AP_N1_HOLD gets logged while I would expect AT_SPEED_HOLD or something similar.
...
NOTHING is logged for C/O
Er ... I can only make that log when I click the N1 button. In N1 hold mode the C/O button and the speed display is irrelevant, which is why the C/O button logs nothing -- it has nothing to do except change the display (which is a local action to the gauge). This is always the case when SPD mode is not enabled.
I really have no idea why on your PC pressing the SPD button is operating N1 ... something is wrong, and explains why none of the speed mode controls you tried changes Mach/IAS. Which button lights on the panel graphic, SPD or N1?
I suggest you double check all this very carefully.
Here is what I log for the default 738:
Pressing SPD button (after enabling A/T):
188343 *** EVENT: Cntrl= 65891 (0x00010163), Param= 0 (0x00000000) AP_PANEL_SPEED_HOLD
Pressing the C/O button:
191515 *** EVENT: Cntrl= 65918 (0x0001017e), Param= 0 (0x00000000) AP_MACH_HOLD
Toggling SPD mode on and off whilst Mach displayed:
199140 *** EVENT: Cntrl= 65918 (0x0001017e), Param= 1 (0x00000001) AP_MACH_HOLD
200187 *** EVENT: Cntrl= 65918 (0x0001017e), Param= 1 (0x00000001) AP_MACH_HOLD
(Not sure why the parameter is 1 here -- I am sure it isn't used anyway).
Pressing the C/O button again:
209859 *** EVENT: Cntrl= 65891 (0x00010163), Param= 0 (0x00000000) AP_PANEL_SPEED_HOLD
Finally toggling SPD mode off:
211328 *** EVENT: Cntrl= 65891 (0x00010163), Param= 1 (0x00000001) AP_PANEL_SPEED_HOLD
The only way I can get N1 hold to be sent is to operate the N1 button. Here is me toggling it on then off:
215890 *** EVENT: Cntrl= 66711 (0x00010497), Param= 0 (0x00000000) AP_N1_HOLD
217281 *** EVENT: Cntrl= 66711 (0x00010497), Param= 0 (0x00000000) AP_N1_HOLD
The only obvious difference I can see with my installation is that I am running FSX SP2. Not sure why you've not yet updated yours, it is well recommended for SimConnect improvements.
Maybe you have something else installed which is interfering with the default 738? I'm sure that FSX worked okay, setting SPD mode correctly, right from the original release and before.
You aren't using TO/GA are you? If TOGA is set you probaby get N1 mode I think.
Regards
Pete