91 Legacy LSI 4EAT question

[SUBARU3]

  (1991 Legacy 2WD 2.2 106K )

      OK.... so today I went to drive the car and I only have 3rd gear. (Reverse too). (Car is not a daily driver, but does get driven a few times a month).

   Anyway, no matter what I do, it's 3rd gear ONLY. Starts form stop in 3rd and never changes with speed. Car was fine last time I drove it. Any ideas? Limp only mode?  Fuse is OK inside car and otherwise, nothing has changed, (no maintenance, or other work done on car) Low mileage 106K

Weird...Did the trans just blow??

 

  Thanks in advance!

 

 Todd

 

[idosubaru]

That’s how the trans operates without any TCU input.  Or, yes, it could be a hosed trans.

Does the AT light blink 16 times at first start up?

Any check engine lights?

Id read the TCU codes. If there are none, I’d heck the trans connectors for power or rodent damage. 

[SUBARU3]

Nothing blinking and no rodent damage. Car was just driven without issue a week before. Sitting on concrete driveway in suburban neighborhood. Just weird. Is there some other fuse for TCU someplace?

[Rampage]

From the 4EAT manual.

If shift solenoids #1 or #2 malfunction, the TCU deactivates the other.

This results in either 3rd gear or Reverse (when selected).

Edited August 6, 2020 by Rampage

[idosubaru]

10 hours ago, SUBARU3 said:

Nothing blinking and no rodent damage. Car was just driven without issue a week before. Sitting on concrete driveway in suburban neighborhood. Just weird. Is there some other fuse for TCU someplace?

Sounds like the TCU codes need to be pulled.  I haven't looked in a long time whether these have a "memory" mode or diagnostic mode or not but I would wonder if you can learn something even if it's not tossing an active code, from a memory functionality or pending code like the ECU.

No there's no fuse.  Sorry if you already know this, I don't know if you do or not. Are you sure the TCU light isn't blinking - it only does it immediately at start up, really fast and is easy to miss.  It's not like the check engine light which stays on all the time.  If you don't look right away at first start up, you won't see it. 

It seems unlikely to go into fail safe mode without a code but stranger things have happened. 

Have you tried turning it off and restarting it multiple times - does it do it every single time?

I assume the fluid level is fine? 

Check the trans pan and make sure it's not dented at all, even a tiny bit.  There's almost no clearance between the pan and solenoids, wiring, and sump.

Rodent damage is unlikely but I've seen it more than once.  Chipmunks, even in squeaky clean residential neighborhoods, can crawl into areas and gnaw on wiring that's unseen. I've seen wires chewed under carpeting that seemed impossible to access.  But this is, of course, unlikely. 

[john in KY]

Check the linkage under the car. Have someone move the shifter and verify that whatever thing the rod connects to moves through its range.  That whatever fan-shaped thing may have failed and check its wiring harness.  

 

I have a FWD 4EAT from a 94 model with 84K miles that has been sitting in a shed the last 20 or so years.  PM me if you should become interested in it.

[SUBARU3]

 

 I appreciate all the feedback! Working on this. I will keep you posted!

[SUBARU3]

 I don't see a  "TCU" light on this dash. Nothing in manual either. Am I missing something?

[SUBARU3]

  Ah... I see it's the "power" light that is used for codes.

 

[idosubaru]

Yes the power light that sounds right for 80s and early 90s. Subaru has given that transmission light a few names over the years - Power, AT, AT Temp or Temp.  If there’s a code, itll be the only thing flashing 16 times at start up.

If you start the car while looking at the instrument cluster, it’s the only light that will blink 16 times (or really at all - so 16 is quite obvious - if there’s a noted code).  

it is also the light that will flash, and you will count, if you try to retrieve any codes.

good to check to make sure the light is working, in which case you do need to know the name. 

