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Bright Sparks
Classic American issue189


How many of you have ever faced the following scenario. You are out driving at night enjoying the peace and quiet of the country roads when suddenly the headlamps on your muscle car are stolen! Or at least they might as well have been, because they have both failed without warning. This is an all too familiar scenario especially for owners of vehicles from the sixties through to the eighties. These niggling electrical problems are all too often brought about by ageing components such as the headlamp switches, which on some vehicles may have a circuit breaker incorporated within it. Other possibilities are incorrect components such as headlamps and, or fuses with the wrong rating or a mismatched lamp upgrade. If components have been replaced with parts rated at either too low or too higher wattage, systems may overload and cause circuit breakers to open, fuses to blow or in the worst case cause a small fire under the hood or under the dash. Of course if you were planning to cook out or have a roadside ‘Bar-B-Q’ this is not too much of a problem. However, if all you wanted to do was complete your journey, you’re in trouble. So this month we will mostly be looking at electrical problems, fault finding and hopefully showing you various ways to solve some of these problems. We will be looking at amps, watts, voltage and candlepower, and no this is not the power rating of the new Chevrolet Kalos engine.

Before we go any further maybe we should just cover the differences between fuses, circuit breakers and relays. Let’s begin with a brief look at fuses. In electrical terms a fuse is basically a type of over-current protection device. A fuses ‘critical component’, the part that breaks, is usually a metal wire or strip that melts when heated by a pre-determined electric current, this then opens the circuit of which it is a part and so protects the circuit from damage. Properly rated fuses are an essential part of a power distribution system. They are used to prevent fire or damage due to overload or short-circuit. An over-current device should normally be selected with a rating just over the normal operating current of the downstream wiring or equipment, which it is to protect. Fuses are often characterised as fast-blow or slow-blow according to the time they take to respond to an overload. For automotive use there are two basic types of fuse, the cartridge fuse, fig 1, and the spade fuse, fig 2. The cartridge fuse usually comes in three different lengths. Likewise the spade fuse comes in three different physical sizes. Firstly the mini-fuse which measures 10.9mm x 3.6mm x 16.3mm, next the ATO or ATC fuse which measures 19.1mm x 5.1mm x 18.5mm and finally the maxi-fuse which measures 29.2mm x 8.5mm x 34.3mm.

Fig. 1

Fig. 2

Next let’s take a look at circuit breakers. These basically do the same job as a fuse but can be reused. Most automotive applications use what is known as an auto-reset compact circuit breaker, fig 3, although some applications do use a manual reset circuit breaker. Most of these compact circuit breakers are designed to with stand a direct overload but will trip, or open quickly on short circuits. The circuit breaker will latch into the off position under fault conditions and automatically resets to the on or closed position after a set period of time. However, if the fault that caused the trip still exists, the unit will immediately open again. This will continue until the fault is put right. The only difference between these automatic circuit breakers and the manual type, is the addition of a reset button on the manual type. If a manual circuit breaker trips it must be closed by pressing the reset button. In some situations it is possible to replace ATO or ATC type fuses with a circuit breaker as long as there is room in the fuse holder.
Finally let’s go over the relay, fig 4. Whilst not technically an over-current device, relays play an important role in all automotive electrical circuits. In its simplest terms a relay is just an electrical switch. However, rather than being operated manually a relay switches under the control of another electrical circuit. In its most basic form, the switch is operated by an electromagnet to open or close one or many sets of contacts. Joseph Henry invented this basic type of relay in 1835. It is worth remembering that a relay is able to control an output circuit of higher power than the input circuit. For example the high current starter circuit on your vehicle is controlled using a circuit with a much lower current.

Fig. 3

Fig. 4

If you are going to seriously consider getting into fault finding it is almost essential that you purchase a multi-meter, fig 5. Once you have one of these you will quite simply wonder how you ever got by without it. Even with the simplest meters you can carry out numerous tests on resistance, voltage, amps and continuity.

Right, if your headlamps have gone out, how do we set about finding the problem? Well faultfinding needs to be carried out in a very logical manner. The first question we need to ask is: do they work all right if you turn them off and leave them for a while and then try them again? If the answer to this is yes then the chances are you have a problem with either a circuit breaker or a relay. If the answer is no then the system probably has a blown fuse or blown bulb or lamp. Initially let’s assume that we think the problem lies with a blown fuse or bulb. We shall begin by checking the fuse. We now need to locate the fuse box. If you have the vehicle’s handbook the location of the fuse box can usually be found here, or at leased a picture of it. In some handbooks and manuals the fuse box maybe referred to as an Electrical Power Distribution Centre. If you don’t have the handbook there are a few common places that vehicle manufacturers like to put the fuse box. Many are situated under the driver’s side of the dashboard or in the end of the dashboard to be found when the door is opened, fig 6.


