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Electro-Tech

Rewiring your Land Rover

The easy way. Use a complete, ready made wiring harness for your particular model. Specialist manufacturers can offer original-looking harnesses with braided textile loom covering and textile conductor sheaths, all in the right colours. Alternatively, you can specify all-plastic insulated conductors and vinyl loom tape. This is a fairly foolproof method provided you have the correct new harness and label everything as you remove the old one.

When I first wrote this section, I could not find any websites belonging to makers of reproduction Land Rover harnesses. Now I have three links to offer you, one in the UK, one in the USA, one in Australia.

Auto Sparks - The worlds leading classic vehicle wiring harness manufacturer.
Long established United Kingdom business. Good catalogue on website. Ask them about any special requirements such as a revised wiring plan for an alternator conversion.

British Wiring Inc. Well known firm located in the USA. Supply Land Rover wiring harnesses and original specification wiring accessories.

Vintage Wiring  Respected Australian specialist located in Bellingen, NSW and specialising in original wiring harness for many types of vintage vehicles.

The hard way. You may wish to rewire from scratch for any number of reasons. Perhaps your Rover is  far from standard anyway with a motor conversion, additional lighting and electric radiator fans. Maybe you have a well-used working vehicle which just needs a reliable, safe, low-cost electrical system. Think carefully, as future buyers may be prejudiced against an obviously non-original wiring harness, especially if your Rover purports to be a restored example.
It is more time consuming, the few specialised tools required are essential and you must be willing to learn new skills.
If you still want to go ahead, it really isn't very difficult.

On this page:

• Selecting your wire colours.
• Selecting your wire sizes.
• Current Draw and Voltage Drop.
• Useful Links!

On associated pages:

• Planning and installing Series IIA wiring
• Making terminations.
• Digital Multimeter and its uses.
• Why and How to Upgrade your Headlamp Circuit
  

Selecting your wire colours

• The stuff used in your original Land Rover harness followed a Lucas colour code in which the colour of the wire and its tracer stripe indicated its function. I have never found it worthwhile to obtain a myriad of cut lengths in all the correct colours. I use four or five colours at most, all obtained locally in 30 metre reels. I take care of further identification by using four different heat shrink colours on the terminations.

• I also make a list of all my wires in which I assign a alpha/numeric code to each wire and record its colour, shrink colour, size, terminal type and where it starts and finishes. In practice, if you can see where a wire starts you can look up the wire-list and find out where it goes. The wire list is an integral part of my planning method and you can read more about it in Planning and Installing Series IIA Wiring.

Selecting your wire sizes

• The Australian made automotive cable I use appears to follow the British system of varying numbers of strands of 0.30mm copper. This wire is plain copper but many builders like the more expensive marine grade cable with tinned copper conductors.  There's no doubt that it would be very suitable and long lasting.   I've even heard of some builders using aircraft quality wire with very special insulation and tinned conductors but I think that is going to extremes. These vehicles are glorified agricultural machinery, not the Space Shuttle.
  Here's a table of the wire sizes used by me, all made by Tycab Australia.

• This wire has the lowest grade of PVC insulation at 75°C rating. PVC is a fair general purpose choice as it is flexible, resists chemical attack and is reasonably tough and chafe resistant. You can also obtain wire with PVC insulation of 105°C rating which is good for use in engine compartments. The more sophisticated non-PVC insulation types have higher temperature ratings and can be made thinner too. Modern car makers like thinner wire because it reduces the bulk of complex wiring harnesses. Often, the conductor sizes used in modern cars and many older cars are barely adequate thanks to the quest for slimness and also cheapness.
• In my latest wiring project, I only used 3mm, 4mm and 8 gauge cable plus a piece of 00 battery cable for the starter motor connection. I never have figured out what the 3mm/4mm/5mm designations mean but I suspect they refer (roughly) to the circumference of the bundle of copper strands. These Australian/British wire sizes are all half a gauge smaller than the nearest American equivalents shown. Putting it another way, 14 AWG wire is actually a bit thicker than Australian 4mm Auto cable. You can apply these Amp rating/resistance figures to American wire and have an additional built-in safety factor. If you want to be exact, ask your wire supplier for figures or visit one of the manufacturers websites listed in the Electro-Links section.
• If purchasing ordinary auto cable, please choose a well-known brand name. The 25 foot mini-reels sold at your local El Mucho Cheapo auto emporium could contain cable made anywhere and it is hard to tell just by looking whether the copper is annealed properly or the insulation meets minimum standards. It is much safer to shop at a specialised supplier of wiring products.
  

