Jaguar wiring deconstructed

Jag_Blog-207My Classic Technologies (CT) fuse panel arrived over the Christmas break. The CT fuse panel is a great solution for upgrading the wiring in classic cars to modern standards. It has 15 fuses, 7 relays and 2 flashers. A vast improvement over the one relay and two fuse arrangement of a 1962 Jag!

Here’s some more detail on the CT fuse panel and its application in other Mk2 restorations:

Reference: Valve Chatter and Jaguar Mk 2 Restoration. Lin Rose and Eric Kriss have both provided a huge amount of detail on their sites about how they integrated the CT fuse panel into their respective Mk2 Jags. Check out the Classic Technologies site for more information.

I’ve decided to rewire my car with a “home grown” loom. The main changes to the original design will be the inclusion of the CT fuse panel and new circuits required to support the installation of a later model engine and overdrive gearbox from an XJ6. I’m also considering powered seats from a later model Jaguar, reversing camera, modern sound system, and a few other “bits and pieces.”

When I disassembled my car, I removed the main wiring loom intact. The main loom connects the engine bay, front lights, dashboard, steering column components and provides connections for the remaining wiring: the door switches, interior lights, rear lights, fuel pump and fuel tank sender. I didn’t label the connections (although I took lots of photos) as the plan was always to build a new loom.

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Main loom as removed from car

To be sure I understood the layout of the current wiring I laid the main loom out on the garage floor and began the task of identifying the key routes and connections. To do this I used the wiring schematic from the service manual printed out on A2 paper and a continuity tester (multimeter), the trick with this approach is to start with something easily identifiable. I chose the horn wiring, which was a mistake. The wiring schematic for a 1962 Mk2 Jag makes no reference to a horn relay, and I had assumed that my car didn’t have a relay powering the horns, or more correctly that it wasn’t wired up. Having owned the vehicle for a long time such things become ‘fact’.

I kept going around in circles as the horn wiring didn’t seem to connect to anything. Finally, I realised that the relay must have been connected and everything made sense!

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This image also led me astray, clearly I had disconnected the wiring before taking the picture

The wiring diagram is in the supplementary information section for the 240 and 340 models in some later manuals. I have included a diagram of the horn circuit with relay below.

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Horn relay wiring

Once I had figured this out, I decided to carry on and decode the greasy mess of wires that connect to the regulator and fuses. By this stage, I had developed an obsessive need to carry on and identify all the wires in the main loom. Getting to this point was tedious and required patience, but it sure gave me a good understanding of what went where. It wasn’t necessary or practical to go to these lengths. However, this is a hobby, so it doesn’t have to be!

I put together a diagram showing the layout of the wiring loom. The drawing is roughly to scale, but I had to increase the width slightly to get the required detail. I used an application called AutoDesk Graphic for Mac to draw the layout, which was perfect for what I needed. I will use this application to create the schematic for my wiring as well. Click on the diagram (and the relay diagram above) for a Vector PDF version that can be printed or zoomed-in without losing quality.

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1962 2.4 litre wiring layout diagram

See below for some images that help clarify the routing of the original wiring harness. These have the same reference numbers as the wiring layout diagram.

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Main harness – firewall
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Main harness RH engine bay
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Main harness LH engine bay

The numbers (on the layout diagram and the pictures) are referenced in a spreadsheet I created to record the wiring connections. Why bother with a spreadsheet? I figured that it gives me the ability to filter on any aspect of the wiring. It also provides a simple model to build on when creating the new layout and helps keep track of everything.

The first column in the spreadsheet records the circuits that form the heart of the wiring: the battery, generator circuits, regulator and fuse wiring. Circuits with components connected in series are in rows. Components connected in parallel appear as one connection with multiple components vertically. The interior light circuits are an example. Trying to get this all to make perfect sense in a spreadsheet isn’t that practical, but it works for what I need. But it’s no replacement for a schematic!

Here’s a link to the spreadsheet

The Jaguar service manual also has good (if not very detailed) views of the harness layout.

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Harness layout from Jaguar service manual

Overall condition of loom

The overall condition of the loom, considering it’s over 50 years old, was pretty good. There were some parts: the front lights, horns and the multiway connector blocks feeding them that were in bad shape. The rest, even the very discoloured wires feeding the fuse and regulator circuits, were all good low resistance circuits, as were the bullet connectors. I also found that the wiring matched the colours (when I could tell the colour) on the schematic almost exactly, one difference was the wires to the front indicators were both Green rather than Green/White (RHS) and Green/Red (LHS).

