94 Legacy, AWD failure, tranny codes

[erecker]

Hello, I'm the proud owner of a new '94 awd turbo wagon.  The thing is, I believe the AWD is not working at all.  I punch it on snow from a stop, the front wheels spin and spin forever.  It does not feel like my previous subies and I don't feel the rear end at all.  The front wheels will burn all the way thru 1st gear on dry pavement. 

 

I've verified the proper hardware is installed underneath - tail end of tranny, drive shaft, diff, rear axles.

I've verified the FWD fuse is not installed.  I've verified installing the fuse will light the FWD indicator on the dash, and I pulled the fuse back out.

 

The transmission code procedure is giving codes 24 and 32:

Duty C solenoid

Speed sensor 1 (VSS1)

 

Additionally the Power light blinks 11 times when turning on the car.

 

CEL light is off, ABS light is off, and ABS functions properly.

 

 

 

I'm preparing to take the following action and would appreciate input from more experienced members of this community...

 

1) pull the main harness from the tranny to enter 'limp' mode forcing 50/50.  Drive and see if rear wheels provide power now. 

-If so, look for TCU/harness issues and VSS1 issues.

-If still no power, install an oil gauge at the test port on the tranny tail, expecting 0psi indicating solenoid stuck Closed(bypass/fwd).  Replace solenoid, flush tranny, test again.

-If PSI is present, flush tranny.  If still no power, tear down tail inspecting for missing gaskets, torn basket, missing plates, check plate thickness, grooving thickness, oil feed line, etc.

 

 

 

Does this sound sensible?  Is there anythign else I should check before tearing the tranny down, laying in the snow?  Finally anyone in Denver area with a garage I can use? haha

 

 

Thanks in advance
Eric

[johnceggleston]

first double check that you got a VSS 1 trans code.

these are rare, unless it is a wiring / connector issue.

i have never heard of the VSS 1, rear speed sensor, failing,

but maybe.

 

VSS 2 code is much more common,

does your speedometer work correctly?

 

the TCU need both speed sensors to operate the AWD .

if one is signal missing, there is no AWD.

 

the 94 has a speedo cable that runs from the trans front diff to the speedometer.

and the ''speed sensor'' is a part of that in the dash.

i do not know enough about that set up to help more.

but if your speedo is not working that could be the problem.

[erecker]

Thanks for the quick reply and great info John!

 

Yes the speedo and odometer work correctly all the time.

 

Yes the TCU 'power' codes flashed are definitely 24 and 32. 

I can add the following:
I didn't know there was an EXISTING and a PREVIOUS codes registry...

 

EXISTING codes read D-3-2-1:

code 24, Duty C

 

PREVIOUS codes read 1-2-3-D

Code 24, Duty C

Code 32, VSS1,

Code 24, 32, 24, repeats.

 

 

 

I'll visually check the VSS1 now, and its wire, that sounds simple.  Is there an electrical test I can perform here to verify function?

 

If I unplug the large transmission harness to enter 'limp' mode, verify if it 'should' still split 50/50 torque even with this VSS1 is failed?

Edited January 7, 2014 by erecker

[idosubaru]

yes with the harness disconnected the trans should drive in 3rd gear and "locked" 4WD - which would verify the components are working properly mechanically.

 

you sound like you know what you're doing, sounds like yo'ure on the right track.

 

could probably test the Duty C electrically.

[erecker]

Oh fun.  Unplugging the harness to 'limp/3rd gear/4wd' mode worked, locked the system in 4wd.

 

I ohmed out the harness and the duty-c, all's well there.

 

Pin3 the sending wire on the TCU for duty-c showed B+ 14v at all times, and duty-c flash code persisted.

 

I traced the wires out of the TCU, and found the previous owner had spliced the wire to B+.  After removing this splice and restoring the harness to spec, 4wd is locked and voltage is 0v to duty-c.

