my '69 420G has an open diff.
somewhere I thought I read that LSD was deleted or made optional from 420G sometime in 1967
but I cant find it mentioned in Skilleter, Thorley, or the Parts Manual.
as far as I know S1 XJ6 was not LSD from the outset ('68) and the above sources allude to certain mechanical standardisation with XJ production.
Is anyone aware of a reference or source of information on this
Tony I checked J37 as well as the IPL so then checked the SPC for the DS420–I also find no mention of an open diff option. As import of the 420 “G” stopped in the US in 1967 I am afraid none of my sales materials reflect an open diff either.
It may have been deleted at the same tine as it was on the ‘S’-type.
Jaguar began reducing costs on the Mark 2 early in 1967 – deleting items and installing cheaper components such as interior trim. The ‘S’-type remained unaffected until late in the year when the 1968 models were being planned. Cost reduction pressure became even greater when Browns Lane finally lost their independence in May 1968 and Jaguar was rolled under the banner of the British Leyland Motor Corporation (BLMC).
The Thornton Powr-Lok differential was deleted from the standard specifications of the ‘S’-type (along with other cost-reducing changes) on September 17, 1967,
Pardon my ignorance but I have never heard of the term “open diff”. Could someone explain.
an open diff allows both axles to turn completely independant in the diff centre, so if one wheel loses traction, the vehicle has no traction at all, (but you can lightly apply the handbrake as a defacto LSD/locker)
'LSD" diff has internal clutch plates that partially transfer torque from one axle to the the other if SOME traction is lost (useless if one wheel loses all traction)
a “locked” diff has several methods of locking the axles together, so they MUST turn at the same speed, used by off-roaders and dragsters.
I am personally not all that fond of LSD, because I have found they can be unpredictable in the wet, causing uncontrollable fishtailing
thx George, and S-type…I have carefully re-checked ALL my books and manuals on 420G, and I cant find reference, so its got me perplexed
Thanks, Tony. I was aware of LSD and locker diffs but never heard the “open diff” term which I always understood as just non LSD. My 3.8S has an LSD known a Thornton Powr-Lok, but not sure about my 1990 XJ6 as I’ve never had to put it to the test. 
dont know about 1990 XJ, except if you jack the car up and rotate one wheel, you can tell by the opposite wheel rotation whether lsd or open
interesting they are called Powr-Lok, as I dont think they are “lockers” in any sense (altho I could be wrong) in that they have clutch plates only, and will not transfer toque across a diff where the other axle has lost all traction
there is a lsd type, known as (Detroit) Truetrac, previously called Thorsen, that has gears, not plates, and is “in between” lsd and locker in effectivness.
I have had low-geared high power 4wds and XJ12 manual, very squirrely, in the wet, especially on corners when the the lsd kicks in (auto diff locks are even worse)
Could you share where that info comes from. I can not find it in either the service or parts bulletins. If I can zero in on it I may be able to help AWG
George.
I did the research and articles on the ‘S’-types’ production changes about
10 years ago. My primary sources were the Parts Manual and Parts and
Service Bulletins.
I do not remember exactly where the “September 21, 1967” date came from,
but Taylor (Jaguar S-type and 420, The Complete Story) gives August 1967
for the 3.4-litre and September 1967 for the 3.8). I wouldn’t have written
it as the 21st if I hadn’t found it in another source.
I am going to rte-visit ‘production changes’ later this year. If I come
across it I’ll let you know,
The Newsletters containing the ‘S’-type Production Changes are attached
for your information.
All the best
Newsletter Vol_5_No_12.pdf (845 KB)
Newsletter Vol_6 No_2.pdf (984 KB)
Newsletter Vol_6 No_3.pdf (1.03 MB)
Newsletter Vol_6 No_6.pdf (460 KB)
Hi George, I noticed in the IPL it shows 2 different different part numbers for the final drive unit with a chassis num split as attached, dont know whether this is any indication, or what the ~date of those chassis num are?
on a related matter, I just rebuilt the IFS for my 420G, and found the FSM does not mention at all several important points regarding R & R, installing the IFS back into the vehicle was a real pig of a job
when you own a 420G, you get used to having to find the answer by yourself,
which isnt to say it wasnt staring me in the face all along
those chassis nums would mean later than “the beginning of 68” (engine mount change) but before July, as per Thorley MKVII-420G page 133 (although he does not mention this change)