Mystery connections

I did find some interesting choices which I assume Jaguar engineers made in the interests of efficiency.

I discovered that the wiring to the ignition coil also contained a mystery wire that went nowhere. From the colour of the wire, it seems it may have been for the automatic choke on the 3.4 and 3.8-litre cars. But that’s a guess on my part. You can see from the image that it was simply doubled over in the loom.

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The wire labelled “choke” was cut and doubled over in the loom

The other mystery was the circuit for the handbrake and brake fluid warning lights. This had me puzzled for a long time. It seems (to me) that the handbrake wiring is included in the loom for both left-hand and right-hand drive cars. I can’t understand what other reason there was for the seemingly endless amount of red/green wire I discovered on both sides of the vehicle. Just as confusing was the mystery connector in the left-hand side of the engine bay. After cutting the loom and tracing the wire by sight (continuity testers are great, but you need to have some idea of what you are looking for), I discovered that it connects to the handbrake/brake fluid warning light wiring, so I assume that it’s for the brake fluid reservoir on left-hand drive vehicles. I’d be interested to know if this is correct.

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Double bullet connected to the brake warning circuit

If this is correct, the circuit connects the +12V supply (via the ignition switch and the A3 Fuse terminal) to the brake warning light and then to the left-hand sill wiring bullet connectors. From there the wiring travels around the vehicle to the various switches: the left-hand handbrake, and the left-hand fluid reservoir, and back over to the other side of the engine bay to the right-hand fluid reservoir and then to the right-hand handbrake! Anyway, this was the last piece of the puzzle that I bothered checking out – it was annoying me that I couldn’t sort out what was a very simple circuit.

I have recently discovered by looking at the various loom manufacturers that the main harness is the same for the left-hand and right-hand drive vehicles. So, maybe it does all make sense.

After many hours of detective work here is the main loom laid out roughly in the shape it was when fitted to the vehicle. Most of the connections are identified.

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Everything identified!

Having got this out of my system system, I can now focus on figuring out the paths and connector options for the new loom.

My plan is broadly similar to the approach taken by Eric Kriss. I intend to locate the CT fuse panel where the regulator and fuses are located in the original Jaguar design. As I only need to consider a right-hand drive setup I hope to be able to simplify the wiring layout.

Update 26/06/17 — Steering column wiring

Kjell, a follower my blog, made a comment (below) re the horn wiring in the steering column. The diagram and the commentary in the manual are fine if you have experience with these types of systems. If you don’t (like me), then it’s a bit of a head scratcher figuring out how it all works.

I decided to pull mine apart and send some photos which I hoped would clarify how it all fitted together. I have now decided I may as well add these (and a brief description) to this post.

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Steering componenets

I started by carefully separating the inner column complete with the steering wheel from the outer tube. These pulled apart with little pressure.

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Inner column and outer tube

I removed the steering wheel from the end of the inner shaft (20) by undoing the locking nut (31) with a 15/16″ socket and then the nut (29) which holds the steering wheel in place with a 1″ socket. With the steering wheel removed, the wiring to the horn push on the steering wheel can be seen, along with the inner shaft, and the two halves of the split cone (27). The locking ring (24) for the steering wheel height adjustment is still in place.

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Lock nut removed
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Horn push wiring and split collet

The horn switch wiring consists of a soldered terminal, a circlip, washer, spring, and an insulating bush. I cut the wire just below the terminal and removed these components.

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Terminal, spring, and insulating bush

I then unscrewed the two small set screws holding the indicator striker (46) in place and removed it. I unscrewed the locking ring and removed it as well.

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Indicator striker
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Steering column lock-nut

I removed the stop button (21) that holds the steering shaft in place and limits on the steering wheel height adjustment.

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Stop button
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Stop button, inner shaft and other components
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Inner shaft

From the horn switch, the wiring travels down the inside of the inner tube and terminates at the slip ring (33). The manual states that one should carefully prise up the serrations and then remove the slip ring. I approached this with some trepidation, but it all seemed to go OK. I don’t know if I will reuse this part, but I sure didn’t want to wreck it during the removal process.

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Horn wiring/inner column
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Slip ring
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Slip ring removed

With the slip ring removed, the top and bottom halves of the rubber rotor (37,38) can be separated. The top half holds the horn wire lower contact in place. This contact is embedded in the upper rotor and is spring loaded, maintaining electrical contact with the slip ring. I cut this wire to remove it and the top half of the rotor assembly. I then removed the remains of the horn wire (34) from the inner column.

