Seems to be an ongoing struggle with this…there are three things that matter re rewiring, depending how you count…
- Having the correct WIRING DIAGRAM is a start, and the best by far is the factory original, as produced by LUCAS in conjunction with Jaguar, there are various after-market efforts of greater or lesser accuracy unfortunately, more concerned about apperance/presentation than technical accuracy. The wiring looms fitted by Jaguar into an XK140 were indeed manufactured and supplied by LUCAS as per their LUCAS produced Wiring Diagram…
There were two only versions of the LUCAS XK140 Wiring Diagram No.W26880 as issued in November 1954 but actually printed in December 1954, so very much to suit the release of the XK140 in October 1954, and the second version No.26880A as issued in March 1955 that is simply an update, but usefully on the rear of the single sheet diagram now provides the connection details for the Headlight Switch, the Windscreen Wiper Switch and the actual Windscreen Wiper Motor…
See below a copy of the March 1955 Lucas Wiring Diagram…
I dare say these diagrams can be copied and expanded, as needed to read properly, if not send me a direct email - rogerpayne@bigblue.net.au - and I will send you a 2MB scanned copy…
Note however, this Wiring Diagram suits all XK140, OTS, DHC and FHC, RHD and LHD, including Fog Lamps (if fitted) and Interior Light (if fitted), but it does not include anything for the OVERDRIVE (if fitted) nor AUTOMATIC TRANSMISSION (if fitted). I am not aware of any LUCAS diagrams specifically for XK140 Overdrive nor Automatic, albeit they must have existed somewhere within Lucas and/or Jaguar, but seemingly never produced as a readily available Wiring Diagram or update of the March 1955 Diagram. However when I did all this WIRING work for XK140 EXPLORED I did draw up two separate Wiring Diagrams for both the early and the later Overdrive arrangement - see Plates 8-c8 and 8-c9 that again I can send copies direct, for those that do not have XK140 EXPLORED. I am loath to supply copied pages out of XK140 EXPLORED onto these public JagLovers web sites for my own-intellectual property reasons given the rampant unauthorized plagiarism by some individuals…
But no, I didn’t get around to doing similar for XK140 Automatic Transmission, as in the scheme of my then priorities and publication deadlines, I ran out of time…
In XK140 EXPLORED I spent a lot of work better illustrating all the separate circuits, components, connections and colour coding of wires being easier to read and understand than the correct, but basically presented LUCAS WIRING DIAGRAM - there are some NINE pages detailing all these separate circuits (including the two Overdrive circuits)
Next step was to turn this all into a physical practical reality regarding the looms, and practical routing of the looms, locations of all the components, and wiring connections to these components, but although a good start, this was not finished (HUGE amount of work, competing priorities, and a publication deadline, but certain work was done and included in XK140 EXPLORED, including the Plate 6-a2 shown in earlier posting - what did I say about ‘unauthorized plagiarism on a public web site’! 
But the question about the ‘Loom inside Chassis Rail’… that is of course the Petrol Tank sender unit/Petrol Pump loom… If you look at the discussion on (as it turned out) XK120 Petrol Tank Senders in another thread, I did post three or four very detailed/accurate photos of this looms connections to the petrol tank sender unit, and its subsequent routing up and over the side and rear cross member and entrance into the right side chassis rail - but again I do have more photos I can send direct… But yes, this is a cow of a loom to correctly route on top of the rear crossmember and then inside the right side chassis rail, even when the body is off the chassis…
But another comment is all of this is easier or harder, depending on the source/manufacturer of your new reproduction wiring loom, not just correct wire colour codes, and correct layout/dimensions of all the various wiring looms, and all the correct connectors for each wire…, but also the rating and diameter of individual wires and the resultant diameter of the looms, as an oversize loom can be impossible to fit through small holes designed for original Lucas loom, and there are also some tight corners and restricted space to route correctly many of these looms…
I had a very close look at this some years ago now, so qualify my comments that some of the lesser quality/accuracy looms on the market may (hopefully) have improved, but back then found the BEST loom in all respects was made/supplied by Rhode Island Wiring (in USA - and as it happens, in Rhode Island 
), and indeed second best was made in Western Australia, and others then checked from both Australia and UK to be kind ranged from the ‘rest’ to ‘dont touch’
So I purchased my new loom from Rhode Island Wiring, that the observant may appreciate is in a LHD dominated market rather than my RHD XK140 loom requirement in Australia, so again some research, which revealed that the LHD XK140 loom(s) were 100% suitable/same as that for a RHD XK140 apart from THREE only wires outside their looms were too long, and for my RHD car simply had to be cut shorter to suitable length. If the Rhode Island LHD loom was not 100% suitable for my RHD XK140 I would have purchased second-best from Western Australia…
Others may have different experiences/recommendations than I…
Roger Payne
(co-author and intellectual property owner of XK140 EXPLORED)
