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How to Rewire Your Jaguar

How to Rewire Your Jaguar

by Bill Bassett

(click here to download MS Word Version)

It is highly recommended to rewire almost any car that is going to be driven regularly or restored. Even though complete rewiring is not inexpensive, it is far less expensive than completely rebuilding your car after a fire. There are 2 basic types of wiring harnesses available: a plastic wire tape wrapped universal type and an authentic reproduction of exactly what was originally used in the car.

The plastic type is usually one or two main pieces to be routed through the car. The main advantage of this is the low cost. The main disadvantage is that it is not complete and requires much improvising to completely hook up all the wire ends. Another disadvantage, particularly if you have intentions of showing your car, is that the plastic type is unauthentic in construction and selection of materials used. This will cost you at shows.

The authentic reproduction harness is made up of braided and lacquered wire with the outer covering braided in the original colors and patterns which match up with the cars wiring schematic. The main advantage with this type of harness is installation ease. All wires are marked, have proper terminals, and are of the correct length. Authentic reproduction harnesses are much easier to install because original routing can be used. These harnesses also come with wiring diagrams and proper bullet connectors; the plastic ones don't. You will also get full scores at shows. The main disadvantage of these authentic type harnesses is their cost. They are quite a bit more expensive than the plastic type. The choice of which type of harness used is yours. We tried to briefly cover the advantages and disadvantages of both types to help you make a choice.

For the basic rewiring, we'll use an XK 120 Roadster as our model. This is the most popular and is very similar to the XK 140 and XK 150 models. We will also use the authentic type of wiring. Here is the step by step procedure:

1. Make sure you have the proper wiring, connectors, and schematic for your car. Study the schematic to familiarize yourself with the colors used. Many of these colors are the same for almost any British car from the 1940's to the present.

2. List any discrepancies with any part of the electrical system including dashboard gauges before disassembling the car. This eliminates many future memory problems (what was or was not working before rewiring) and may help you avoid disassembling the car a second time. Rewiring will cure many problems.

3. Disconnect the battery (batteries) at this time. Do not work on the wiring with battery connected.

4. Remove the necessary upholstery panels to gain access to the wiring, underdash panels, passenger side rear kick panel, seats, passenger side floorboard, trunk floorboards, trunk panel, and transmission tunnel cover. This removal procedure will apply to most older XK's with the exception of the Coupe and Drophead models, which additionally have overhead lights.

5. If you have a Polaroid or other type of camera, now is a good time to take some pictures. These will help in reassembly.

6. Remove the center dashboard at this time. The Roadsters have four ¼" B.S. nuts holding the center panel to the main dash. If your car is an XK 140 or XK 150 it will be much easier for you to install new harnesses with the main dash removed.

7. I suggest that the old harnesses not be taken completely out of the car until the new replacement is ready to install. It is very difficult to remember the exact routing of the harnesses.

8. Start at the rear of the car and remove and replace harnesses (one at a time) starting with the driver's side taillight. Do not hook up lights at this time. It is easiest to cut the wires away from the lights and harness clips, leaving enough wire at the lights and other equipment so you can readily see which color wire hooks to which terminal. Next replace the trunk and license harness, making sure you properly position the clips in order to properly hold the harness in place; this will keep the trunk panel from bulging when refitted. Leave the light wires unattached. Next install the passenger side taillight harness as the others.

9. At this point you are ready to remove the old body harness and install the new one. You will need a roll of mechanic's wire and a roll of black electrician's tape for this project. The body harness is the most difficult one to install. The body harness runs from a junction just behind the windshield washer bottle and bracket to the rear of the car and the gas tank sending unit. At this time, remove the washer bracket and bottle otherwise you'll waste a lot of time trying to work around them. The negative battery cable and body harness runs side by side down the firewall, underneath the toe board and into the front of the passenger side sill assembly. Cut the body harness six inches in front of where it disappears into the sill.

10. Next, open the passenger side door. You'll notice wires coming out of the sill, through the frame, then through the harness clip located on the back of the transmission. The wires are connected to the reverse light switch. Cut the wires approximately 10-12 inches after where they exit the frame.

11. The body harness also comes out of the back of the sill and up the passenger side wheel well arch. Halfway up the arch, cut the harness off again. You are now left with a body harness cut off at three ends. Cut 2 pieces of mechanic's wire approximately 8 feet long. Locate the harness's cut end at the wheel well arch, measure back approximately four inches from the harness end, and make several very tight loops with the mechanic's wire. Twist it tightly against the harness. Next, run the mechanic's wire back the four remaining inches to the end of the harness and tape it tightly with the vinyl tape. Go through this same procedure with the other wire on the reverse light switch end. It is important to have made tight small loops around the harness and to have taped it securely. These have to go through fairly small holes in the sill, bulges tend to get caught.

12. Having 2 people at this point helps a lot. One person should lie at the front of the sill and the other person should be in the door opening. The one at the front CAREFULLY pulls out the old harness. The one in the door opening carefully feeds the cut ends with the mechanic's wire into the sill. Pull the old wires out until you have a foot or two of mechanic's wire coming out the end. Secure these wires so no one will helpfully pull them out for you. You will now have 2 wires (mechanic's) coming out of one of the holes at the front of the sill, one running the length of the sill and the other coming out of the sill and through the holes in the frame.

13. Again take your roll of tape and tape up all the body harness that runs through the sill and under the trunk floorboards and the part that runs to the reverse light switch. All of these areas cannot be seen and by taping the harness will add wire protection from the elements. Do not tape areas that show. Hook the straight through pieces of mechanic's wire to the two terminals that go to the fuel tank-sending unit. Tightly wrap the other wire around the reverse switch branch. Tape both of these tightly. With the person inside carefully pulling the wires and the person underneath feeding the harness evenly it will take about 2 minutes to pull the new harness in place.