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Upper rotor
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Upper contact showing spring loaded terminal
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Top and bottom rotor components

The horn power feed/earth connection block mounts on a bracket the at the lower end of the outer column. The purple/black wire in the image below is the power feed which connects to the slip ring via a contact (41). The lower connection, which is a bit difficult to see, contacts the inner column to provide the earth connection (32).

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Horn +12 Volt and earth connection
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Slip ring contact
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Earth contact

The final task was to remove the upper and lower steering column bushes; these are shown as felt bearings in the diagram (15, 16, 17,18). My car has plastic bushes which were a replacement for the original felt bearings. They push into holes in the outer tube. I remember fitting these and then having to sand them down as the steering was binding due to the new bushes being a bit tight.

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Steering column plastic bush – upper
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Steering column plastic bush – lower

Horn circuit – operation

Refer to the horn wiring diagram above. Although this is a straightforward electrical circuit, it can be a bit tricky trying to figure out how it all works.

The horns, or the horn relay, depending on the vehicle, are connected to the battery via the fuse terminal A4. The “other side” of the relay coils (or horn) circuit continues to the bottom of the steering column where it connects to the slip ring contact, and from there to the green/black wire which runs up the centre of the steering column via the spring loaded connection in the rotor. The circuit is completed (grounded) when the horn ring on the steering column is pressed, and the copper cup on the underside of the horn ring makes contact with a matching copper ring on the steering wheel. Phew!

 

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Horn ring components

The horn activation is a “classic” example of 1960’s electromechanical technology!

 

10 thoughts on “Jaguar wiring deconstructed

  1. Phil,
    Congratulations and wonderful job, you certainly took what I did to a whole new level. Wow! Just very impressive. By the way, I am finding that I really like having the spreadsheet. I have consulted it a number of times. I also discovered after some noodling and a little frustration that the wiring loom does have some duplications so that it can be used in LH and RH drive cars. Great work!! Lin

    Like

    1. Lin,
      Thanks for the great feedback! Yes I’m sure the spreadsheet will be useful for keeping track of things, I can use it to estimate quantities of wire and connectors I need as well. Cheers.

      Like

  2. Phil,
    Thank you for this effort! As I started to install a new standard loom (from SNG Barrat) some time ago and also stumbled on the horn problem immediately. Yesterday, as I went about with side body harnesses, I came across the handbrake contact mystery you mention too. As everything was dissembled by pre-owner your engine bay pics and “physical” schematic helps a lot translating Service book schematics to the real world. Also I have got an improved original schematic from Tom Jenkins jagschemes@live.com.au which make color understanding faster/easier for aging eyes.
    Kjell

    Like

    1. Hi Kjell,
      Yes, I’ve heard of other people (as well) restoring cars they have bought semi or completely disassembled. Those of us who have pulled our own cars apart have it easy! When I was drawing the diagram I wondered what information comes with a new harness. Do you just have to figure it all out, or are there instructions?
      Cheers Phil

      Like

      1. No instructions follows. So there is a lot of wholes where wiring shall pass…or not. And worse, while fixing rust in floor and sills preowner did a great job but didn’t always care to reinvent old wholes. Took a while before I realized there should be wholes into sills for side harness to pass thru . Right now I am trying to put Steering-column together and how hornpush is supposed to work. Servicemanual shows a cable nr 34 i fig 4 on page I10 and I am uncertain how that one connects i reality. I might mail you some questions as projects moves along 🙂

        Kjell

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      2. Hello Phil Seeing the date you have probably moved on, however I bought a new loom from Autosparks UK. for the Mk2 great product. They offer at extra cost, a manual showing all the detail. YOU NEED THIS
        Regards

        Hal

        Like

      3. Hal, thanks for the link. My problem is that I started my working life as a Radio Tech, therefore I just have to over complicate things when it come to wiring. Looks like Autosparks looks like a great product. Hope the wiring goes well!

        Like

  3. Absolutely great idea labelling the wiring diagram and providing the spreadsheet. Wished I’d seen this a little while ago whilst trying to sort my door switches. Great job all round.

    Liked by 1 person

  4. Thanks Phil, great explanation of how the horn works, at last I’m beginning to understand it. Next problem is that I’m fitting a Moto Lita steering wheel, and I think that I’m going to have to modify the system, so that the centre contact in the horn boss is able to reach and operate the horn push. It will be a few weeks hence as whilst the column is out, I’m fitting a new wood interior and am going to start with the facia panel and instrument panel. Once again many thanks Phil. Patrick

    Like

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