Current Draw and Voltage Drop

• It is easy to figure out on paper the current draw of electrical accessories rated in watts, such as lighting equipment.
  If you feed the accessory with cable which has an adequate nominal current rating but is too thin for the length involved, you will have excessive Voltage Drop.
  Consider these facts! A Halogen headlamp is rated to produce 100 % of its rated light output at 13.5V. When voltage at the headlamp drops by 5 % to 12.82V the light output drops to 83 % of normal. When voltage drops by 10 % to 12.15V the light output drops to just 67 % of what it ought to be. A mere 1.35V of lost voltage has robbed you of 33 % of your light output. And it gets worse as voltage is further reduced. These figures come from a leading headlamp manufacturer.
  The situation is no better for the electric motors in your starter system, screen-wiper, heater blower or on-board tyre inflator. Motors with chronic undervoltage supply run slower, hotter and fail prematurely.
  
• All conductors have a resistance to current flow which depends on the material of the conductor, the cross section area (CSA) and the length. In a circuit consisting of wire running from the system supply to the accessory and then from accessory to ground return, the resistance of the wire reduces the voltage at the accessory. Reduced voltage = reduced energy available to make accessory work.The difference between system voltage and voltage at accessory is the voltage drop. If we reduce the resistance of the wire by either reducing its length or increasing its CSA we can reduce the voltage drop.
  Thats why cable makers quote the resistance per metre of their cables, to assist you in calculating which cable size will give you the minimum voltage drop over the required cable length. 
  You'll need to use some simple electrical formulae and a cheap pocket calculator. High School physics students can skip this bit. Those who need reminding may go on to this table based on Ohms Law which states, in effect, that an electric potential of One Volt will cause a current of One Ampere to flow in a circuit with a resistance of One Ohm.

It's a simple arithmetic relationship. For example:
     5 Volts = 5 Amps x 1 Ohm
OR: 5 Amps = 10 Volts ÷ 2 Ohms
OR: 10 Ohms = 50 Volts ÷ 5 Amps
Power in Watts is a function of Voltage x Current.
100 Watts = 10 Volts x 10 Amps
15 Volts = 150 Watts ÷ 10 Amps
15 Amps = 180 Watts ÷ 12 Volts                                                                                               

This colourful diagram is a handy way to recall the formulae. Chalk it up on your workshop wall.
To find the formula for, let's say Current, when you know Voltage and Resistance, place your thumb over CURRENT.
VOLTAGE is over RESISTANCE which tells you to divide Voltage by Resistance.
To find Power when you know Current and Voltage, place your thumb over POWER.
VOLTAGE is next to CURRENT which tells you to multiply Voltage by Current.

IMPORTANT NOTE: Many authorities on wiring recommend a maximum voltage drop of 0.5V or around 3.7% of 13.5V system voltage. Because it's my truck and I'm paying for the wire I like to work on a figure of 0.2V maximum or 1.5%. This gives me a little leeway to compensate for switch resistance or less than perfect ground connections.

• Lets suppose you have a 65 watt headlamp. When your truck is running, the system voltage ought to be 13.5 volts or better. If Current = Power ÷ Voltage then Current = 65 Watts ÷ 13.5 Volts which = 4.8 Amps.
  If it is a 100 watt headlamp then Current =100 Watts ÷ 13.5Volts which = 7.4 Amps.
  
  Looking at the Wire Data Table we might imagine that the 16AWG / 3mm wire rated at 10 Amps would be sufficient for either lamp. However, we need to think about the resistance of the wire and the likely voltage drop between battery and headlamp.
  So, lets consider the wiring to that 65W headlamp drawing 4.8A. It goes from your fusebox to the light-switch, then down to the floor mounted dimmer then up again and forward to the headlamp. Maybe as much as 7 feet (2.13metres) of wire. If the ground connection, lamp to chassis is good, the return path to your battery can be assumed to have negligible resistance. If ground return is through a second wire all the way back to a central ground point, then the resistance of the ground wire must be taken into account.
  
  The table says that 16AWG / 3mm wire has a resistance of 0.0165 ohms per metre. Thats .035 ohms over a 2.13 metre run.( 0.0165 ohms x 2.13m )   If Volts = Current x Resistance then Volts = 4.8A x .035 Ohms which = 0.16V. Thats less than 1.2 % of 13.5V so everything should be OK.
  
