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Tutorial - car electrics.

Discussion in 'Common Faults and Questions' started by cowasaki, Jan 3, 2014.

By cowasaki on Jan 3, 2014 at 9:41 PM
  1. cowasaki

    cowasaki Well-Known Member

    Jun 17, 2013
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    North West England
    Cowasaki's electrical tutorial part 1

    Part 2 of the tutorial has now been written with specific instructions for tests and checks.....


    Having seen numerous simple questions on car electrics/electrical systems I thought I would write a simple tutorial. I will create a few more on more specialised topics later if people find them useful. My way of thanking people for all the help they have given me over the past 6 months since I got my first landy.....

    Vehicle electrics:

    1 - The basics

    Electrical systems are basically simple if broken down into small manageable parts. A basic electrical circuit consists of a power source (shown here as the battery), come connections (the wires shown here as green lines) and a load (shown here as a bulb). This is basically the simplest circuit.


    As you can see from the above diagram the bulb is connected to the battery by two wires which creates the circuit. One side of the battery is shown as positive whilst the other is shown as negative. For our means, working on cars, there are several different ways of referring to these sides of the battery:

    Positive = 12v, live, battery
    Negative = 0v, ground, chassis, body

    Bulbs are examples of devices that require being in the circuit but do not require being in the circuit in any particular direction so will also work if the positive and negative are swapped around as long as there is still a circuit. Other examples of components that can be wired either way with the same effect would be a heater element, relay or a buzzer.

    Other components can be directional in that their operation reverses if the connections are reversed or that they only work one way or the other. Two components that you will often see in a car which have this characteristic are solenoids and motors.

    If we add another bulb to the same circuit it can be connected in two ways:


    This example shows what we call series, each bulb is connected one after the other.


    This example shows what we call parallel, each bulb is connected independently to the battery.

    In a car we like to use the parallel method when connecting bulbs as with the first method if the bulb failed then both bulbs would fail. Old Christmas tree lights used this method whereas the indicators on your Landrover use the parallel method so if one fails the other still works.

    If you follow the two examples you can see that you can get from the battery to the bulb and back to the other side of the battery only by going through the other bulb in the series example but in the parallel example you have two paths. By removing one bulb completely you could still get around from the battery through the bulb and back to the battery.

    When we have components in parallel we can treat each section as a separate series circuit. By breaking this circuit that component fails to operate and this is how things turn on and off in our cars.

    The simplest way of breaking the circuit is to remove a wire so here is the circuit again twice with one of the wires disconnected in each example:


    Both these circuits do the same in that they do nothing until the wire is reconnected. It does not matter that the disconnected wire is the negative or positive.

    The first example has the negative wire disconnected and this is referred to a negatively switched, the second example has the positive wire disconnected and this is referred to as positively switched. Cars use this all the time and a good example are the dash warning lights in the Defender. Some such as the “side light warning” are positively switch i.e. the negative is permanently connected whilst the positive is only connected when the side lights are switched on. Other circuits such as the diff lock warning light have the positive permanently connected and the ground or negative is connected by the action of activating the diff lock.

    If we introduce a new component, the switch, then this circuit becomes a circuit you will be familiar with:


    This is effectively the circuit diagram of a torch! We have a battery, a switch and a bulb. The switch breaks the circuit and stops the bulb lighting. The switch can be anywhere in the circuit as unless the circuit is complete it will not light.

    In cars we have a term “negative earth” or VERY rarely “positive earth”. This refers to the fact that on a car we often make one side of the circuit using the body shell. In the usual negative earth systems such as those used by Landrover we can treat the body of the car as if it were one giant negative wire.


    In this example the battery is connected to the earth/ground/body and the positive goes via the switch to the bulb which is in turn connected to earth which makes the circuit. This explains why the lights often stop working on a Landrover because the earth connection gets broken. It also explains why often we have strange faults which appear to be unrelated but are in fact a faulty earth. From your knowledge now you should be able to check this, connect a jump lead from a good earth point to our suspected bad one. If the fault goes away you have found the fault.
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Discussion in 'Common Faults and Questions' started by cowasaki, Jan 3, 2014.