 

The power transistor inside the TCU that drives that pin3 was totally fried, scorched the circuit board, blew out a 391ohm 1watt resistor, and a diode.  I cleaned up the circuit board and replaced the burnt out components, everything now ohms out good on the circuit board, but still no voltage on pin3.  So the TCU is toast It is the HU model for 3.90 ratio turbo 4eat's, everything else works good as far as shifting so if anyone has a use for this TCU let me know I'll pass it on to you.

 

For now I have installed a switch B+ to Duty-C to allow fwd and 4wd switching, this is inconvenient but I'd rather deal with fwd than with torque bind tearing up my clutch pack and tires.  On the plus side this says the duty-c in the tranny is good, right?

 

 

So this leads me to needing a new TCU with 'HU' on it for a 1994 Subaru Legacy Turbo Wagon AWD, 3.90 ratio (I believe..).

Can anyone advise what part#'s to be looking for for the replacement?  As I understand it I cannot put an NA TCU in due to gear ratio differences...  Does this limit me to 1990-1994 Turbo AWD sedans&wagons for an acceptable donor?

 

Thanks in advance!

Eric

Edited January 12, 2014 by erecker

[presslab]

The turbo trans has more clutch discs and a different brake band servo, so to have it shift right you'll need the same TCU. But you could use an early < 92 Legacy AWD TCU, IIRC t

hese are 3.9 ratio, just the shifting will be off.

 

I could use the TCU for parts, and I'll pay shipping. Send me a PM. Also I will dump the ROM and email it to you.

[nipper]

Before you plug in another TCU, make sure all the wiring is good. Having something shorted out in the TCU makes me wonder about the rest of the harness.

 

If the car doesnt read a VSS (automatic) you wont have AWD. You can have just one VSS working and still have a spedo and normal shifting, but should have a trouble light on the dash illuminated.

[johnceggleston]

 

As I understand it I cannot put an NA TCU in due to gear ratio

differences...  Does this limit me to 1990-1994 Turbo AWD

sedans&wagons for an acceptable donor?

first, the hardware on these is all the same, only the programming is different.

and i have run a 96 lego TCU in a 95 lego with no issues.

i think i even tried the 95 lego TCU in my 97 GT to see what difference that would make, none.

i was hoping to cure a hard 1-2 shift.

 

but none of these are turbos , so maybe that makes a difference.

but it would not cause the engine, trans, or computer to blow up.

it just might not drive the same.

 

anyway, www.car-part.com will have a few.

either look for one close to home or look for one cheap and ship it in.

i doubt they are expensive.

they almost never fail.

 

my guess is the ''switch'' the PO installed may have caused the damage to the TCU.

as nipper said, make sure the wiring is right.

Edited January 12, 2014 by johnceggleston

[erecker]

Update: Bought identical replacement from car-part.com, removed all 'extra' wiring and switch and ohmed out the harness, all good.  Installed new TCU and AWD is now working beautifully!  Shifting still sucks but at least now I know that's not the TCU's fault.  [lazy off the line unless preloaded, feels like too low line pressure at idle.  shift 2-3 actually shifts 2-4-3.  shifts around 6000rpm at full throttle and in 'power'.  already changed fluid, added treatment, changed transfilter]

 

PM to Presslab if he still wants the old TCU, everything works except no output to duty-c ever, so no AWD.  I don't need the rom dump as I got an identical TCU - but I could provide the dump to you and save shipping if that's all you need?

[presslab]

Hey thanks for the offer of the ROM dump, that would be awesome.

 

Lazy off the line is probably just because it's a turbo.  Mine is like that too, and I got my torque converter modified for higher stall that helped some.  Full throttle 6000 RPM shift sounds right, but worth checking the TPS adjustment.  I have adjusted my shift points in the TCU to around 7000 RPM.  But the 2-3 shift sounds like your high clutch is shot, I'm dealing with that right now too.

http://www.ultimatesubaru.org/forum/topic/145520-performance-4eat/