How To Rewire a Car -- The Easy Way

8/11/2019 How To Rewire a Car -- The Easy Way

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How to rewire car the easy way

Often amateur car builders will spend an enormous amount of money and effort on suspensionand engine modifications, then totally bodge the cars electrics. I have in the past witnessed some trulyamazing lash-ups, using household 3 core wiring and even the offcuts of ring mains wiring. I think thisis because people just decide that electrics are to complicated and they cant or are not able to under

stand them, so they dont even try to. If it helps, you can think of electrics in a similar way to plumbing,!as it used to be at the beginning of the "#thcentury$ the voltage can be thought of as pressure andcurrent !amps$ as flow. %hus if you need to fill a receptacle with water in a fi&ed amount of time youcould use a large pipe at low pressure !'"v$ or a small pipe at very high pressure !"(#v$ this may helpyou to under stand things later on.

)ere I hope to show you how easy it is to rewire a car from scratch. *irst it is generally not agood idea to try and reuse an e&isting wiring loom on your kit car or special. !unless its essentially thesame car, i.e. +utton ierra.$ However engine management wiring tends to be largely separatefrom the donor cars main wiring loom, and should be reused unchanged or slightlymodified.ngine management systems can be very fussy about the uality of the wires and connections.

I will describe a basic wiring installation and tell you how to fit it into your chosen vehicle. *orthe purposes of this article I will use a +utton /haeton ' ! 0 1otus even style kit car$ as an e&ample,but it is easy to apply to any car. 2sing this method I have rewired kit cars and specials, once over aweekend at a car show.

It is worth noting, that in my e&perience the vast majority of electrical faults are caused by pooror faulty earths, a lot of the rest by loose or faulty connections. It is rare for a wire to fail, unless it hasbeen overloaded. !by usually faulty earths or connections.$ so spend some of your hard earned on somedecent crimping pliers to make the terminal connections, it is very important these connections aregood. 0nd it will pay dividends in the long run. %he only other really important tools are some decentwire strippers, and a cheap multimeter for basic voltage and continuity checks.

ome of the things I am going to say here might not be actually '## technically correct, but toprovide you with an understanding they are good enough, and we dont want to confuse things toomuch. 0lso the diagrams given are very basic. !%he sort of thing you would e&pect to find on manycars up to the early 4#s$ )owever if you feel able to cope with this level, then the e&tra complicationof a more modern wiring loom is not a problem to add on. ome of what you may need will bee&plained by myself later on.

+o not be tempted to try and reuse an e&isting wiring loom from a donor car, especially if yourbuilding a kit car or radical special, and dont try to salvage the individual wires. %here is a case forsaving the connector blocks. ! e.g. back of the headlamps$ In which case cut them from the donor loomwith about 56 of wire still attached, and join them to your new loom. !by soldering preferably.$

)owever as previously noted it is definitely worth reusing engine management wiring looms as theseare often largely separate from the donor cars loom, and the whole .7.2 and wiring is usually bestused intact and unchanged.

%he ideal method of rewiring a car is to buy several bulk reels of wire all with the same colourinsulation. !black is best$ ither rated at the highest consumer amperage, or in several differing ratings.e.g. 40 , '(0, 3#0. One reel each of about (# meters length will do for starters, and select accordingly,it will not be e&pensive.

0t this point I should point out that wires and switches are rated in amps but consumers !bulbs


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and motors etc.$ are rated in watts. 8atts are simply volts multiplied by amps, so to determine the amprating of any wire or switch needed to supply a consumer of '"# watts, !two headlamp bulbs$ youdivide '"# watts by '" volts and get '# amps, easy huh9

0s a rule of thumb consumers will be the same rating in watts at whatever voltage they operateat and switches and wires will likewise keep the same amp rating at whatever voltage you use them.

0 good e&ample of this in action is to compare your cars starter motor to your house holdvacuum cleaner they both are rated at about '### watts but whereas the vacuum cleaner reuires only

wires and switches rated at : amps, your starter motor needs close to '## amps. 0nd thats why thestarter motor lead is very thick, in fact as you operate the starter motor it can draw as much as '###amps9 ;ou should plan your wiring harness route !see sketch for suggestion$ and run all of your wiresalong this route where possible. ven if it means that the wiring is ( or 5 times longer than the directroute. e.g. Oil warning lamp to oil pressure switch direct might be only " or 3 feet.


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that under ideal and damp free conditions it will be the case that scotch locks will make very good andlong lasting connections. )owever my years as a mechanic, fitting alarms and tow bars etc. and thensubseuently fault finding on them lends me to believe that these conditions do not e&ist in theautomotive industry. *rom personal e&perience it seems that scotch locks eventually have a similareffect as hammering flat a section of water pipe would do, so benefit from bitter e&perience, and takemy advise on this matter.