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    1. cowasaki
      Vehicle electrics:

      2 - The definitions - Voltage, current, resistance and power


      In most circuits we refer to the voltage of the power supply. In most cars this is nominally 12 volts (most cars reach to nearly 14 volts in operation) this figure refers to the potential difference between ground or earth and the output of the battery or power source.

      If you look at the simple circuit for a small light and we consider that this is a 12v standard car then the switch and bulb need to be rated for the voltage in question so again 12v.

      What happens if you use the wrong voltage?

      Well if you connect a 6v bulb into the circuit it will either blow up, glow more brightly or glow brightly for a short time and blow. Remember though that we have 12v….. If you connect TWO bulbs in series then the 12v goes across both the bulbs which between them can handle 6v each so 12v in total.

      And if you connect a 24v bulb into the circuit it will either not light up or glow dimly.


      Checking or measuring voltage


      Voltage is the potential difference between two points in a circuit and is measured in VOLTS (V – in equations). We measure this voltage in reference to ground/0v/earth so with our multi-meter set to VOLTs we connect the black (ground) to earth which is shown as (A) on our diagram and then connect the red to the place we wish to measure. So to measure the battery we would connect the red probe to (B). This should show 12 volts (or more like 13.5v) if the battery is charged. If we now move the red probe to location (C) the reading will be 12v with the switch switched ON and 0 volts with it switched OFF. If we follow the circuit round and connect the red probe to (D) if will show…… 0 volts because there is no potential difference between that part of the circuit and earth, that part IS earth. If we, however connect the black to this point and the red to (C) we again get 12 volts.

      Looking at a series circuit containing two 6 volt bulbs:


      The values are the same with black connected to (A) and red connected to (B) because the potential difference between (A) and (B) is still 12 volts. From (A) to (C) again we get 12 volts.

      Now we get a little more complicated. If we connect black to (A) and red to (D) the potential difference is the difference across the 6v bulb (1) which will be 6 volts.


      If we go to Wikipedia we will find the definition:

      “An electric current is a flow of electric charge. Electric charge flows when there is voltage present across a conductor.”

      So a 9v battery is a 9v battery even if it isn’t connected to anything because there is still a potential difference of 9v from one side to the other. If you place the 9v battery into a circuit and connect the circuit up (electricity) electrons flow around the circuit. Current is what we call this flow of electricity around the circuit. The amount of current flowing around the circuit is measured in AMPHERES or AMPs (I in equations – Blame the French). As you will remember from the voltage section the voltage around a circuit changes depending on where you are in the circuit and is usually measured against ground or 0 volts. With current the measurement is the flow of electricity around the circuit and so the current is the SAME everywhere around that circuit.

      In the voltage example I stated that the voltage rating of the components in the circuit must add up to the voltage supply from the power source. With current we need to make sure that the components all need to be rated high enough for the current to flow.

      To measure current we must BREAK the circuit and install the meter (set to AMPS) into the circuit! So if we again look at the example:


      The current at all points is the same (A), (B), (C), (D) or (E). To measure it we break the connections and include our meter so:



      Resistance is futile…… Well no, resistance is what is attempting to stop the flow of electricity. All components have a resistance (although this might be tiny) and this is measured in ohms (R in equations). This is something we don’t necessarily need to know as much about whilst fixing or upgrading our cars but a knowledge of it is definitely worth having. If we take our switch out of the circuit and connect our meter (set to ohms or resistance) across it the display will show close to 0ohms if the switch is ON and --- or infinity if the switch is OFF. Ie the switch is either allowing electricity to flow across it or not. Using this you can test components such as wire or switches quite easily.

