the EA/ER stuff. if there's a way i'll do it.


The EA/ER 4EAT code output is done like this

key off, insert FWD fuse(skip for FWD)

Put shifter in D.

Turn key to *run*/no start

depress 1st hold button(on)

Shift to 3, depress 1st hold(off)

Shift to 2, depress 1st hold(on)

now push gas pedal more than halfway down.

POWER light will come on steady for 2.5 seconds, then blink codes, then repeat.

There are 11 blinks after the 2.5 second initial Pulse. Each of the 11 blinks represents a component.

If all 11 are quick, .2 second blips then everything is normal.
If say the 3rd blip is slower, almost a full second, then you have a code 3. 8th blip slow, then you have code 8, and so on.

90-94 Legacy 4eats use a similar, almost identical proceedure, using the *manu* button.

#5 grossgary

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Posted 10 April 2008 - 12:20 PM

90-94 Legacy 4eats use a similar, almost identical proceedure, using the *manu* button.

so i'm hosed on 1995+ 4EAT codes? dealer only here? this is a 1995 TCU so i'll need those procedures.

#6 Gloyale

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Posted 10 April 2008 - 01:44 PM

so i'm hosed on 1995+ 4EAT codes? dealer only here? this is a 1995 TCU so i'll need those procedures.


IDK? The 95 and later might still have a way of outputting codes via the AT temp light? I just don't know it. Does the 95 have a *Manu* button and a *power* light?

#7 grossgary

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Posted 10 April 2008 - 06:33 PM

i have a 95 TCU in a 96 vehicle, but it doesn't have the manu button.

#8 johnceggleston

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Posted 10 April 2008 - 06:37 PM

gary, i think your idea of the pin maybe on track. the recent thread about how does the TCU / ECU know if it's a manual or auto trans was ther same idea. maybe there is a wiring harness difference. i would think this would be cheaper than a TCU difference... maybe.

keep us posted.

#9 grossgary

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Posted 10 April 2008 - 08:15 PM

the recent thread about how does the TCU / ECU know if it's a manual or auto trans w

yeah that was for the 95 ECU...same part numbers but auto's have EGR and manual does not...so somehow "it knows".

so how does it know FWD to AWD? i need to pull the code and find out what it thinks the problem is. runs and drives fine.

#10 johnceggleston

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Posted 11 April 2008 - 07:55 AM

Duty C wiring and FWD switch...both of which aren't working so it shouldn't "see" those anyway.




since you used a FWD trans i would have thought any necessary 'pin' work would have been covered unless the harness in the car that the trans connects to is different. but then i don't really know how it works. i can't believe they manually adjust the pins for each trans install.

do you have the donor car connector to look at?

did you remove the FWD socket & wiring? have you tried putting in a fuse?

#11 crazyhorse001

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Posted 11 April 2008 - 08:18 AM

did you remove the FWD socket & wiring? have you tried putting in a fuse?


How simple, it may be looking for the duty C that isn't there.

#12 grossgary

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Posted 11 April 2008 - 08:56 AM

do you have the donor car connector to look at?

did you remove the FWD socket & wiring? have you tried putting in a fuse?

i installed a FWD trans into an AWD vehicle. i don't have the donor car to look at.

i did not remove the FWD fuse holder and wiring. i have put a fuse in....doesn't matter either way.

funny thing - the first time i put the fuse in and started it up the trans light (POWER or ATF TEMP, whatever it has) didn't flash. but it did every time after that.

interesting note.....the vehicle is OBDII, but the trans and TCU are OBDI (someone told me all FWD trans are OBDI). does that matter?

#13 johnceggleston

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Posted 11 April 2008 - 10:39 AM

the interchange software @ www.car-part.com says there was a FWD trans in either 97 or 98 for one year, but i've never heard anyone talk about one.

so chances are good that you have obd1 vs. 2, but will this cause a real problem? you really do need to read the code..... did you try it with the AWD TCU? with the fuse in?

maybe the pin you use for the 4wd locked switch is involved.

http://www.ultimates...ead.php?t=50264

#14 johnceggleston

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Posted 11 April 2008 - 10:57 AM

maybe the pin you use for the 4wd locked switch is involved.

http://www.ultimates...ead.php?t=50264[/quote]

i just read through andyjos details and it sounds like pin 11 with a 13 ohm resistor might do it. this is just a wild a$$ guess on my part, i don't really know much about electronics.

