420 automatic ignition cut-out safety switch

Can anybody help me out with a part number for the ignition cut-out safety switch for the Model 8 transmission as used on the 420. Or can someone point me to where I might find the switch. My car is a 3.8S but its transmission is a Model 8 and I don’t have the correct switch.

The automatic trans position switch on BW8 MKX/420G is located on the steering column, and controls a relay, in a secret hidden location

You should refer to the 420 Service Manual which will give instruction on this matter

(If you do not own one, I can look it up)

I have taken apart and cleaned one to make it functional

This part is NLA, and can only be obtained used

What is your problem ?

Voltage drop to the hidden relay is the number one cause of “no crank”, as it has madly long wires, and almost no-one knows it even exists,

the easy test is to bridge the relay

to test if your trans switch is dysfunctional, no reverse lights is a clue

I have the Parts Manual for a 420 as well if you want an actual part num, but imo, you will need to integrate the existing system (and new parts are NLA anyway)

did your vehicle originally have an auto trans ?

more info needed from you, but if your vehicle is an “S-type” I can confirm your switch should be located up near the steering wheel, its hard to see, and get at (on my 420G)

It will not be correct if the car previously had a manual, or DG auto trans

dont know about a BW35, which is the other S-type auto trans, it probably should,
the BW8 has L2DNRP configuration

serial number plate in engine bay will say what original trans

here is something i wrote a few years back, almost forgot

Guide to repair NSS and relay on Auto Saloons - Saloons - Jag-lovers Forums (jag-lovers.com)

Thanks Tony. Like everything on my car, it is complicated. The car originally had a DG-250 transmission and had the appropriate NSS in it down by the steering column. When I got it the car would crank no matter what position the shift was in, so I assumed that the switch was faulty and installed a brand new one. Then it still cranked in any position, but the turn signals, brake lights, parking lights no longer worked. That turned out to be because every time I turned on the ignition the fuse blew out. So, after much detective work - I am a novice at auto electrics - I figured out that the previous owner had bypassed the whole NSS circuit and that’s what caused a short circuit. I disconnected the NSS and that at least fixed the fuse problem and now all the lights work properly. It was during this hunt process that I finally determined that my did not have its original transmission. The big clue was the shift pattern, which became apparent once I got the old girl mobile. Then, with the help of S-type register folks I determined that I probably have a BW Type8 trans and because of the different shift pattern that would require the specific NSS for its shift pattern. So now I would like to get the correct NSS for the Type 8 and get the whole thing squared away. I guess it might be possible to reconfigure the old DG-250 NSS and get it to work, but getting the right one seems like less of a challenge for a novice like me. Then I would have to figure out (or get someone else to figure out) how the wiring was bodgied up by the previous owner and set that right. I have a lead on a NSS at a local junk yard that has a 420 that might have one, so all is not lost. If I do get one your guide to repair might be very useful. If I put in the 420 NSS will I also need the “secret” relay? It’s a long way to go and any help is appreciated. Am I correct in assuming you are in Australia?

so long as the trans has a transition of 1(L)-2-3(D)-N-R-P it should work

thats correct for a BW8

I would have to check my book to see if the 420 has a BW8 or BW35

a DG has a different shift pattern, so cant be made to work imo

an NSS switch has the complication that it is two circuits

a 12V one for reverse, and a ground path for starting in the correct gear only

hopefully its correct for your column, as its bolted to a bracket on the column, which is rotated to adjust

In the case of the relay, I recommend replacing the relay with a Bosch, as its unseen, think I show the wiring in my writeup, thats what I did anyway.

It is possible your vehicle does not contain a relay (although it should)
Consult your FSM and circuit diagram

Being able to start in any gear is very unsafe for an automatic. I would fit a switch to prevent this till you get it sorted

yes, I am in Oz

Thanks for all that tony. It is a lot to digest. I am Australian - born in WA, lived in Darwin for 30 odd years. Came to the US 16 years ago.

While researching the gearbox fitted to a 420 I found an old booklet published in 1996 for the 38th Annual General Meeting for the Jaguar Clubs of North America specifically on the S Type, 420 and Sovereign. That booklet does say the 420 used the BW8 gearbox but there was also this correction published for the Service Manual and Owner’s Handbook that may be of use to anyone working on the inhibitor switch and wiring.