%he vast majority of connectors will be the blue colour ones, and of these they will largely be theinsulated female spade connectors. ;ou will also need some male spade connectors, some in-lineconnectors !the blue tubes, not the bullet type$ some piggy back connectors, and some ring terminals ofvarying sizes. 0lso you will need some chock block connectors, but not the smaller sizes. !you mayhave to stuff two or three wires into each connection$. 1astly you will need a selection of differentcoloured insulating tape, namely@ red, yellow, green, blue, brown, black, white, and a roll ofyellow=green earth tape. %hese should be available from where ever you got the bulk connectors from.

0bove are a selection of the most likely used crimp connectors in a wiring loom, they are fromleft to right, large ring connector !for connecting to the battery$ smaller ring terminals for makingvarious earth connections, an in line connector for permanently joining wires together !and making aspur connection.$ *emale insulated spade connector, and a piggyback connector.

Aodern cars and their wiring diagrams in the readily available manuals in most car accessoryshops can be ferociously difficult to follow and understand. Aine I hope, are not. In all of my diagramsthe left hand side will be Blive6 !'" v$ and the right will be Bearth6 !# v$. *or ease of understandingassume every component has a Bhot side6 !>'"v$ and a Bcold side6 !not necessarily at #v$ so even if afuse blows one side of it will still be Bhot6 at '"v. %his is very important to remember, so for e&ample,be careful how you connect things to the fuse bo&.

lectrically speaking one end of a wire is the same place as the other end, thus if multipleconsumers need to be connected to a common supply point you doNOT have to run multiple wires tothe same point but could link them all together with just one wire rated at the total amperage of thewhole number of consumers on it. !so ideally your battery should have only two connections to it.$

figure ' below shows how four different looking circuits, can in fact be electrically the same.It is very important that you realise that these four circuits are in fact the same electrically.


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figure " below shows a very similar looking circuit that is notthe same electrically as figure ', in thatif the fuse blows, two of the bulbs will stay lit.

*our bulbs lit from a single fuse

If for some reason you choose to have, or are obliged to have a earth return system, !i.e. 0n earthwire back to the battery instead of using the chassis of the car$ ensure that the earth wire you choose is

able to handle the amperage rating as if it were to supply all of the components, and this will reduce thepotential for problems at some future date.

%o a large e&tent where you wiring loom will go is decided by your car = engine lay out.)owever one major component is down to your personal choice, namely the fuse bo& !> relays if used$my choice would be the n=s bulkhead under the bonnet as high up as practicable, and close to thebattery if possible. Ideally the battery to starter motor cable should be as short as practicable, but anyother wires should take your chosen route around the engine bay. If for instance your oil pressureswitch was at the rear and offside of your engine then this is where your new wiring loom would start.%he wire would run out to the o=s of the engine bay and towards the front of the car, across the front tothe n=s and back to the n=s bulkhead where it would connect to the chock block under the n=s dash. %he

reason for choosing the n=s dash for the wiring loom route, is for better access. If you have a left handdrive car reverse the planned route as shown, !although the n=s will be on the right in this case$ OC usethe same route but then run the loom across the front of the bulkhead and enter the car on your n=s !theright hand side$

0lso start wiring your car from the engine bay and back to the dash. Its much easier than theother way round. %rust me. 2nder the dash you will see that I have sited four chock blocks all of whichhave numbered connections. !from ' D 3".$ 2sing this method to construct your loom is very easy toboth make and eventually fault find on if necessary.


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to modify at a later date or to correct any mistakes as you construct the loom. On each of the circuitdiagrams you will notice BE6 and a number along side, this corresponds to the chock block connectionunder the dash for that part of the wiring loom.

ee the primary circuit below.

%o the novice it would appear that when looking at the circuit for Bprimary circuit6 that the mainbattery supply lead goes to the starter motor, alternator, fuse bo& and the ignition switch via chockblock connector no.F. /lainly this thick !'#mm>$ cable only goes to the starter motor.


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taying under the bonnet we go back to the alternator and connect the charge e&citer wire!battery warning light$. %he alternator !lucas types$ usually have a plug with 3 spade connectors in it, "large and one smaller. %he larger one!s$ connect to the battery !via the starter motor if it is closer$ the

smaller spade connector is the charge e&citer connection. !battery=ignition light$ Cun this wire aroundthe engine bay as shown in the plan sketch, routing the wire through the n=s bulkhead and to the chockblocks connection no.3. Ge&t the starter solenoid Bstart6 wire via the same route to no.(. %he oil senderas above to no.'. 0nd again for the temp sender to no.". ;ou will notice that the starter motor,alternator, oil sender and temp sender are all shown as having Bwire6 connections to earth, but this isnot the case, as these components earth through their bodies. %hey are drawn like this to basically makethe wiring diagrams tidier.