#15 grossgary

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Posted 11 April 2008 - 11:02 AM

yes - i drove the car with the AWD TCU and the FWD fuse installed at first. ran and drove fine with the blinking at start up.

i thought a FWD TCU would get rid of the blinking at start up...but it did not (with or without the fuse).

i'll check into it more next week, out of town this weekend. i'll check that pin and thread out, good call JCE! thanks!

#16 Legacy777

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Posted 11 April 2008 - 12:52 PM

I doubt the issue is between AWD or FWD.

Johnceggleston mentioned about OBD1 & OBD2. The OBD2 transmissions had more sensors in them then the OBD1 transmissions. So it's possible the TCU is looking for more inputs then is actually there in the transmission.

What year transmission did you use?

#17 grossgary

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Posted 11 April 2008 - 11:00 PM

What year transmission did you use?

95 FWD transmission
95 TCU

both from a parts place, so i'm only at the discretion of their relaying the proper info to me.

i'm hoping this is an OBDI set up and I can indeed trigger the light to flash me the codes. it seems the FWD stuff should be OBDI from what i've heard from others.

#18 johnceggleston

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Posted 12 April 2008 - 10:13 AM

if you still have the old trans hanging around, i'd compare the connectors on the trnas wiring to see if thay are the same. of course the connectors are the same but do they have the same pins.

some one a long long time ago talked about swapping a trans and having to rework the wiring at the connector, because of the change from one gen to the next. i don't really remember. but i think he had major driving issues, not just a flashing light.

a 95 to 96 swap should work, the 96 to 95 did for me. i'll bet it's something specific to that trans like a bad solenoid, not a generic swap conflict.

#19 grossgary

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Posted 12 April 2008 - 12:31 PM

good call on checking the wiring. i'll try and pull codes and do that as well, i still have that old trans.

i think it's swap related...AWD to FWD, EJ25 to a frankenstein EJ18, no EGR, no charcoal canister.

hopefully the TCU will give me the code when i do the secret handshake.

#20 Gloyale

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Posted 12 April 2008 - 01:08 PM

I doubt the issue is between AWD or FWD.

Johnceggleston mentioned about OBD1 & OBD2. The OBD2 transmissions had more sensors in them then the OBD1 transmissions.


I don't think this is true. All the sensors and solenoids in the trans are all the same until at least 98.

#21 grossgary

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Posted 12 April 2008 - 01:14 PM

All the sensors and solenoids in the trans are all the same until at least 98.

when looking through FSM's before this swap it seems the only difference is the FWD fuse holder and the Duty C solenoid, everything else i believe is the same between them.

#22 Gloyale

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Posted 12 April 2008 - 01:38 PM

when looking through FSM's before this swap it seems the only difference is the FWD fuse holder and the Duty C solenoid, everything else i believe is the same between them.



Yeah, 2wds are obviously different. I was just speaking about the majority AWD trans. Unless someone can show me otherwise, All AWD 4EATs from 88-98, use the same set of sensors and solenoids,(different connectors:rolleyes: ) regardless of OBD I or II. As example, 98 OBDII automatic, does not have any extra or different solenoids as compared to say a 92 OBDI auto.

The only difference is in the TCU, Which in later years does use more input from vehichle sensors. But not Any extra sensors in the trans itself.

OBDII cars may not be able to output trans codes manually, without *select monitor* scanner. Although I really don't know. I haven't had to do trans work on any OBDII cars yet. But I don know the transmissions themselves are the same untill at least the mid 98 redesign (external filter, etc..) Not much was really changed, and the newer ones may still have excactly the same set of sensors and solenoids in the trans.

#23 johnceggleston

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Posted 12 April 2008 - 05:00 PM

Not much was really changed, and the newer ones may still have excactly the same set of sensors and solenoids in the trans.

the phase 2 trans, 99 and up, has at the very least, 2 more speed sensors. i think they are internal. they are the simple one like the one on the rear extention housing. this by it self adds 2 more wires to the wiring harness / connector. i don't know what other changes they may have made in 99.

#24 crazyhorse001

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Posted 12 April 2008 - 05:59 PM

Ok, this thread just became relavent to me. I swapped a '95 OBD1 trans into my SVX & have the flashing AT TEMP light @ startup. Mysteriously, I also have no FWD light with the fuse in. Factor this into your thoughts on this issue.
The car drives flawlessly, still does the torque interrupt during shifts, and generally works as advertised.

For the record:
Trans is a '95 Impreza 4.11 AWD
Car is a '96 SVX

#25 grossgary

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Posted 12 April 2008 - 06:14 PM

oh well, at least a friend joins me in the pursuit of this! if you figure out what the code is, let me know. i plan on trying to read mine when i get back in town.




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