XK 140 OTS Overdrive

My '56 140 has the Laycock overdrive but no throttle switch. It has the “willow leaf” switch. Looking through the XK wiring diagrams I see where the 1-relay sketch shows no throttle while the 2-relay sketch does show it. Question; did some OTS come with the kick-down feature and some did not, and why?



You don’t advise which model XK140 you have, whether RHD or LHD, OTS, FHC or DHC, or most importantly CHASSIS NUMBER…
The introduction of the Throttle Switch/2-relay arrangement was an age related improvement introduced for XK140s fitted with optional Overdrive, and factory records advise from which exact Chassis Number for all six model variants… Regardless the Throttle Switch improvement was introduced for all six model variants for cars built from about the last week in November 1955, noting this is their actual date-of-manufacture. You advise you have a '56 XK140 which is presumably date of sale or similar, and not actual date-of-manufacture. If you advise your exact CHASSIS NUMBER then your question can be conclusively answered…

Thanks Roger;
The chassis on the OTS is S811487DN. Without the throttle switch I usually de-clutch, then throw the dash switch to “Off”, then give the engine a little throttle to catch up the transmission; is there a more proper way to downshift from OD without the throttle switch?


S811487DN is of course a LHD OTS, and would have a date-of-manufacture in the second week of June 1955, albeit my data doesn’t reveal an exact date, but close enough…
So it was built nearly 6 months prior to the introduction of the Throttle Switch/dual relay arrangement that commenced for LHD OTS from 811866DN onwards.

There are various driving techniques in engaging and disengaging the overdrive, and with practice/skill can be done perfectly OK purely by throttle control, but until you are skilled/confident with SMOOTH engagement/disengagement of the overdrive its circumspect to use your clutch as though its a normal 5th Gear… Rough/Jerky disengagement and Slipping engagement of the Overdrive can and will damage your overdrive - not a simple/cheap repair…

Many thanks Roger. I’ve just recently acquired this car and with this information I should be able to master the shifting technique you’ve described. We always appreciate your generous response to these Jaguar questions.

ron brazell
vallejo, ca

The throttle switch just ensures that the accelerator pedal is depressed when the overdrive disengages so the engine will make up for the sudden increase in RPM. You can accomplish the same by only switching out of overdrive when you have pressed down on the throttle. How much depends on your set up, but it should be much less than full throttle.

10/4, Thanks Bob, I’ll do some practice.