      Ohms law

      Oh no it’s back to school time….. V=IR or I=V/R or R=V/I


      So if we know two we can work out the third. This can be useful if we need to know the power requirement of the wire needed to hook something up. I don’t want to get too heavy with this tutorial so I’ll leave it here for now.


      This is the RATE at which electrical energy is transferred by an electrical circuit and is measured in WATTS (P – in equations).

      The equations for power is P=IV or I=P/V or V=P/I


      Lots of items in cars are specified in WATTS eg 55w for a bulb so we can take this specification plus the known value of the voltage and work out the current flowing through the circuit……

      I=P/V = 55/12 = 4.58 amps

      We know from the section on AMPS that the same value is all around the circuit. The equation gives 4.58 amps which means that the wire and switch must also EXCEED this value so we would look for a switch capable of 10-15 amps and wire for 15amps minimum. The expected current is 4.58 amps but the bulb may take more power initially and with specification being approximate there current might be plus or minus 10% at least which means that it could quite easily be 4-5 amps. So I would protect this circuit with a 7.5 amp fuse (note we will get on to fuses in components)
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    2. cowasaki

      As we are not planning on teaching design you don&#8217;t need to know a great deal about batteries. You will see a few details in the specifications:

      Voltage (V)

      Capacity (Ah) Amp hours

      This is the number of amps it can supply when fully charged for that number of hours. So my Landrover battery is 96Ah. When fully charged it can theoretically give 1 amp for 96 hours, ½ amp for 192 hours, 2 amps for 48 hours etc.


      Starting battery
      - This type of battery is what is normally fitted to a car, it is designed to be able to provide a lot of current instantly for starting a car. These batteries however do not like to be flattened and recharged constantly and work best if kept at a decent level of charge.

      Deep discharge battery/leisure battery &#8211; This type of battery is what is normally used in things like caravans or often as a second battery in a car. It is not designed to give a massive amount of current at one go but is designed to survive being flattened and re-charged (although it&#8217;s best not to leave any lead acid battery flat)

      Split charge systems

      There are several ways to do this but the simplest way is:

    3. cowasaki

      The simplest electrical device we have on the car but one which creates so many issues. Basically it requires 12v plus GND and the lack of either one of them will stop the device working. The most common problems on a Landrover in relation to lights are faulty grounds and dodgy connections usually the bullet connectors.

      The easiest way to check ground is to get a jump lead and connect it to a good ground source then touch the GROUND side of the light. If the problem is fixed then you have your answer. The easiest way to check the 12v side is with a bulb probe (see tools). Connect the spring attachment to ground and then touch the 12v side. Does the probe light? If not there is no 12v. If it does then it could be the bulb.
    4. cowasaki

      In section 1 we learned about the simple switch:


      This component stops the flow of electricity by breaking the electrical circuit. The switch on the left is a normal light switch, the switch in the second is a switch often used in electronic projects. We cannot see the back of the light switch so cannot say any more about it but we can see that there are two contacts on the second switch. Having two contacts means that it is a simple switch ie it breaks the connection between the contacts whilst the switch is in one position and makes the connection whilst the switch is in the other position. It is also referred to as a SPST switch ie single pole single terminal

      The third switch and fifth switches are normal SPDT (single pole double terminal). This type of switch basically diverts the output of the switch to one of two output connections. Another diagram might help here:


      So you can see here that with the switch in one position bulb 1 lights then with the switch in the other position bulb 2 lights.

      A table showing this looks like this:


      The fourth switch is technically a SP3T switch having three possible positions ie middle, &#8220;I&#8221; and &#8220;II&#8221;. The switch is a special one though with the middle terminals hidden so that it looks like a SPDT. Basically it normally sits with no contacts made and clicked to &#8220;1&#8221; it makes contact in one direction whilst clicked to &#8220;11&#8221; makes contact in the other direction.