%ake a wire from the -ve !negative$ side of the coil and run it to the distributor, this wire will notusually follow the path of the main loom, however it doesnt hurt to do so if possible. *rom the >ve!positive$ side of the coil follow the loom back to connection no.:.Note!this connection can be usedfor the live ignition feed for the 72, and the 72 will control the coil etc. 1eave the fuel gauge and

reversing light until later, but dont forget them.

Gow go to the dash !if fitted$ and take a heavy duty wire from connection no.F !colour code it$and run it to your ignition switch feed. !a "#-3#0 rated wire should do it$ *rom the first switchposition, !usually au&.$ run a similar rated wire to connection no."3 and from there to fuse +. 1abel allfour ends of the wires. *rom the second position !usually run$ again take the same rating of wire andrun it to connection no."" and then to fuse 0, link it to fuse < and fuse 7. 0gain label all ends of thewire the same at each end. 0t the ignition switch end of this wire use a Bpiggy back6 connector, andfrom this link !using a lower rated wire obviously, a 3-( amp rating will do$ over to the batterywarning lamp, use another piggy back connector and link it over to the brake warning lamp, at this endof the wire use another piggy back connection and link this one over to the temperature gauge >ve

terminal and then the >ve terminal for the fuel gauge. !or the voltage regulator >ve connection for thetemp and fuel gauge, which will then link from the -ve connection over to the temp and fuel gauge >veconnections$ these two will then link down from their -ve terminals to the chock block connections no." and no.5 respectively. %he reversing lights derive their power from the same source, but it is mucheasier to just link connection no."# and no."" on the chock blocks !its the same place electrically$.*rom the battery lamp run a wire to no.3, from the brake warning lamp run a wire down to no.3".%hese two wires will be colour coded at both ends to match the chock block connection. Cememberfrom all these dash connections to run your loom in a single neat cable run under the dash and wrap itup when finished. *rom the chock block connection no.3" there will be one wire in and two wires out.7ode all three of these the same, one will go to the handbrake switch.!OC some other remote switch,


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for the sva test$ %he other will go to the brake master cylinder reservoir cap. %he handbrake switchwill earth itself through the chassis, but you will need to provide a small wire to connect to the secondterminal of the brake reservoir to earth. 8hen you route the wire for the handbrake switch, follow theloom routing as much as possible and spur off with the reversing light switch, as these will both likelybe near each other when finally fitted.

%hats about it for the Bprimary6 circuit e&cept to note that the earth lead to the battery should be

left disconnected until every thing is finished. 7onnect the main earth lead to the engine direct, and usea second lead to connect from the engine to the chassis. !or to a metal bracket somewhere convenient$.If you use the metal bracket for an earth return system, connect your earths to this bracket and not thebattery. If you do have anything under the dash that needs earthing, link them together with one wireand run it to the chassis or to connection no.3'.

ust to give you some idea of what the fuse bo& will look like, !only fuse 0 D * in this e&ample$you can see that fuse 0,


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lamp, then connect as shown for the alternative dash light.!it wont need earthing if you do it this way$

Note about Ha"ard and indicator switches.

7ar manufacturers have for years standardized wiring colour codes, here is what the typicalconnections will look like if you dont already know.

)azard switch !and indicator switch$ colour codes.

1ight green - pink tracer *eed to indicator switchKreen 1ive feed for indicators, from relayKreen D white tracer o=s !right hand$ indicator bulbsKreen D red tracer n=s !left hand$ indicator bulbs1ight green D purple tracer hazard warning lamp


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%he circuit from fuse < controls the washers,wipers, heater and brake lights. ;ou will notice thatthe wiper motor and heater motor do not have any connections on the chock blocks. %his is because Imassuming that these components live under the dash and therefore wont need any connections on thechock blocks. Cemember that all these components can be all linked from one wire, piggy backed fromone to other. o from no."F run a wire to the wiper switch piggy backed to the heater motor switch,then on to washer switch !if separate from the wiper switch$ then on to the brake light switch.