Edited August 7, 2020 by idosubaru

[SUBARU3]

 OK...so upon further assessment, I noticed that after starting the car, the Power, Manual and the center drive selection indicator lights, (P R N D 2 1 were ALL out).  All cables intact and no obvious issues. I shut car off, disconnected the battery for like 4 hours. Upon reconnecting, restarting and putting in gear...all is back to normal! Car shifts OK and all the dash light and indicators are illuminating again. But if it was a code, the "power" light would be out, so how would a code be pulled? So it appears that the TCU just needed a reset for some strange reason and being placed limp mode, I lost all dash transmission related lights. Just weird. Guess I will see what tomorrow brings!  I can't believe this trans is bad, it's been so babied and only gas 106K on it! But it shifts fine now...so it can't be bad. An anomaly of some sort maybe...... I pray!

[Rampage]

Wow, I didn't expect a reset would fix it. Cool.

It would seem there is an intermittently poor connection between the transmission and TCU, since it went into "limp home mode". Or the TCU is beginning to fail. Could also be a wire.

There are two fuses for the TCU, 14 and 16. Pull flip and reinsert them to improve their connection. Also remove and replace the connector on the TCU (behind the glove box on ours) and the 16 pin connector located above the bellhousing (TCU to transmission). May as well do the 12 pin connector too (inhibitor sw. and select lever). I would look for any corrosion or discoloration on the contacts of those connectors.

If you want to spend some money, there is a product I use on RAM modules and cards in computers and sometimes on the car made by SMP - Standard SL5. It is a liquid for improving electrical connections on DC, AC and High Frequency connectors. It works, but is not cheap. Don't matter if you are sloppy with it, it will not conduct between adjacent contacts.

Comment from a mech - It is perfect for resistance issues on SRS systems. Putting this product on the pins for the airbag connections under the seat of a jeep cleared the airbag light.

[idosubaru]

Awesome glad that helped. Now you know to try that if it happens again. Which it probably will.

Did it happen more than once or did you drive one time like this?

17 hours ago, SUBARU3 said:

 But if it was a code, the "power" light would be out

Im not sure what your question there is - if there’s an active TCU code, the transmission related light will flash.  

[SUBARU3]

  OK....Car drove last night and shifted fine, after reset via battery disconnect. Now tonight...same thing again! All dash light related to the transmission are out, nothing flashing, no power or manual and no center gear position indicator lights in the cluster. Car drives in 3rd only. Once again, reset with battery disconnect and all is fine. Lights reappear on dash and transmission shifts and drives perfectly.

  What I said earlier, is that when this happens , I lose all dash lights related to the transmission. There would be no way to pull a code, (if present), as all the lights are out.

 

 This seems to be related to a total power disconnect of the TCU, but how come if I D/C the battery, all of a sudden it powers on?

   I will be replacing the fuses and jiggling the connectors later today. Maybe it's just that? It is not the trans is would appear!

[idosubaru]

Okay, I understand what you mean now.

Test the alternator output, preferably immediately after the transmission issues happen again. There’s a chance the alt is overcharging (and maybe under charging) and I would want to rule out overcharging right away. Then test battery condition and connections.

If it was only a poor connection it would most likely cut out while driving, not just at start up.  So something else is the culprit or there’s an additional constraint on the power issue (like a TCU start up routine). I’d check the alt output first.  

[Rampage]

Fuse 16 gets its power from the IGN Switch ON and START positions. Maybe, the Start position of the switch does not have a good connection to supply the voltage needed for the TCU.

The IGN Switch gets it power from SBF 4. (Slow Blow Fuse 45 amp)

Fuse 14 gets its power from SBF 5. (Slow Blow Fuse 45 amp)

The wiring diagram shows lights you refer to are controlled by the TCU. So it would seem the TCU is not getting power or it is shutting down.

