blackroadster Posted September 14, 2020 Share Posted September 14, 2020 Hello Subaru people. I recently bought a 1990 Legacy SW from a friend. Its a FWD Automatic. All maintenance was up to date and had been a reliable car but with the associated 30 year old car problems. Various worn out switches, cosmetic wear and tear and such. The car doesn't leak a drop of any fluid except water from A/C condensation and strangely a lot of water. Without any warning the transmission would only go into 3rd gear (4EAT). Checked the transmission fluid, which is very hard to read and it appeared low but clean. No burning of the ATF and it was a nice red color. I warmed up the trans, shifted it into several gears and rechecked. I added more ATF until it appeared ok. I drove it home and about 10 minutes from home, it got stuck in 1st gear. I limped it home and parked it for about two weeks. I drove it again today and it appears stuck in 3rd still. Park, Neutral and Reverse still works but selecting any other gear does nothing. I tried to check TCU codes but the power light never comes on. I'm thinking a failed TCU may be the culprit. Where is this box located? I have also considered doing a 5MT swap but it is more involved. Since this is just a spare vehicle and my hauler, I want to do the minimum to get it back to being the reliable vehicle it was. Subaru Gurus do your thing. Link to comment Share on other sites More sharing options...
Rampage Posted September 14, 2020 Share Posted September 14, 2020 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). Could be the TCU, wires, connectors or the solenoids themselves. The solenoids operate valves. Go here and download some manuals. http://jdmfsm.info/Auto/Japan/Subaru/ Click on Articles and then Transmission. (click Parent directory to go back) There are several manuals for the 4EAT. I downloaded all of them. You can also get the FSM. I forget which side, but the TCU is under the dash either behind the glove box or steering wheel. Our 95 has TCU on the label. Unplug and reconnect the plug and do the same for the two big connectors on top of the bellhousing. One 12pin is for the inhibitor switch and the other 16pin goes into the tranny. Look for corrosion on the connectors. The manuals will tell you what pins on the 16pin connector to check ohms for the shift solenoids. Link to comment Share on other sites More sharing options...
idosubaru Posted September 14, 2020 Share Posted September 14, 2020 (edited) is anything else not working? No check engine light? does the power light flash 16 times at first start up or come on at all? Check the trans pan and make sure it’s not dented even a little. There’s microscopic clearance between the pan and intent pick up. If this car sat and you’re poking around the car look for areas of custom wiring or rodent damage. 3rd gear only . It’s in a fail safe mode or it’s not being run by the TCU at all. If it’s disconnected (via the plug in the engine bay rear pass side) or the TCU it’ll drive like you described. id want to determine which it is. It sounds like the TCU was doing something at one point and it’s unlikely for something to be disconnected. Check to see if the TCU is getting power. And yes they can fail. Try another used one. GD says the solder joints fail with age and can be reflowed by placing them in the oven. Sounds crazy, I’ve never done it, but there you go! Edited September 14, 2020 by idosubaru Link to comment Share on other sites More sharing options...
blackroadster Posted September 20, 2020 Author Share Posted September 20, 2020 (edited) So... I left the car's battery unplugged for several days and unplugged the harness connection on the top of the transmission. All the contacts were shiny and clean. As I was connecting the battery I noticed a ground wire I had left unattached when I replaced the battery harness a day prior to my problem. Had I just reset the TCU or do you think the unconnected ground wire was the culprit? BTW, the Power light works fine now. I suppose I should check the codes but the transmission is currently working as normal now. Edited September 20, 2020 by blackroadster 1 Link to comment Share on other sites More sharing options...
blackroadster Posted September 20, 2020 Author Share Posted September 20, 2020 Whelp... It's Baaaaack. It was working completely normal when I went and filled up with gas around the corner. I filled the car up and I decided to try and diagnose any TCU codes. I used this method. After performing the steps twice I noticed power light did not illuminate. Car is stuck in 1st gear. I limped the car home in 1st gear. At this point, I'm at a loss. How can it work just fine and then go to complete crap again? No chewed wires that I can see. I'm still at a loss as to where the TCU is located. Under the dash, near the steering column is a gold colored box. I would have thought its the ECU. I don't see anything under the passenger side dash that looks like a computer. The only thing I've found is the small photo but it doesn't really show me where it's located. Looks like maybe the back seat? Almost looks like a power window motor in the lower left corner of the photo. Link to comment Share on other sites More sharing options...
