SubaruFred

When I replaced the duty C I looked at the clutch assembly and it showed some grooving and wear but I didn't have the time, parts or desire to do anything about it. It still locks up solidly when engaged and slips freely when not. I suppose it's possible that momentarily sticking clutches could be causing the TCU to determine that the duty C is bad but the bigger problem IMO is that once the TCU makes thie determination it will not send power to the duty c until the ign is turned off and back on. This makes a moment of TB turn into full blown TB for the rest of the trip or until the ign is cycled off and on. I tired very quickly of cycling the ign while driving or pulling over frequently to cycle the ign. It was especially bad at night since the headlights go out with the ign off. Hence the duty C switch. I know that the correct way to fix this would be to rebuild the MPT clutch assembly and possibly the TCU if the rebuilt MPT didn't fix the issue but the car has 179k on it, rusted rear 1/4 panels and I don't plan on keeping it forever. It's true that running in forced FWD mode probably changes the handling characteristics a little but I think it's less than the 205/75/14 A/T tires and 3" suspension lift have done. So while this manual duty C switch may not be the correct way to fix TB, it is cheap, easy and works great. If the clutches ever do stick in the basket grooves, I can't tell. To me it feels like any other FWD car until I switch into AWD mode. My only concern is possibly shortening the life of the duty C but if it fails, I'll either replace it or just pull the drive shaft.

Could that transistor work fine sometimes but fail intermittently? Sometimes everything worked fine, no TB at all and no TCU codes and then randomly (more often in cold weather) I'd suddenly get full TB and a bad duty C code. Resetting the TCU with an ign off/on cycle always fixed the TB for seconds, minutes or sometimes hours. I had thought that the root cause was the TCU incorrectly reading a bad duty C but maybe it was the TCU itself causing the duty C to fail and then compounding the problem by not even trying to send voltage to it. IMO the default mode should have been 90/10 rather than 50/50.

Just an update to close the thread out. Although my TB symptoms and TCU codes all pointed to a bad duty C, the duty C was fine. Presumably the TCU is bad, specifically the part that controls the duty C. I cut the duty C control wire (+V) to the TCU and wired it to a 2-pole LED switch I installed in the dash. For +12V power I tapped into the main ign-on switched feed wire on the steering column. Now when the switch is on the duty C receives +12V and remains in FWD mode without the slight trace of TB. Flip the switch off and I have 50/50 torque split for snow and off-road use. It's been a few weeks and a few thousand miles now and I'm very happy with this solution. Thanks to whoever came up with this idea originally, to those who suggested it to me and those who provided me with the TCU pinout, etc. It works like a charm.

Thanks again. It's always good to have options.

No, I didn't look there. Thanks for the link! How can I tell which TCUs will work with my car? Will any 92-94 AWD Legacy TCU work?

John, you make some very valid points but I already had the switch, wire and connectors in hand so this mod only took under 2 hours total. I've already spent over 2 hours searching for a replacement TCU and still haven't found one. Do you know where I can find a known-good TCU for a 1993 Legacy AWD non-turbo? Maybe I can leave the switch in for 10 years? We don't really know how long the duty C will last at 100% duty cycle. My original lasted 175,000 miles and 18 years running at 90% duty, 95% of the time and was still good when I replaced it. If I can find a TCU for a good price I may try it but frankly this car will probably be sold or scrapped when the tires wear out in ~40,000 miles. It's rusted, the paint sucks, it's no classic and has very little re-sale value. Thanks again for what I consider to be very good advice, even if I don't take it. That's on me and I may or may not regret it. Only time will tell. P.S., I have a 12V PWM fan speed controller laying around. What if I wired that into the duty C circuit? Instead of a +12-14V switch I could manually control the duty C the same way the TCU does. I may try this. ~1 hour and free.

Thanks guys. I agree that replacing the TCU would be the ideal solution, if that's all it takes. The thing that worries me is that my TCU does control the duty C correctly much of the time. It's only when the TCU decides the duty C has failed and stops sending it any current that my TB occurs and remains until I re-cycle the ign switch (and TCU). I also agree that theoretically it's not a great idea to run a intermittent duty solenoid at 100%. However I was looking more for real world success and failure data like below: Thanks again grossgary. 2 years is a long time and if my duty C lasts that long wired this way I'll be satisfied.

the FWD fuse is left in or the TCU is bypassed and the duty C is controlled by a manual switch? FWIW, I tried flushing the ATF, Trans-X and a new duty C but the only thing that fixed my TB was bypassing the TCU and hard-wiring the duty C to a switched +12V power source. The problem was that the TCU kept determining that the duty C was bad and defaulting to 50/50 torque split. This ensured that TB was always present since the TCU wouldn't send any voltage to the duty C. Sending +12V to the duty C via a switch has eliminated all TB and allows 50/50 AWD on-demand at the flick of a switch.

IIRC with the gear selector in D I measured ~4.5V, in 1st gear it was ~2.5V and with the throttle wide open I measured less than .05V. I forget the measured resistance but it was within spec too. I'm just speculating that since it presumably takes 8-14V to ensure a 90/10 split and less than .05V to ensure 50/50 that perhaps 4.5V is not enough to hold the solenoid closed for 90/10. I'm not very familiar with PWM circuitry but I'm speculating that something may have failed, causing the TCU to duty C output voltage signal to drop. Then again it could be something else entirely. All I know for certain at this point is that bypassing the TCU appears to have fixed all of my TB issues.

Update: 2 days and not the slightest hint of any torque bind, cold or hot. If I flip the switch to stop sending voltage to the duty C I get immediate and severe torque bind. Enough to cause the rear tires to chirp as they slip while turning and very noticeable. Worn or not the transfer clutches really lock up solid. I'm very inclined to blame the TCU for all of my TB problems. The TCU reported a failed duty C but when I replaced it, nothing changed. Byp*********ing the faulty TCU has made all the difference. I also noted that with the fwd fuse installed the duty C was getting 10V+ but without the fuse it only got 2.5-4.5V. I suspect that a problem within the TCU had reduced the voltage going to the duty C to a level insufficient to acheive a 90/10 split. Thanks again Grossgary and others!

Not a problem, no one else drives the car and it will be left in 90/10 mode 95% of the time. The switch has a very visible LED to remind the driver that the proper fwd mode is selected. I probably didn't explain my TB symptoms accurately enough or with sufficient detail. In 70F+ weather TB occurs only randomly and can always be cured by cycling the ign off and on again to reset the TCU. In <50F weather TB occurs constantly until the ign is cycled off and on after the trans has warmed up. If the TCU is not reset the TB is just as severe when the trans is hot as it is when cold. It always takes a TCU reset to fix it. It doesn't lessen at all as the ATF gets warmer. 50/50 AWD works the same, hot or cold. Obviously something is causing the TCU to "fail" the duty C and it occurs more often in cold weather than hot but I can't say exactly what that is. It could well be a worn transfer clutch *********embly combined with higher ATF pressures when cold but the consistent "fix" is to simply reset the TCU. Rather than continue to turn my engine off and on again to reset the TCU, I felt that establishing manual control over the duty C would be preferable. I could have installed a switch on the TCU power circuit to reset it without turning the engine off but I wanted to try this method instead. Besides, I felt that the TCU did a lousy job of managing F/R torque split anyway. This way I have FWD normally with AWD on-demand, on-the-fly.

Thanks! It worked like a charm!