Originally Posted by Chirpy

Extended PIDs are supported using UDS service 0x22 (0x means hexadecimal value, so 0x22 is 34 in decimal). Requests are sent to the ECU, TCM, or whatever module, and the targeted module sends back the appropriate responses.
(A slightly updated list for the AJ27 ECU PIDs is here)

This thread relates to the AJ27 ECU and TCMs, and if you have a 2004 XKR per your signature, then I guess you have an AJ33/34 family engine, and that ECU is a completely different design.

I don't know to what extent the firmware implementation is similar, as I've never studied it personally - others can chime in on that.
I assume Jaguar tried to get some consistency, but I've no idea on any similarities regarding the request / receive canbus addresses (AJ27 only supports UDS service 0x22 on canbus, and not on serial), service unlock protocols, or the actual PID codes.
Some aspects of AJ26/27 canbus OBD2/UDS implementation are not fully in line with the standards, and this might have been cleaned up for the AJ33/34

I personally don't think UDS service 22 requests are going to do anything bad to the ECU, at least they won't as far as I can determine on the AJ27 ECU, but don't take my word for it.

Originally Posted by LMG

Dibbit, did you ever get this to work?
I'm toying with the idea of adding a few extra displays to Torque, but the warning that pops up that you need to know what you're doing before adding custom PIDs is a bit intimidating, especially considering the diagnostic start and stop commands we apparently need to include. We're not actually sending commands back to the ECU, just to the app itself, correct?

Originally Posted by JagV8

You're safe enough using 0x22

You may want 7e8 not 7e9 and 7ec instead of 7ed

I have a somewhat dim memory that there's a (spreadsheet?) of PID values elsewhere, try some searches.

Originally Posted by dibbit

To answer your question - no I never got this to work.

The commands are being sent to the ECU or whatever module they are addressed to. Now in theory the ECU will simply reject any incorrect commands, but it's impossible to know how robustly this was tested in development.

I know my car (2001 AJ27S engine) has at least one bug that I found to do with reporting the engine temperature, so there could well be others. I also know that I can get the car to show Restricted Performance if I send queries too often - I'm assuming it simply doesn't have enough spare bandwidth to do too much. That was one of the reasons I gave up experimenting, so I wouldn't really recommend playing around with this unless you have a good level of knowledge.