Rampage Posted September 20, 2020 Share Posted September 20, 2020 (edited) The Power Light is controlled by the TCU. If it worked earlier but is not working now, then I would suspect the TCU or a power wire to the TCU. There are 3 power wires to the TCU. I do not suspect a fuse because the 2 fuses that power it also power the ECU and Fuel Pump Relay and they appear to be working. The TCU Sockets jpg shows the location of the 3 power connections. Looking at the wire side of the plugs they are numbered from right to left, top row then bottom row. Pin 1 and Pin 6 are Yellow wires. Pin 14 is Yellow/red wire. They should show 12v with the key on. The FSM does not give the location of the TCU. It is under the dash on our 93, 95 and 97 RHD wagons. I'm attaching a PDF page from the 1990 Parts Manual that show 9 different part numbers for the TCU used in the 2WD station wagon. They are marked with a "1". It is probably one of the first three numbers. I did not look up the remarks. That part number "31711 xxxxx" will be the top row of numbers on the label on the TCU. You can open and print the PDF file and print the JPG too. TCU part numbers.pdf Edited September 20, 2020 by Rampage Link to comment Share on other sites More sharing options...
heartless Posted September 20, 2020 Share Posted September 20, 2020 check under the drivers seat... IIRC that is where the TCU was on the 90 LS that I had.. possibly under the carpet... it has been a long time tho, so I could be wrong Link to comment Share on other sites More sharing options...
Rampage Posted September 20, 2020 Share Posted September 20, 2020 Found this on a search. https://www.ultimatesubaru.org/forum/topic/21011-help-where-is-transmission-control-unit-tcu-located-on-a-1990-legacywagon/ Link to comment Share on other sites More sharing options...
blackroadster Posted September 21, 2020 Author Share Posted September 21, 2020 13 hours ago, Rampage said: Found this on a search. https://www.ultimatesubaru.org/forum/topic/21011-help-where-is-transmission-control-unit-tcu-located-on-a-1990-legacywagon/ Thank you. Got the TCU out. The ECU looks buried! Link to comment Share on other sites More sharing options...
blackroadster Posted September 25, 2020 Author Share Posted September 25, 2020 So I bought a used TCU on eBay and replaced my TCU. I get a POWER light on start up. I test drove the car and transmission is shifting through all the gears. The shifts are a little harsher than I remember but maybe the new TCU is learning? Anyways, I warmed up the car and ran through the test to check for TCU codes. Ignition off / Ignition on / light on / drive above 12 mph / Ignition off / Ignition on / Shift to 1st / Ignition off / Ignition on / Shift to 2nd / Shift to 1st / Shift to 2nd / Shift to 3rd / Shift to Drive No codes and no POWER light. Started car and got 16 quick flashes of POWER light. I tried pulling the codes again with the same results. Transmission still working in all gears after about 3-4 miles. I'm nervous to drive it far. The 16 flashes has me concerned but no TCU codes. Link to comment Share on other sites More sharing options...