      A switch can however change more than one circuit at the same time and an example of this are the DPST or DPDT switches:


      This switch is again best shown using a circuit diagram:


      A table showing this looks like this:


      You can see the switch in the diagram, which looks like two normal switches but with a dotted line between them. This shows the switch turning on two bulbs independently. This is just to show how it works but in reality you wouldn&#8217;t do it like that. In this diagram bulbs 2 & 3 will light with the switch in one position and bulbs 1 & 4 will light with the switch in the other position. These switches are typically used to change the direction of a circuit but we will come to that in the section on motors.

      Switches, especially in cars, often have other contacts, which illuminate the switch and are regularly designed specifically for the purpose. You will usually be able to work out what each connection is and can find the illumination normally using a 9v battery with a snap connector attached, by looking on the internet or getting the data sheet.
    5. cowasaki
      Solenoids/Motors/Relays and electro magnets.

      A solenoid, an electric motor, a relay and an electro magnet are basically related devices. An electro magnet is a magnet that becomes a magnet only when a current is attached. A solenoid is an electro magnet that pulls a plunger in or pushes it out depending on the current’s direction, a relay is a switch moved by a solenoid rather than a finger and a motor is a perpetual rotating electro magnet. This is why these names often are interchangeable.

      You can make an electro magnet using an iron core some wire, a switch and a battery. Just to show this here is one I created earlier. This is a long iron bolt with solid single core wire wrapped around it. Then one end of the wire goes to – whilst the other end of the single core wire goes to + via a simple switch. This is powerful enough to pick up a nail or something like that.

      Solenoids are used in a cars in several ways for example in a central locking system, valve or solenoid controlled switch/relay.


      A just to show you the circuit diagram of the above device:

      Last edited: Jan 3, 2014
    6. cowasaki

      Motors attach to a circuit just like a bulb but unlike a bulb a motor’s connections are directional. So if we swap the connections around the motor rotates in the other direction.


      It would be a pain having to detach and re-attach wires to change direction but we can do that with a DPDT switch:


      This circuit looks complicated BUT if you follow the circuit it is not THAT complicated. Basically the + and – enter the switch via contacts 2 & 5 so if we look at the table from the DPDT switch:


      And then look at where they go to we can see that 1 & 6 are connected to one side of the motor and 3 & 4 are connected to the other the table can be seen as follows


      So basically in position 1 the + goes to the motor’s + and the – goes to the motor’s – but with the switch in position 2 the battery’s plus goes to the motor’s – and vice versa.

      If we use a normal DPDT switch then the motor will always be going one way or the other but if we use a switch like the 4th one in the switches section ie one that has no contacts in the middle position then it will not move at all unless the switch is pressed to “I” or “II”. You COULD use this to control a window motor for example.

      **NOTE** we need to consider the current of motors. A switch will have a maximum current rating and this needs to be safely more than the maximum that the motor uses. If not then we can use a RELAY or a solenoid controlled switch.
    7. cowasaki
      Electric window motors

      Electric window motors are just normal motors so if we connect them to 12v they go one way and if we swap the connections around the motor rotates in the other direction and the window goes the other way.

      The normal way to connect an electric window motor is to wire it using the universal 5 pin window switch


      The connections of these standard window switches is as follows:


      So you can see that when nothing is pressed both sides of the motor get GND and nothing happens. When DOWN is pressed 12v from pin 3 goes to pin 5 and the motor rotates sending the window down whilst when UP is pressed 12v from pin 3 goes to pin 1 instead and the motor rotates the other way sending the window up. I am showing this with the supply connected to the battery via a fuse and during these tutorials I will do that just to show it’s getting 12v but in reality I would connect it to IGNITION LIVE which is the 12v power connection when the key is turned to ON.

      There are alternative wiring diagrams using different switches.

      Universal illuminated 7 pin switch

      This will be as above with the 2 extra pins for the light. Connect as per suppliers instructions although this will normally be on connection to ground and the other to the side light on OR ignition on circuit depending on how you want them to light.