  Now, about that 100W headlamp drawing 7.4A.  The calculation is Volts = 7.4A x .035 Ohms which = 0.26V. That's 1.9% voltage drop which is not acceptable in my opinion, especially when we haven't even thought about the resistance of the two sets of (old?dirty?) switch contacts or of the ground connection from the other side of headlamp filament to the chassis. Go to 14AWG / 4mm wire which will reduce the voltage drop to 0.17V. Much better. Even better for a high wattage lamp will be a relay. See Why and How to Upgrade Your Headlamp Circuit for more about adding relays.
  
  Maybe your 100W lamp is really a work-lamp mounted on the back of a 109 with 14 feet (4.26metres) of cable fed neatly through the chassis rail and up the rear bulkhead to the lamp. Even if you use 14AWG / 4mm wire the resistance will be .0101ohms x 4.26metres = .043 Ohms so the voltage drop will be 7.4A x .043 Ohms which = 0.32V. No good!
  Wire of 12AWG / 5mm will give a voltage drop of 7.4A x .027 Ohms = 0.19V. Thats acceptable to me.

• These worked examples have shown you how to figure the current draw of your various accessories and select the wire sizes you will use.Most individual electrical accessories draw far less than 10 Amps.
  I use 4mm cable for short runs to lights, ignition systems and electric motors in and around the engine bay.
  The 3mm cable is useful for low current things like instrument sensors, small instrument lights and relay coil feeds.
  The 8 gauge battery cable is for high current applications like the main battery feed, the alternator output, the electric radiator cooling fan and the main feed to the headlamp relay. In theory, there is no reason why you should not wire the whole vehicle in 50 Amp cable except that it would be expensive, pointless and the harness would be impossibly thick in places.

• Use a cheap Digital Multimeter to check current drains and chase voltage drops.

Useful Electrical Links

• AUTO SPARKS - The worlds leading classic vehicle wiring harness manufacturer.
  Long established firm located in the United Kingdom. Good catalogue on website. Ask them about any special requirements such as a revised wiring plan for an alternator conversion.

• British Wiring Inc. Well known firm located in the USA. Supply Land Rover wiring harnesses and original specification wiring accessories.

• Vintage Wiring  Respected Australian specialist located in Bellingen, NSW and specialising in original wiring harness for many types of vintage vehicles.

• Daniel Stern Lighting
  Commercial site with many no-nonsense articles on automotive lighting.

• Bayou Rovers
  Enthusiasts site with useful articles on alternator conversion, ground conversion and wiring.

• C.J.'s Junkyard
  Contains a small archive of articles on 12V Yacht electrics.The discussion about voltage drop, wiring sizes and fuses can be related to Land Rover electrics.(Yachts and LR's both damp, draughty, rot-prone, bottomless money pits, right?) Look out for the very interesting on-line wiring calculator.

• Ancor Marine Grade Products
  The place to go for tin coated marine grade primary wire and high-grade marine wiring terminals. Many Land Rover owners say that the extra expense is worth it. Extensive technical articles here.

• Outback Marine Australia
  Gold Coast based Australian distributor for Ancor Marine Grade Products.

• General Cable Corp.
  US based company. More than you can possibly need to know about higher grade insulation for automotive primary wire.

• Industrial Electric Wire and Cable Inc.
  Another US based maker of automotive primary wire.

• Chris Dows Land Rover Pages
  Enthusiast with a Series IIA 88. Much information about the work he carried out on his electrics including alternator conversion and conversion to single-motor windshield wipers.

• Land Rover FAQ Part IV
  Managed by members of the OVLR Club. This link takes you direct to the Series electrics section with valuable information on alternator conversions.

• The Series Shed
  Fine enthusiasts site conducted by Joseph Broach. Excellent full colour circuit diagram for early petrol IIA with more to come, he says.

• Triumph Maintenance Handbook
  Conducted by the Vintage Triumph Register. Not a Land Rover site but the Lucas electrics on Triumphs were closely related. Good electrical section on Lucas alternators and other topics.

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Legal stuff: These pages have no connection with Land Rover, BMW or any of their authorised agents. The information presented herein comes from my personal experience and I guarantee the veracity of none of it. Visitors should exercise their own judgement and seek expert advice about local vehicle laws before applying any of my practices to their own vehicles. The fearsome copyright notices seen on some web-pages probably aren't worth the bytes wasted on them. If you want to rip off any of my stuff for non-profit purposes please let me know and give me credit.