Rampage Posted September 25, 2020 Share Posted September 25, 2020 The 16 flashes means that a problem was detected on the LAST power on cycle of the IGN Key, which would have been you checking for codes. I don't know if that is normal or not. I read that you can remove fuse # 14 for 1 minute and it will reset the TCU. Try that and see what it does on startup. Link to comment Share on other sites More sharing options...
idosubaru Posted September 25, 2020 Share Posted September 25, 2020 13 hours ago, blackroadster said: Anyways, I warmed up the car and ran through the test to check for TCU codes. Ignition off / Ignition on / light on / drive above 12 mph / Ignition off / Ignition on / Shift to 1st / Ignition off / Ignition on / Shift to 2nd / Shift to 1st / Shift to 2nd / Shift to 3rd / Shift to Drive No codes and no POWER light. Started car and got 16 quick flashes of POWER light. I tried pulling the codes again with the same results. Transmission still working in all gears after about 3-4 miles. I'm nervous to drive it far. The 16 flashes has me concerned but no TCU codes. There are TCU codes - that's what the 16 POWER light flashes at start up indicate. Getting it to flash the code(s) is a work of art. It's usually a shock if you get it to work right, not the other way around. Double check your procedure and make sure you're not missing a step or there's not multiple ways to interpret a given step in which case you try the other. Some of the steps if you're being very meticulous to details, do have some ambiguity in them. Link to comment Share on other sites More sharing options...
blackroadster Posted September 27, 2020 Author Share Posted September 27, 2020 On 9/25/2020 at 10:03 AM, idosubaru said: There are TCU codes - that's what the 16 POWER light flashes at start up indicate. Getting it to flash the code(s) is a work of art. It's usually a shock if you get it to work right, not the other way around. Double check your procedure and make sure you're not missing a step or there's not multiple ways to interpret a given step in which case you try the other. Some of the steps if you're being very meticulous to details, do have some ambiguity in them. I did list out the steps as I understood them in my post. Can anyone clarify the process? Link to comment Share on other sites More sharing options...
Rampage Posted September 27, 2020 Share Posted September 27, 2020 The steps you took are correct. I just looked at the chart. Maybe one of the switches in the selector switch (inhibitor switch) is not making contact so the process does not complete. Or if the switches are good, the TCU is not responding. Think about it, how often does 1,2.3 get used. Move the shifter through the range a bunch of times and then try the process again. Here is a link to the chart. http://jdmfsm.info/Auto/Japan/Subaru/--Articles--/--Transmission--/Self diagnosis.pdf A Subaru dealer should have the Select Monitor to check it, but I have no idea what they would charge. Probably too much. Link to comment Share on other sites More sharing options...
idosubaru Posted September 27, 2020 Share Posted September 27, 2020 5 hours ago, blackroadster said: I did list out the steps as I understood them in my post. Can anyone clarify the process? It may be right, it looks like it is. I don’t know why but it’s easy to go through those steps and not get it to show codes. It may be timing - need to do each step in 1 second - and you do one in 1.2 seconds or one step doesn’t register because it was too quickly done, or maybe it’s due to age, TCUs fail in that way, I have no idea. It just happens all the time on these older 4EATs. It’s so much the case that I’d be surprised if someone did the steps and got the code the first time. It is by far the least consistent code acquiring Subaru system out of all years and ECU TCU ABS SRS VDC, all of it, hands down. Those older TCUs are nothing like all those other systems which consistently give up their codes. The TCU is for the correct vehicle right? I doubt it matters because you’d have encountered something if it was wrong but did you run the parts number on the TCU through a Subaru parts site to verify years/models? Link to comment Share on other sites More sharing options...
idosubaru Posted September 27, 2020 Share Posted September 27, 2020 Too review the current knowns: 1. Two TCUs both flash the Power light. 2. The current TCU drives without issue so far (although you haven’t driven it much?). 3. The first TCU went into limp mode sometimes and had drivability issues. Some unlikely guesses: Make sure to read codes on the cycle immediately after symptoms. Maybe you’ve only tried reading codes when the intermittent codes aren’t set? They were set and flashing and then when you went to read them they had cleared? Doubtful. Both TCUs are problematic. This is like the 3rd 30 year old Subaru TCU issue I’ve seen this month. Internal trans issues. Link to comment Share on other sites More sharing options...
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