      Universal illuminated 6 pin switch

      This is as per the 7 pin switch but with this switch the ground connections are fixed so it takes ground from there and the 6th pin is connected to side light or ignition

      Discovery pre D1s 1995

      as per 5 pin universal switch.

      Discovery D1s 1995 onward 6 pin switch

      These are a great option for defenders as the switch is virtually the same size as the emergency light/ rear heater switch. For this one there are three options depending on how you are connecting it:

      (A) if you are connecting it into a discovery D1s 1995 onwards OR using a discovery window ECU then follow the following wiring - 1 = out A, 2 = out B, 3 = supply from window ECU, 4 = ground, 5 = supply from window ECU & 6 = illumination

      (B) if you are using it to replace a universal switch AND NOT using a roll up interface then you need to connect it as follows: 1 = out A, 2 = out B, 3 = ignition live, 4 = ground, 5 = Ignition live & 6 = illumination

      (C) if you are using it to replace a universal switch AND using a roll up interface then you need to connect it as follows: 1 = out A, 2 = out B, 3 = ground, 4 = ignition live, 5 = ground & 6 = illumination/ground. This will work fine and the switch will illuminate as soon as the ignition comes on. If you want to illuminate it by the side lights then you need to connect the side light live to a relay as follows: 85 – side light live, 86 & 30 ground & 87 – switch pin 6.

      I will talk about roll up interfaces when I talk about alarms in a later article.
    8. cowasaki
      Central locking motors

      Called a central locking motor this is actually a solenoid and locks or unlocks the door. Solenoids are usually made specifically for the manufacturer to go in their particular door but you will often see universal central locking motors which are available as 2 or 5 wire versions:


      A two wire motor is normally connected by a green and a blue wire and is a solenoid which is activated simply by attaching a voltage across it in exactly the same way as a motor so refer to the section above for MOTOR. Using that setup the switch would lock or unlock the door when pressed to &#8220;I&#8221; or &#8220;II&#8221;. I once broke down and didn&#8217;t have a meter with me. I had a central locking motor in the back so used that as a voltage detector!

      A 5 wire central locking motor is the same as a 2 wire motor with the blue and green wires doing exactly the same as they would in the 2 wire version. The extra 3 wires are a SPDT switch and are used by the central locking controller to check when the door is locked or unlocked manually. You would normally use one of these on any door with a key lock.

      The hole in the white plastic at the top of the motor connects to a metal rod which in turn attaches to the metal rod inside the locking mechanism of the door. So that when the motor activates it pulls the lock open. The motor is normally supplied with a length of mechano strip but it is often better to make your own more rigid bracket.
    9. cowasaki

      A relay is simply a solenoid that moves a switch. The switch itself can be like any other two position switch ie single or double pole and single or double terminal. The most usual are SPST or DPST.

      So using the same symbols we brought up earlier here we have a relay, note that I will be automotive standard connection labels of 30, 85, 86, 87 & 87a


      The input &#8220;30&#8221; normally outputs to &#8220;87a&#8221; but if the electromagnet is energized by applying a current across &#8220;85&#8221; & &#8221;86&#8221; the switch switches and &#8220;30&#8221; now outputs to &#8220;87&#8221; instead.


      85 & 86 solenoid circuit
      30 input
      87a NC output (normally closed output)
      87 NO output (normally open output)

      You may ask yourself WHY would I bother doing this as I might as well use the positive going to the electromagnet to control the component rather than using a relay. Well relays in cars have two practical uses, the main one is to switch larger currents using small ones but you can also use them to invert a signal ie 12v in 0v out or 0v in 12v out.

      Using a relay to control a large current:

      So we remember from the basics section that the current around a circuit is the same around the whole circuit. Now imagine that we have just added 4 100w spot lights to our A frame and 6 to the roof bar. We want one single switch to activate all these lights&#8230;.