14. Use the remains of the old body harness in the trunk as an example for proper routing to the tank-sending unit. Let the front part of the body harness lay under the car until later.

15. Remove the wires from the voltage regulator and from the fuse boxes. After removing wires, remove the 2 screws holding the regulator in place.

16. Remove the 3 nuts holding down the windshield wiper motor. Take off the top cover and remove the cable rack. Cut the two wires and leave the ends in place. Check your rubber shock mounts that hold the wiper body. These are usually rotted and need to be firm for proper operation of the wipers.

17. Underneath the voltage regulator is where the cowl harness comes through the firewall. Using a heavy pair of cutting pliers cut this branch off under the dashboard behind the firewall. Also, where the main harness goes to the dashboard center panel, 3 or 4 inches behind where the braiding stops, cut the cowl harness again. Remove your oil pressure temperature gauge, tachometer and speedometer cables. Leave the wires on the instrument panel. Cutting the main harness and removing the gauge and cables will enable you to remove the center panel from the car completely - then you can do whatever repairs or polishing you want. 18. The instrument panel harness can be installed at this time. I suggest leaving the wires from the cowl harness on the dashboard until you're ready to hook up the ends. The reason for this is that there are many terminals that are unused on the back of some switches. It's much easier to change them one at a time from old to new using your instructions as a cross-reference. It's almost impossible to read the numbers upside down and from underneath.

19. Get your roll of vinyl tape out again and get your new cowl harness and start taping. Tape all the wires that go to the fuse box and voltage regulator to the main harness. Do this with the other ends that are pushed through the firewall. The tape can be removed after pulling harnesses through the holes. You can also spray silicone on the outside of the tape to make the harness easier to pull through. CAUTION: If you are planning on doing any paint work on the car DO NOT use the silicone. This can cause the painter much grief. Also do not spray silicone directly on the wires as it causes damage to the finish on the old type wires. Hook up the wires to the voltage regulator before putting it back on the firewall. Note how the new wires are all correct length going to the fuse block; they will only hook up the proper way. Connect dimmer switch wires, solenoid wires; put body harness connection through firewall; hook up windshield wiper pigtail; remount wiper motor. You are now ready to wire center dash panel to the main cowl harness. These new wires have been designed so that they are long enough so you can install them with the dashboard laying face down on top of the transmission tunnel on a pad or blanket. Hook up your wires and leave it.

20. Your engine compartment and headlight harness is next. Again using your old one as a guide, install the new one in the harness clips. Using a small (stubby) straight slot screw driver, unscrew the machine screw in the top of each horn cover. Remove these cutting off the old wires first. You will then have enough room to work around the bolt that holds the ground wire (black). The headlight bucket harnesses and turn/park harnesses connect readily to the main headlight harness

21. The right and left flasher harnesses should be installed directly after taking the old ones out. Again, these are quite simple and straightforward.

22. There is a separate harness available for the manette control assembly. This is released by unscrewing the 3 grub screws on the steering column hub and slightly withdrawing it. Cut off other end outside steering gearbox and withdraw manette complete. The easiest way is to stagger splice the new wires to the ends of the original wires. The wires are never damaged up inside the column and splicing will keep you from trying to take apart this assembly and possibly damaging it. When they can be found, Manette control assemblies are very expensive to purchase.

23. Using your old electric fuel pump wire, draw the new one down through the frame and hook it and the ground wire up to the pump.

24. Disassemble your lights, clean contacts and by using your instructions and the old wire ends as reference, begin hooking up previously left wires to their respective terminals.

25. Hook up fuel tank sending unit, trunk compartment junction, reverse light switch, and body harness to cowl junction. Hook up your brake light switch, generator, horn relay, sidelamps, manette harness, and headlamps. Also remember to hook up the coil, distributor and starting carburetor and the pig tail from wiper motor to the cowl harness.

26. At this time check to make sure all your wires are connected. Assuming they are, turn on the ignition. Remember, not to start car now, as oil pressure gauge is not hooked up! Start checking out your lights and equipment. Do not get excited if everything does not work. Make a list of everything that does not work properly. Compare this list to your original malfunction list; some will probably be gone. Next, start checking the problem areas. The most common cause of lights and horns not working is poor grounds. Poor grounds are especially prevalent in freshly painted cars or very dirty ones. Check your bulbs next.

Approach the problem areas logically 1 is there power? 2 do I have good grounds? 3 is the piece of equipment malfunctioning? 4 repair or replace as necessary.

When all the electrical components are operating properly, you can then start the reassembly of your car. Remember to disconnect battery before installing center dashboard. After installing, look up underneath with a drop light to make sure no connections are grounding. Stay there, and have someone else apply power to make sure there are no sparks. The oil pressure line and temp cables bend when pushing the dash into place and can cause short circuits.

This article has been written as a guide to assist you in rewiring your Jaguar in conjunction with the installation instructions and schematic.

It may seem that it will take longer to leave pieces of harnesses connected here and there. Please take my word for it, it won't. All the harnesses are duplicates of original and have to be routed properly to be able to hook up the ends.

All of the trouble areas have been listed to make you aware of them before you tear all the wires out and throw them away.

As a last note, take your time while doing this and don't get too excited. It isn't something that can be done in an evening or two; it takes me fourteen hours or so to make a complete installation and checkout. Good Luck! Bill Bassett

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