Hi goz,
Many thanks for all your input. I will talk to Peter next week.
Meanwhile, had to run into town at lunchtime. Started logging on the way back, and soon snagged the worst example I've had so far, on a 4-lane road. Had to slow for traffic and she jumped 3rd to 1st and got "stuck" there. Revving along at 20km/h, I had to shove into the left lane and off into a side-street, although it found 2nd just before the turn.
This time, I remembered to check for
TCM DTCs pronto...
===TCM DTC P074A:00-27=== Code: P074A - Unable to Engage Gear 2Status (-27): - DTC Present at Time of Request
- Malfunction Indicator Lamp is Off for this DTC
Module: Transmission Control Module Freeze Frame #1:
-EVENT_TIME: 371491145 s (Fri Jan 03 13:34:56 2025) -Event time
-TOTAL_DISTANCE: 134446 km - Total Distance
-MODULE_VOLTAGE: 14.3 V - Control Module Voltage
===END
TCM DTC P074A:00-27===
You might say this changes everything.
I've had a quick look at the graph,
TCM PIDs especially, and they don't seem to show any attempt to "Target" 2nd gear. TGT_GEAR simply drops from 3 to 1, but that happens
after GEAR_SEL hits Neutral, which happens
after the engine starts to rev up, which happens just
as foot-off-brakes.
Whether the brakes had anything to do with it, the
TCM apparently knew it was planning to change into 1st as soon as it started to rev up the engine, while TGT_GEAR was still 3. Now suspecting that TGT_GEAR is completely misnamed, just to confuse us. To me, a Target is what you're aiming for, not what you actually hit, so I've been misunderstanding what it was showing us. German designed, probably Lost in Translation, but maybe TGT_GEAR should really be called NXT_GEAR to indicate that the gear is actually engaged and ready to be used.
Also from the data captured, can see that just before the engine started to rev up, the accelerator was at 0, the engine fuel was 0, MAP is at atmospheric (100kPa), and the brakes were still only lightly (166.7kPa) on. There seems to be nothing to show that the
TCM was attempting to engage 2nd gear at this point or earlier. That must all be happening inside the mind of the
TCM and no PID for that is exposed.
To take all this any further, I imagine we would need to capture the HS-
CAN Bus data, as there would have to be a command from the
TCM to the
PCM to "Rev Up!" But I don't think that would get us any further ahead.
It really does seem like the transmission is having trouble engaging 2nd, and that 1st is the last resort. Maybe all that kicking and bucking did do it some damage.