      So that is 10 x 100w or 1000w or 1Kw and we are using 12v.

      Using the conversion formula from earlier we have

      I= 83 amps !!!

      You try and find a nice looking switch that can control THAT amount of power! Also remember that your cable needs to be twice the maximum power or more so 200amp cable and likewise the switch. This just isn&#8217;t practical.

      So how do we use a relay?

      I will first show you the diagram of a simple relay circuit:


      Here you can see a simple circuit as per previous examples in which the battery negative is connected to the chassis as normal with the positive going to a switch (thin green wire) then on through the coil of the relay and on to ground through the blue wire so completing the circuit.

      This is just the same as this circuit:


      But in our circuit the switch is energizing (switching on) the relay rather than the bulb.

      If we now look at the rest of the circuit we can see a heavy cable going from the battery through a fuse (we will talk about those later) and on to the input of the relay&#8217;s switch. This cable then comes out of the switch through terminal 87 (normally open) and on to the lights before returning to ground to complete the circuit.

      We should not be connecting the switch circuit to the battery though because that would allow us to switch the light on at any time. What we need to do is connect the switch live connection to a connection that becomes live ONLY when it would be ok to turn on the light we want to turn on. This example shows the circuit modified for a spot light:

      Using the correct control line


      You can see that the thin green wire now attaches to the 12v output from main beam switch when main beam is on and so this switch will only turn on the relay when the main beam is on.

      If this were a front fog light then we would connect that thin green wire to the side light circuit because we should only use fog lights when we have normal side lights on.

      If this were a high power reverse light then we would connect that wire to the reverse light switch.

      The current through the switch circuit is tiny whilst the current through the light/bulb circuit can be massive. The above example talks of 83 amps and this is perfectly fine, we could connect ten lights just as above and use a monster 200 amp relay which are available for around £10-12, so long as the relay and wiring is up to the task. Personally I wouldn&#8217;t actually control 10 lights from one relay but I&#8217;ll show you a better way next.

      Controlling multiple relays

      There is no reason why you can&#8217;t control multiple relays from a single switch and the diagram below shows this:


      or again using the correct control input from the main beam/side light/reverse switches:


      If we use our original example of 100w bulbs we will still require 2 x 100 amp relays but to be honest we will most likely be switching normal 55w bulbs. Based on that we have

      For the A frame:

      I=18.3 amps

      For the light bar:

      I=27.5 amps

      So we can use two normal 40amp automotive relays or at a push 30amp ones and much less thick wire!
    10. cowasaki
      Relays as logic circuits

      Relays are simple logic gates and we can do a number of useful things with them.

      Relay as an inverter ie GND in 12v out, 12v in GND out:


      Output of 12v unless GND is added:


      Output of 12v if GND is added:


      Output of GND if 12v is added:


      Output of GND unless 12v is added:

    11. cowasaki
      Tools at about £5 each or less

      There are a few tools that will make dealing with the electrics on the Landrover easier and these are:

      Bulb tester/probe

      These are VERY cheap but very useful. It's true that a multimeter can do the same task (and a lot more) but a lit bulb is a quick and simple test and it doesn't have batteries which can go flat.



      These are cheap but very useful too. *I make plenty of use of a multimeter in the tutorials I am writing. You can get one for about £5 but I would pay a little more and get something half decent it should last years and help save hours of work


      Whilst you are at it additional probes with crocodile clips etc are also very useful


      Crimp tool

      Get a decent crimp tool that ratchets and crushes the entire crimp so making sure that the crimp doesn't simply fall off the wires under load. You should not be able to pull a crimp off the end of a wire easily

      Soldering iron

      A 12v soldering iron is very useful and you can get one of these for about £5 too

      Side stripper

      A tool that splits the insulation on a wire bearing the inside conductors. Very useful for making a connection to existing wiring whilst adding extras. Again these only cost about £5 from ebay

      Last edited: Jan 3, 2014
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