Making or Repairing Wiring Harnesses

by Dan Masters,

Whether it is advisable to make your own harness, as opposed to repairing the existing harness, depends on a number of factors. If you have no harness at all, it would be much easier and cheaper to buy a ready made harness. Building a harness from scratch, without a pattern to go by, is extremely tedious, to say the least. If, on the other hand, you have a fairly good harness, with only a few wires damaged, then it would be cheaper, and almost as easy, to do a repair. These two extremes make the choice fairly easy; it’s when your situation is somewhere between that the decision is difficult.

A common situation is to have one or more harnesses that have been salvaged from the junkyard. In this case, it may be desirable to save as much of the harnesses as you can, re-using the connectors and wire to make a new one, or to repair the better of the harnesses to make one good one out of those that you have.

As for re-using the connectors, I would advise caution. Of all the pieces of the harness that give problems, the most likely is the connectors. If you do re-use them, make sure they are clean and corrosion free. The bullet connector sleeves, the black pieces that are used to connect two pieces of wire, are particularly trouble prone. Often, the metal sleeve inside will crumble with age, and will no longer have sufficient tension to make a good connection (not being a metallurgist, I can’t explain this, but on about six of them on my ’71, when I removed the bullets, pieces of the metal sleeve fell out on the ground).

Salvaging the wire is not a problem, as long as the wire has not been damaged. If you can, i.e. the wire is long enough, I recommend cutting the wire an inch or two from any connector. Often, moisture will wick up the strands and corrode the wire, making it difficult to get a good connection. You should cut back to where the wire is clean and shiny.

The easiest way to repair a harness is to clear out a large space on the garage floor, and lay the harness down, stretched out, with each “leg” of the harness laid out by itself. Strip back the tape surrounding the damaged area, being carefully not to displace the wires from their original position. Lay the new wires down, one at a time, alongside the original wires, to make sure they are the same length (allowing 3/4 ” overlap for the splice). Strip off 3/4″ of the insulation from both the old, and the new, and slip a piece of heat shrink tubing over one of the wires. Twist the wires together, and solder. After the joint has cooled, slip the heat shrink tubing over the joint, and apply heat to shrink. Attach whatever connector is appropriate to the free end of the wire. Continue with each wire, till that section is done, and then go on to the next section. Use plenty of cable ties to hold the wiring in place as you work, prior to re-wrapping. As you come to each cable tie, during the re-wrapping process, cut it off, and discard it. Be prepared to use plenty of cable ties.

Don’t make all the splices in exactly the same place. If you do, the wire bundle will be very thick at this point, and all the stress will be concentrated in this area. Space the splices out over the length of the harness, at least six inches between each splice, if you can. Don’t worry about making splices, the factory did it themselves.

When you do the re-wrapping, begin with the terminal, or connector end, and work back towards the main harness. That way, you have fewer loose ends of the wrapping tape to worry about, as the wrapping for each leg gets wrapped again by the main wrap. Start by running a short piece of the harness tape parallel to the wires, heading towards the connector end, and start wrapping back over this piece, back towards the main harness. This way, the wrapping secures that end of the tape. Use the special harness tape supplied by British Wiring, which has no adhesive, but adheres to itself. When you get to the last piece, use electrical tape to secure the last wrapping.

Be careful to note any pre-formed bends in the harness, and reproduce them when you do the repairs. If you bend the wires, and tie them tightly with the wrap, they will hold their shape surprisingly well.

Making a new harness will be exactly the same as above, except you will run the wires from one end to the other, using the existing harness as a guide. Even if you are making a new harness, there will be places where you will have to splice, just as the factory did. Just make your splices in the same place and manner as original.

If you don’t have a harness to use as a pattern, you can still make your own, but the degree of difficulty goes up significantly. In this case, you will need to route each wire individually, in place, in the car, connecting each end as you go. If you can, It’s a good idea to look at a completed car, to see how the harness was ran by the factory. The factory provided sufficient supports, and these should be used for the new harness. The location of the supports provides a good indication of the correct harness routing. Connect one end, then route the wire to the other end, and make that connection. As you lay the wires down, pay particular attention to the need to remove and replace both the harness and the electrical components. There must be sufficient slack in the wire to allow for this. Also, great care must be given to the routing of the wire to avoid any possibility of damage from rubbing against sharp edges of the body, and making sure that the wire is properly supported to eliminate strain on the connectors. As you run the wires, use cable ties to hold everything is place. When you have completed and tested the wiring, very carefully remove the harness from the car, and wrap as described above.

Whichever route you take, you will need to obtain the required supplies. There are five different sizes of wire used in a Triumph (or most British cars for that matter), and over 50 different color codes on the later models (out of a possible 144, 106 of which are available), the exact number depending on the model year. [For an excellent description of the color codes used, and their application, refer to the Wiring Table by Chris Kantarjiev.] British wire, as used in Triumphs, MGs, etc., is not sized by gauge as is American wire. The size of British wire is stated as the number of individual strands of 0.30 mm copper used to make the wire. The sizes available are listed below, along with the maximum current rating of each size:

# of strands current rating

9 strands 5.75 amps
14 strands 8.00 amps
28 strands 17.50 amps
44 strands 25.50 amps
65 strands 35.00 amps
84 strands 42.00 amps
120 strands 60.00 amps

A typical TR6, for example, will use the first 4 sizes, and either the 65 or the 84 strand wire, depending on the alternator rating ( I think the later models with the larger alternators use the 84 strand – I haven’t confirmed this. I know the earlier models use 65 strand.) For any given wire in your existing harness, just strip a short length and count the strands. This will tell you what size to buy for replacement. (you may find some oddballs – somewhere in my collection of British wire, I ran across a piece of 21 strand wire! I have no idea where it came from). It is very unlikely that you will encounter the 120 strand wire, but if you should ever upgrade your alternator to a more powerful unit, this is the size wire you should be using. This wire is available from:

British Wiring
20449 Ithaca
Olympia Fields, Ill 60461

Call them and ask for a catalog. They also supply all the associated wiring supplies, such as connectors, bullet terminals, etc., as well as complete harnesses.

The above article has been provided courtesy of Dan Masters.