[SUBARU3]

 Played with everything tonight. Unplugged the TCU and replugged, disconnected the battery and replugged, played with fuses and pushed back in. Now I get the lights on the dash, BUT the "power" light will not work. Manual lights and the gear selector lights DO work. Now stuck in 3rd only, no matter what. Last night trans shifted fine. I'm guessing the TCU is failing. It has to be getting power for the dash lights to light...correct? I guess I will replace TCU....It's part # 31711AA921 if anyone has one.

 

Todd

[SUBARU3]

 Found TCU on eBay...worth a shot!

[GeneralDisorder]

Yeah the TCU is probably on it's way out. Could be solder joint problems. You can try baking the TCU circuit board in the oven to reflow the solder. And if there's any bulging caps, replace them. But that may only be a temp solution. Unfortunately this is what happens with 30 year old electronics. The ECU in my 90 Legacy loaner had about 6 codes that would never go away - put a LINK ECU on it ($2,000 in standalone hardware.... but it's an R&D car for EJ swaps into Vanagons....) problem solved. The original ECU in my 86 Trans Am died from a solder joint or circuit board crack issue. Seen plenty of newer Legacy models (05 to 09) have dome light passenger airbag indicators fail from solder joint cracks as well as the entire HVAC/Stereo units in those cars. 

Aging electronics will be the death of most cars that have no support for new replacement modules and can't otherwise be repaired. The 4EAT.... unfortunately there's no standalone controllers so once all the original TCU's die or aren't obtainable, etc..... basically it's a manual swap or scrap it. Unless someone wants to build a controller with an Arduino....

GD

Edited August 9, 2020 by GeneralDisorder

[idosubaru]

I had a 89 TCU fail a year or two ago. 

Used ones are everywhere for pocket change. They’re so worthless I wouldn’t offer to sell the extra legacy TCU I have, it’s not worth my time for as little as they’re worth. They fail so rarely that right now they’re easily obtainable. Www.car-part.com 

[Rampage]

GD nailed it. Old electronics will have problems.

Electrolytic Capacitors (the ones that look like an aluminum can) will have a longer life span when their circuit is working. If they are left sitting for years (even new ones on the shelf) the electrolyte can dry out on some and they become useless and the circuit they are used in will not work like it should. I have bought a few of them. They puff up for two reasons. One least common reason is they develop a short internally between the two sheets of foil and get hot or heated by an external source. I have seen the electrolyte leak out onto the board and eat the copper traces. The other reason is, voltage above their rated voltage. If AC voltage higher than their rated voltage is applied to them or if the DC voltage is reversed, they will explode.

I have seen solder joints that look perfect, but the solder is just laying on the copper trace not connected to it. Using a pick it will move. Also, broken joints that you can see a ring around the pin.

[SUBARU3]

 Guess I better stock up on these TCUs!!! I will let you know how things go when I swap this one out!

    Any other years that interchange with 91?  It's looking like they are all different. (2WD 4WD  and changed after 1995)   I have 2 1995 autos too. Hope their controllers last! All low mileage cars, but they are getting old too!

 

  Thanks guys!

[idosubaru]

6 hours ago, SUBARU3 said:

 Guess I better stock up on these TCUs!!! I will let you know how things go when I swap this one out!

    Any other years that interchange with 91?  It's looking like they are all different. (2WD 4WD  and changed after 1995)   I have 2 1995 autos too. Hope their controllers last! All low mileage cars, but they are getting old too!

 

  Thanks guys!

I’ve swapped 95-99 legacy FWD and AWD TCUs and noticed no difference. Wouldn’t surprise me if 91 is the same  

TCU failure is not common. There’s 100 others things more likely to fail before a second TCU failure in three Subaru’s. 

[Rampage]

There is one quick voltage check I think you should make.

Unplug the ign. coil. Hook your volt meter to a good ground and fuse # 16. Turn the key to on. What is the voltage? Turn the key to start. What is the voltage?

Reconnect the ign. coil.

[SUBARU3]

Looks like 12 volts. 

Todd