From what I can gather the gearbox ECU will tell the engine ECU to reduce performance if the gearbox oil overheats, but don't quote me on it. Random faults like this are often down to electrical gremlins which are usually a pig to trace. I would imaging a gearbox overheat condition would register a code in the ECU which could be read on a Lexia, but then again I'm not an expert. The gearbox ECU does have several modes which it will flip to depending on driving style so may not always react in the same way. Mine used to be permantly in old man mode, i.e. get into top as soon as possible and stay there. If you are feeling let down on the performance then there's always that little button marked 'S'
Problem is when the car is really sluggish as it was on the way home today, even in sport mode its quite flat in performance. Obviously sport mode helps as it gets the revs up and keeps it out of "old man" gearshift mode, but its not just the gear selection that's the problem...
Mine also frequently ends up in "old man mode" where it shifts up early and stays there... I've found that it's not so much wider throttle openings that train it to stay out of old man mode, but how quickly you move the throttle. So if you drive smoothly and gradually open the throttle wide it might not change down and will stay in that mode however if you open the throttle quickly to the same percentage it will both kick down and switch to a more sporty mapping... personally I'd like to disable SK1 (old man mode) or write the same values as SK2 into SK1 to neuter it, but I can't see any way to do that without serious and risky firmware hacking...
Regarding the gearbox oil temperature, according to the training manual it does have two mappings that try to protect against the oil overheating, however they're only triggered at 118 and 120 degrees, which I don't believe mine is reaching. They trigger the gearbox to hold a lower gear more to keep the engine revs up to circulate the gearbox oil faster, and also eliminate the limited slip mode (using full lock up instead) to reduce heat generation in the converter. Above 125 degrees the full slip "open" mode is also disabled in most conditions.
Funny thing is even though I've never seen over 105 degrees reported on the Lexia during fast country driving, when misbehaving it does seem to severely limit the torque converter slip keeping it locked up most of the time. Normally when you accelerate you get a "half change down" when you kick down the throttle a bit particularly in 4th at 60mph with about half throttle... this is actually the torque converter unlocking which boosts the revs about 500rpm and gives torque multiplication and a nice extra boost in acceleration at the expense of worse fuel consumption... if you use even more throttle you get an actual gear change down to 3rd.
When misbehaving this "half change" doesn't happen, the torque converter stays fully locked up until you press it far enough to trigger a change down to 3rd...(at which point the converter is still locked)
Regarding whether the gearbox has control over the engine power output for reasons other than gear change torque reduction, the answer on studying the documentation again seems to be yes. On page 127 of the manual the table shows that "transmission oil temperature" is one of the inputs that affects "Torque reduction".
So if the gearbox thinks its overheating it can instruct the engine to reduce torque by retarding timing... I wonder if this is the source of general lethargic performance when well and truly warmed up after half an hour or more of motorway driving ? I haven't actually check the oil temp in these conditions, and it would take a long time (hours) for the oil to cool down once the gearbox was really piping hot.
Since the heat exchanger cools the gearbox oil via the engine coolant, could a thermostat that is running 10 degrees hotter than it should be be enough to push the gearbox oil temp to the point where the gearbox starts taking protective measures like reducing engine torque ? According to the Lexia my coolant temperature reaches as high as 98 degrees before the fans start dragging it down again, perhaps that's about 10 degrees too hot ? What's the correct 'stat temperature for this engine ? (Perhaps a new 'stat isn't such a bad idea)
Another interesting quote from the manual that I'd somehow missed before:
C - AUTOMATIC TRANSMISSION PROTECTION At very low vehicle speeds (Vveh < 5km/h approximately) or when there is a fault on the vehicle speed sensor line, if P/N is changed to D or 1st or 2nd or 3rd or R, a timer f (T°coolant) is triggered. During this timer, if the driver accelerates and if N > limit f (T°coolant) or TB > limit f (N, T°coolant), the ECU cuts off the injection so as to cause a large drop in torque in order to protect the transmission. Injection is restarted when the engine speed gradient reaches a limit f (T°eng). Injection picks up progressively until it reaches Ticalculated. The increment is of the type Ti calculated x Ktake-up where Ktake-up depends on the number of ignitions which occurred since the injection was cut off which represents the time for which the fuel was cut off. The table which Ktake-up comes from is specific to the "Automatic transmission protection" function. In addition, the optimum advance is applied from when injection is restarted to when Ti calculated is reached
Although it looks like this would only be used in case of a road speed sensor failure this is saying that the gearbox even has the power to command the engine to shut down or severely limit the injector pulse times, in addition to retarding timing... although its not clear how it might signal this.
Looking at the waveforms on page 167 on the torque reduction line from the gearbox to engine it looks fairly simple - high voltage (5v ?) is normal operation, low voltage is retarded timing commanded during gear change. If I can somehow tap into this line (not easy when both ECU's plugs can't be back-probed) and run the signal into the cabin I could potentially monitor that voltage while driving thus have a heads up display of what the gearbox is commanding in regards to torque reduction request. That could be very enlightening...