My XJ6 to XJ12 project has progressed to the point where most of the big projects have been scratched off the list and I'm down to small repairs and adjustments.

The TH400 transmission, used as-is from the Canadian-spec donor car, shifts well and generally seems Ok....but the 'part throttle downshift' feature is inoperative. Or....is it missing?

I found something strange in the Jaguar service manual. Two 'shift speed' charts are given for the V12 HE with the TH400 transmission, Canadian specification and ROW. First, there are minor differences is shift points. Second, the ROW chart has a column specifically dedicated 'PTKD' (Part Throttle Kick Down) speeds . In the Canadian-spec chart there is no mention of part throttle kick down whatsoever. No such column.

In the written text of the manual there is no mention of Canadian-spec transmissions. The parts catalogs do not call out any differences in transmission parts for Canadian spec transmissions.

Any thoughts on this? Why would Canadian-spec cars have different transmission calibrations/specifications in the first place? Makes no sense to me.

I intend to shrug-off the obscure shift chart as a 'Jaguar weirdness' thing and will proceed with diagnosis and repair. However, I can't help but be slightly struck by the fact that I don't have PTKD, I do have a Canadian-spec transmission, and the shift chart sort of implies that this is normal.

Has anyone ever come across this?

Cheers
DD

 

Not me, for sure. But, I do have an idea as to a place to research the point. Trouble is it will probably take you far and wide.... The TH400 was used in so many different critters.


Google PTKD in the TH400 or similar?


True Google knows all or thinks it does!!! Some sifting needed.


Carl

 

I've been doing some Googling....and you're right.....lots of sifting required in this case. Sometimes there's just too much information out there


Cheers
DD

 

Doug,

What I have found on a few, mine included.

Remove the vac modulator, look inside that hole, you will see the slide shift valve, pull it out of the casing/valve body.

It must be a smoooooth sliding fit in that channel, and they do stick. Shitty fluid mainly.

There should also be a spring behind this valve and many I have "played" with have had no spring. Apparently, no big deal, as line pressure is meant to hold that valve up to the modulator. Maybe it is still available from a Trans Parts Supplier.

That spring sorted mine, iffy down shift. I had done a new governor, valve body clean out, etc etc, and the Trans guy told me of it, and fitted a spring to mine, and that fixed it.

Some I have changed the Modulator on have had a spring, some have not, but they all get that spring.

It simply holds the shift valve in a fixed position against the modulator, so any slight movement by that modulator moves that valve precisely.

 

Could they have maybe removed that feature in Canadian-spec transmissions because of more likely cold weather? I know trans fluid takes much longer to warm up than coolant or engine oil. Maybe they worried there could be problems with PTKD with very cold thick trans fluid?

Just throwing out UNeducated guesses.

 

A guess, perhaps, but not an unreasonable one by any means.

At least one fellow with a Canadian Series III V12, contacted off-list, has confirmed that I should indeed have PTKD but he, too, opined that Jaguar may have considered Canadian climate when calibrating any number of things on the cars, including transmission shifting.

I'll inspect the slider valve as Grant mentioned

Cheers and thanks

Doug