%he circuit for fuse 7 is very simple, it just goes direct to the engine cooling fan sender and fromthe -ve side of the sender you could connect two wires !if you are the usual paranoid kit car owner$ onegoes to the >ve side of the radiator fan, !the fans other connection goes down to earth$ the second wireruns back to the chock block connector no.4. 7hock block connector no.J gets its power from asecond connection to fuse 7. If I have to e&plain fuse + and fuse ) then your reading the wrong


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article. 1ikewise fuse shouldnt pose any problems

*use is for the horn, and here is the odd ball in the automotive industry. )orns can be singleterminal or two terminal and they tend to be wired differently. ingle terminal horns are supplied with alive via the horn switch, !upply from *use , run to ": and on up to the horn switch and then backdown to '# and on down to the horn.$ twin terminal horns can be either as above OC have a live feedto them and the horn switch provides the earth. !horn buttons in center of steering wheel are this type.$

o either wire the horn as shown above or run a live from *use to the horn, and then connect back upto chock block connection ": or '#, and on to the horn switch.!which will earth the horn$

*use * is for the lighting circuit. It should present no problems if you have tackled the rest of thewiring loom so far.


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"" Ignition switch to fuse 0, '" are linked together. 0s are connections '( to '5, and 'J to '4. 0s could be "#to "" if reversing lights are used. !these two are on sepperate chock blocks.$

Once you have all of your connections have been made, gather all your wires together and wrap

a short piece of black tape around the loom at various intervals especially at any junction = spur off. %hewhole loom will need wrapping in something to protect it. %he best thing in my opinion is a type oftube cut in a continuous spiral which will easily wrap round the loom. It will make the loom uite stiffand tidy. 0ll that is then reuired is to secure it to the bodywork = chassis at various intervals. +ont betempted to use ordinary black insulating tape to wrap the loom as you wont be able to do the job in onego. 0nd as a result it will come undone uite uickly, leaving a sticky mess. /rofessional wiring loomsare wrapped in a kind of black cling film tape, which you may not be able to easily find. ;ou could uselong lengths of shrink wrap tube.

sing relays.Aost modern cars use switch gear which is not able to handle the current reuirements of the

switched consumer, unlike most cars prior to the early 'F4#s. o as a result it is necessary to use relaysto switch the current loads. Celays are basically remotely operated switches and are very easy to add into any circuit. Celays can be earth switched or live switched !see fig. 3 below$ it makes no difference tothe relay. 0lso using two relays in series can make a circuit conditional on two devices being switchedon to operate it. e.g. )eadlights !but not the sides$ only to come on when ignition and side lights arealso on. !see fig :. below.$ Celays should be placed near to the fuse bo&, and need to be properlysecured to the body work of the car. ither by the tag on the relay if supplied, or by using a cable tie tothe actual relay body its self. It is not a good idea to just leave them dangling. 0nother way to make arelay conditional is to choose the switching current to come from a separate fused supply to theconsumer load, which may also be switched.


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Aost relays will have a small wiring diagram printed on their side and very often the terminalswill be numbered on the underneath. %hese numbers will either be some thing simple from '-: !or ($ orwill follow an international standard, the supply and output connections are numbered 3# > 4J, thetrigger connections are numbered 4( > 45. ome times you will come across a ( pin relays, in whichcase the fifth connection is a second output !4Jb$, this will either be a change over relay, where by

activating the relay you switch between output 4J and 4Jb, or activating the relay will switch both on atthe same time. %he relay will usually say on its side somewhere.

*ig :.


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1ets take the lights circuit as an e&ample of the need to use relays. In the top most circuit above,I have shown you how to add relays to the dip lights circuit and the main beam circuit. If you are going

to add relays in to any circuit it is best to ensure they are supplied by their own separate fuse, hence thefuses are labeled I > . %hese are best supplied from the >ve !hot$ side of fuses * K ), the batterysupply direct. %he lights are powered from the relays not the switch, the switch is used to trigger therelays. Instead of the dip lights and the main beam connecting directly to the chock blocks at no.'3 andno.':, connect to the relays, the switches trigger the relays via the chock blocks at '3 and ':. If youwere to connect the lights in this way the lights will still come on when the switch is thrown.

If however you would prefer only the side lights to stay on when the ignition switch was off andthe lights switch was on !as parking lights$ then add the relay shown in the lower half of the circuitdrawn above. %his will supply the power for fuse I > . %he advantage is, that when the ignition is offthen the dip and main beam lights cannot come on because the relays for fuse I > are not activated. 0s

soon as you turn on the ignition then the lights will come back on. )owL 8ell because fuse 0 < 7 areonly active when the ignition switch is on, then by connecting the relay to the >ve !hot$ side of thesecauses the relay to come on only when the ignition is also on.

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How To Rewire a Car -- The Easy Way