Looking for some constructive criticism here. The tranny in my wife's '91 Postal Legacy (2.2L AWD Auto) had been shifting poorly and finally died. She uses the car as backup to her '97. The remanufactured transmissions seem unavailable, so I installed a salvaged transmission and got torque bind and a 24 transmission code. (The old transmission gave that code after it died as well, but my wife did not describe torque bind in its operation.) I purchased and installed a new Duty C solenoid and valve. Still got the torque bind and code, even with the FWD fuse in. I checked the resistance to ground through the connector at the firewall and got around 14 ohms. I cut the wire there and installed crimp-on connectors on the two cut ends. I ran a wire from a headlight fuse to the transmission end of the cut wire. With the FWD fuse in and the  headlights on I have good FWD and no torque bind. With the headlights off I have AWD and torque bind. I take this to mean that the TCU or the wire from it to the connector is bad. The TCU is fairly inaccessible for diagnosis or removal. (I dislike taking things apart unnecesarily on an old car with cooked brittle plastic and wiring). Replacement TCU's for the RHD 91's look impossible to obtain.  Have I missed any possibilities? Will we be damaging the car by driving it with the FWD fuse in and headlights on, turning the headlights off only if the car needs 4WD? 

Can you post a pic of it? Are you saying basically you just spliced in a headlamp voltage wire to the FWD fuse receptacle?

Edited November 2, 201312 yr by darsdoug

Can you post a pic of it? Are you saying basically you just spliced in a headlamp voltage wire to the FWD fuse receptacle?

I'll take a photo, but no, the wire that I spliced is the wire in the bundle that runs into the transmission and supplies voltage to the C Duty solenoid.

Edited November 2, 201312 yr by lneulicht

Photos

 

Edited November 2, 201312 yr by lneulicht

Hmmm?  The AWD 96 Legacy my wife drives still has torque bind (even with a FWD fuse in) and has been that way for a long time. I might try your idea too.

Hmmm?  The AWD 96 Legacy my wife drives still has torque bind (even with a FWD fuse in) and has been that way for a long time. I might try your idea too.

I only tried my approach after I eliminated a faulty solenoid as the cause. In fact, I tried the work around to confirm that the transfer case and solenoid themselves were not the problem. Have you checked the resistance of yours? You can check it between the appropriate contact on the connector and ground.

Edited November 3, 201312 yr by lneulicht

You probably burned out a transistor inside the TCU.

Seems likely. Are there businesses that diagnose and repair TCUs?

Junkyard TCU would be a better option.

 

Diagnosis would be visual, a component or a trace will be cooked.

 

There is a thread somewhere where someone repaired their TCU.

 

If you can have the car out of service for a bit I would say pull the TCU and open it up and start taking pics for us distant viewers.

 

Or, just wire up the right switches to give front-wheel-drive or locked all-wheel-drive.

A USDM LHD TCU will probably work, as I assume the trans you installed was not from a RHD. What's the letter code on your TCU?

The writing on the TCU is not visible without removal. It is way up under the dash behind the ecu. Searches online last week found TCU's that specified they were for LHD cars, that's why I figured the RHD was different. I don't see why it would be other than that it is physically situated differently than the LHD cars so it might have a different shape or attachments rather than different electronics. I suppose I could ask the stealership to look up part numbers next time I'm buying parts there. (I paid $300+ for the PS hoses last week). My wife only uses this '91r when her '97 is out of service and I think I've got the 97 running well now, for a car with 420,000 miles on it and a freshly installed salvaged engine.  I'm thinking that if nobody suggests problems with my work-around I will just go with it. We don't often need 4WD in the Carolinas. As I say, I dislike messing about with the wiring under the dash on a 22 year old car since I might damage other brittle bits.

applying power to the duty c all the time activates it and puts it in FWD.

 

before you did the re-wiring, did the FWD light come on with the fuse in?

 

did you replace the ATF when you installed the trans?

 

buy a used TCU.

Have 5 tcus from 90-95 4 eat check that the case of trans has a ground wire the tcu powers the soiliniond to hold it open untill tcu gives it signal to turn on awd then it closes

applying power to the duty c all the time activates it and puts it in FWD. Yes, is there any harm in that?

 

before you did the re-wiring, did the FWD light come on with the fuse in? FWD light came on but the torque bind remained. This and the success of the workaround makes me think that the FWD fuse is sending a signal to the TCU rather than directly powering the duty C

 

did you replace the ATF when you installed the trans? Yes, and I changed it again after running it for a while to flush things out well.

 

buy a used TCU. Not sure I can get the proper one for this car, nor that it is worth the risk involved in excavating the current TCU from above numerous obstacles under the dash unless the workaround is somehow damaging to the transmission.

Edited November 5, 201312 yr by lneulicht

Have 5 tcus from 90-95 4 eat check that the case of trans has a ground wire the tcu powers the soiliniond to hold it open untill tcu gives it signal to turn on awd then it closes

That there is good ground is confirmed visually and with an ohmeter.  My understanding is that when the solenoid is powered by the TCU or direct wiring it opens the valve and releases the line pressure that has been activating the transfer case clutch that engages the rear wheels.

