Jaguar wiring deconstructed

I plan to rewire my car with a “home-grown” loom. This will be based around the Classic Technologies fuse panel. With 15 fuses, 7 relays and 2 flashers it’s a great solution for upgrading the wiring in classic cars to more modern standards. A vast improvement over the one relay and two fuse arrangement of a 1962 Jag!

Here’s some more detail on the Classic Technologies 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 vast amount of detail on their sites about how they integrated the Classic Technologies fuse panel into their respective Mk2 Jags. Check out the Classic Technologies site for more information.

Contents

Overview

The main changes to the original design will be the inclusion of the Classic Technologies 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.

Identifying routes and connections

We all know that things take longer than anticipated when dealing with our beloved classic cars. With this in mind, I decided it would be a good idea to understand and document the connection paths and layout of the original loom, before developing the replacement. I laid the main loom out on the garage floor and began the task of identifying the key routes and connections. 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!

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.

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!

Physical layout of loom

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 for a Vector PDF version that can be printed or zoomed-in without losing quality.

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.

The numbers (on the layout diagram and images) 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.

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.

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.

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.

Having got this out of my 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 Classic Technologies 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.

Steering column and horn 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.

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

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.

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.

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.

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

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.

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.

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).

The final task was to remove the upper and lower steering column bushes; these are shown as felt bearings in the diagram (15,16). My car has nylon 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.

Horn circuit – activation

Update October 2018 – Refer to the horn circuit diagram. 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!

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

Front light connections

Update December 2020 – Readers of this blog have asked me a couple of times about the layout of the front lighting circuits. Specifically, how do you figure out which wire goes where – if things aren’t working as they should? This gets tricky if either you or the previous owner has pulled things apart to replace the connector blocks or the bullet connectors. It’s easy to get lost when faced with a sixty-year-old mess of greasy wiring with colours impossible to decipher, not to mention crumbling connector blocks.

In the hope that this might be useful, I’ve added some notes from these discussions and what we learned. The Mk1 and Mk2 seem to have the same, or very similar, wiring layout for the front light circuits.

What’s happening here is that the wiring from the OSHF (off, side, head, fog), dip, and indicator switches comes through the bulkhead and runs around the car’s LH side and terminates to a 10-way double connector block on the LHS inner wing. See the images “Main harness LH (and RH) engine bay” above. Each of the wires on the top of the LH connector block connects to the corresponding connection at the bottom for the LH lights — Indicator, Side, Dip, Main, and Fog.

Each wire connecting to the top of the LH (double) connector has a corresponding parallel connection that runs back up the wiring loom, which is routed around the front of the car and terminates on the 5-way RH connector block (for the RH lights). Hence the 10-way connector on the LHS and the 5-way on the RHS. The reason for this (I assume) is to reduce the size of the loom coming through the bulkhead.

I’ve attached a (very much) simplified view of this which hopefully helps explain what’s going on. The Mk2 schematic has the parallel bullet connections drawn quite clearly when you know what to look for.

Note that the RH indicator wire comes direct from the main loom and doesn’t drop down to the LH connector. Also, the horn wiring goes directly through the inner wing.

Fault finding
Ground connections are often problematic, so it pays to check that all are making good contact. To check the wiring I like to find one thing that I know is correct and take it from there, e.g. the sidelights could be a good starting point.

Caution! the front lights on a standard Mk2 are un-fused, so care needs to be taken when testing these circuits, a sleeve over any exposed wires is a good idea when testing these circuits with a voltage source that won’t current limit in the event of a short circuit. Continuity testing is safer.