My high mileage '96 Legacy (2.5/Auto) is starting to buck after about 20-30 minutes on the highway at speed. Almost feels like an ignition problem, e.g. bad wires, which I've had before and fixed with new wires, but this time it seems different in that it only happens in 4th gear, at speed. I have received a CEL of "740", which if was: "torque converter clutch circuit malfunction", (or STTE...), which I assume is related to the solenoid circuit which controls the lock-up feature. What I need to know is: is this error code a direct fault indication, i.e. a circuit is actually sensing an electrical characteristic not in spec, which would logically be either too high a resistance (open coil, bad connection, etc.) or too low current (once again, open coil, bad connection), or is it an indirect fault indication, such as a mismatch in engine RPM and road speed from the ideal? If it is the former, an indication of an electrical fault, I can concentrate on checking the connections and/or changing the solenoid. Which begs, how does one go about changing the solenoid? With some cars, the lock out solenoid can be changed without removing the trans, via an access panel, but I've been unable to discover any information on how this process works with the Subaru 4EAT. Any help or insight will be greatly appreciated, and "thank you" in advance.

Search under my user name and "lock-up mod," you should find a post where I talk about the TC lock up mod and which particular CEL it threw.

I am mentioning that thread b/c I cannot remember if it were the same CEL as you have. That CEL was thrown when the TCU to Lockup Solenoid (LS) connection was severed (intentionally) and 9 volts was sent to the LS. The AT Temp lamp flashed as well. So... if you have the same CEL and error behavior, check for a short.

Sorry I cannot be more helpful.

Thanks so much for your feedback. My CEL is 740, whereas yours appears to have been 743. They appear to be related, i.e. they both have to do with failures in the TC Lock-Up circuit, but I'd love to know how they differ, i.e. what is different about how they are triggered and what they signify. If code 743 is triggered when there is a break in continuity of the circuit, as you created with your mod, what could be different about code 740? I'm hoping that someone with intimate knowledge of the Subie transmission and error code can help explain this difference (and tell me how to change the solenoid, if need be...).

You know, this probably wont help at all, but if you have 10 minutes and some wheel ramps, why don't you check the condition (e.g., clean them if they are dirty) of your grounds on the tranny case.

Thanks, I'll check out the grounds, but I doubt that's the source of the problem. Oddly, the symptoms seems to be getting better as time goes by. These days it only bucks for a few seconds after about 30 minutes of driving, then seems to smooth out, i.e. back to normal, whereas when the problem first appeared, it would keep hesitating. I would think that if the TC Lock-up solenoid would be bad, it would just fail completely, and the car would not go into Lock-up mode...

I was really hoping that someone with experience troubleshooing TC lock-up circuit failures would jump in here? Surely, there is someone who has fixed one of these or replaced the solenoid? Thanks!

Still looking for advice on how the change the torque converter lock-up solenoid.