I appreciate all of the comments and suggestions

have you taken it apart at all ? most time I can see damage

 

applying power to the duty c all the time activates it and puts it in FWD. Yes, is there any harm in that?

 

before you did the re-wiring, did the FWD light come on with the fuse in? FWD light came on but the torque bind remained. This and the success of the workaround makes me think that the FWD fuse is sending a signal to the TCU rather than directly powering the duty C

 

did you replace the ATF when you installed the trans? Yes, and I changed it again after running it for a while to flush things out well.

 

buy a used TCU. Not sure I can get the proper one for this car, nor that it is worth the risk involved in excavating the current TCU from above numerous obstacles under the dash unless the workaround is somehow damaging to the transmission.

 

 

Yes, the FWD fuse just tells the TCU to disable the AWD, and it powers the dash light.  It does not directly power the Duty C solenoid.

 

Hmm, removing the TCU is not so hard, especially compared to the trans!  The wiring won't be brittle as you say, it's not hot like in the engine compartment.  Why do you think the TCU is RHD specific when the trans is not?  You have a lot of excuses for not wanting to touch the TCU. 

 

Your bodge wire won't damage anything more.  Running in FWD temporarily is fine, but neutered permanantly is a waste as you have all the AWD stuff with more drag and higher weight for no reason.  Unless you're wheeling, might as well buy a Honda.

Yes, the FWD fuse just tells the TCU to disable the AWD, and it powers the dash light.  It does not directly power the Duty C solenoid.

 

Hmm, removing the TCU is not so hard, especially compared to the trans! I'll take a picture of the TCU location on this beast  when my back  recovers from the transmission  replacement and later transfer case work. I  am admittedly adverse to lying on my back across the seat with my head hanging under the dash and digging into its innards. 

 

The wiring won't be brittle as you say, it's not hot like in the engine compartment.   This car has been sitting in the North Carolina sun for  22 years now. Plastic parts that snap together now crumble when pulled apart. The wiring itself is not as brittle as the various brackets and mounting I'd have to go through.   Why do you think the TCU is RHD specific when the trans is not?   http://opposedforces.com/parts/info/31711AD590/   $600-400 new. Google doesn't find a used one. This is actually the part for our '97 but I'm thinking the '91 will go a similar way. You have a lot of excuses for not wanting to touch the TCU.  You're right!

Your bodge wire won't damage anything more. Great!  Running in FWD temporarily is fine, but neutered permanantly is a waste as you have all the AWD stuff with more drag and higher weight for no reason.  Unless you're wheeling, might as well buy a Honda. Know where we can buy a Honda with the driver's seat on the right side? She'd wear a disguise and use it as a back-up car on the mail route for the five years she has until retirement.

I do appreciate your posts.

Edited November 7, 201312 yr by lneulicht

Haha well I guess it'd be hard to drive the Honda in reverse all the time...  Although I do remember our mail person drove a Buick or something and just sat sideways across the bench seat.

 

I wouldn't be surprised if the TCU is a different part number, due to the mounting being different.  There's a metal bracket tack welded to the back of the TCU case, I've seen them in different shapes.  It's real easy to swap the circuit board from one case to another, though.

 

Of course who knows, maybe the TCU is all different, I've never seen a RHD Legacy.  But knowing Subaru I doubt they'd do something like this.

 

It's real easy to swap the circuit board from one case to another, though.

 

been there done that.

 

programming aside,

the circuit boards do not change much from one year to the next.

and i would guess not much at all in the 90s.

 

i swapped a 96 TCU, in to a 95 lego L,

even though the outer case was different, and had a different part number.

the circuit board inside was a perfect match for the one i was replacing.

same design, same connectors, same components.

 

and since all the auto trans 90 - 98 were the same electronically,

it makes sense that all the TCU were the same,

unless the GT / outback had different shift points programmed.

Edited November 7, 201312 yr by johnceggleston

Something to watch out for is if the replacement TCU is for a different R&P ratio. This can throw a code, or just cause funky AWD.

 

To a lesser extent the speedometer drive gear can cause this problem too, but I think this is less of a problem because it's normally related to the R&P.

 

Anyway, just get a TCU for the car the replacement trans came from.

Since I am a new poster here I don't want to give the impression that I am lazier and more obstinate than I am. Here is a photo with the knee bolster removed. The Ecu is visible to the right.

Here is what I take to be the TCU back behind and above the ECU. The top of the photo is toward the firewall.

 

Here, on a piece of paper, is the plastic that broke off when I unscrewed the knee bolster this time

Edited November 7, 201312 yr by lneulicht

Yeah, looks like the TCU right in the middle of the 2nd pic. At least the plugs are right there.

 

I've fixed numerous ECUs by replacing components, usually the damaged part is burnt or has a crater. Sometimes it's less obvious, but most multimeters can test transistors using the "diode check" function. Well, assuming you are adventurous and want to try fixing it yourself.

You folks have about convinced me it would be wise to have a salvaged one on hand to transplant next time I get bored and want to gamble at turning a working 2WD car into a non-working AWD. I sent a PM to Ivan about the ones he mentioned having.