Fig. 5

Fig. 6

Many newer cars and trucks will have a fuse box and an Electrical Power Distribution Centre in the engine bay, fig 7. Once you have located the fuse box and the fuse, check to see if the correct value fuse was in place. The value of the fuse is usually marked on it. The correct value for the fuse can then be found in the owner’s handbook. Some fuse boxes also have the fuse values marked on them, fig 8.

Fig. 7

Fig. 8
Obviously too lower rated fuse will blow quickly because it cannot handle the continuous demand. At the other extreme a fuse with too high a value and it will not blow when the system has a problem, this in turn can lead to hot or melting wire, blown bulbs or worse still, the inevitable fire. If you are unable to see if the fuse is blown, you can use your multi-meter to perform a simple continuity test. A blown fuse, fig 9, should simply be replaced with a fuse of the correct value. However, if the fuse was not blown put it back in the fuse box and let us move on to the headlamps.
It should be fairly clear to see if the lamp or bulb has blown, fig 10.

Fig. 9

Fig. 10

However, if you are not sure you can use your meter to test the continuity of the bulb, fig 11. Obviously if it’s blown simply replace it with a bulb or lamp of the correct value and type. Just because two bulbs look the same, it doesn’t always mean they are the same, there are many different types of bulb, fig 12. Remember that the cause of a blown bulb could have been a fuse of incorrect value. Most standard headlamps and headlamp bulbs used are rated at approximately 55 watts for dipped and 60 watts for main beam. Usually the value is marked on the back of the lamp or the base of the bulb, fig 13. You can obviously use this information to check whether or not the lamps have been changed. As we have already mentioned, this value will normally be given in watts, a measurement of electrical power. Some light sources such as auxiliary lights or headlamps are rated in different units other than watts, candlepower or candela are also common units. To be able to compare different types of light sources, it would be necessary to convert between these types of units when a rating of both candlepower and watts is not given. So now the question is how do you convert between candlepower and watts? Unfortunately the short answer is, you cannot directly convert between these various units. Unless the manufacturer of the lamp or bulb already has calculated both candlepower and watts, you're pretty much out of luck. Watts and candlepower are two very different measurements.

Fig. 11

Fig. 12

Let’s just give you a brief explanation of what watts and candlepower are and hopefully this will help to provide the reason. So for the number crunchers amongst you, Watts is a measurement of the power needed, in our case, to light the bulb to its best brightness. One watt is the unit of electrical power equal to 1 amp under a pressure of 1 volt. Amps are the rate at which electricity flows through a wire. A good analogy is water flowing through plumbing. When you turn the tap on, water flows out at a certain rate. The same thing occurs when you turn on your headlamps. Remember that electricity flows at a certain rate, this is amps. Watts are the amount of power a device uses in performing its function. If for any reason you need to calculate the Amps, watts or volts in a circuit there are a simple set of equations that can be used. As long as you know two of the values you can always calculate the third. Electric power is represented by the letter P in electrical equations, and as we already know is measured in watts, named after Scottish engineer James Watt. In resistive circuits, instantaneous electrical power is calculated using Joules’s Law, which is named after the British physicist James Joule, who first showed that electrical and mechanical energy were interchangeable. Joule’s law gives us the following equation P=IV where as already mentioned P=power, I=Current measured in amps and V=potential difference measured in volts.

For example if you know that the wattage of a bulb is 55W and the circuit is run on 12V we can find the current draw using the following equation: I=P/V. Therefore I=55/12, this gives I=4.583A. So we now know that each bulb draws 4.583 amps. Alternatively we might know that our headlamp circuit typically draws 4.6A and is on a 12V circuit. Therefore P=IV giving us P=4.6x12 which results in P=55.2W. The higher the Wattage, the brighter the bulb will be in use and it’s this rating that is generally used for all incandescent lamps (ones with a filament).
Candlepower is different from wattage in that it is a measurement of light intensity. When shown as a value on a bulb candlepower is often shortened to CP. It is defined as follows: One candlepower is the radiating power of a light with the intensity of one candle. This unit is now considered obsolete after being replaced by the candela in 1948, though it is still in common use. One candlepower is approximately equal to 0.981 candela. The candela is now considered the standard unit for measuring the intensity of light. A light emitting diodes (LED’s) brightness is measured in candela with some lower brightness LED’s being measured in millicandela (thousandths of one candela). The candela is defined to be the luminous intensity of a light source producing single-frequency light at a frequency of 540 terahertz (THz) with a power of 1/683 watt per steradian, The steradian is the unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space. This can also be translated as 18.3988 milliwatts over a complete sphere centered at the light source…. Sorry we got a bit carried away there, I think we will leave it at that and just believe that some things are brighter than other things and we are able to measure it.


Fig. 13

Fig. 14

Now where were we, oh yes, we have examined the headlamp or the bulb obviously if it’s blown replace it with one of the same type and value. Most bulbs and headlamps will have a part number on them, fig 14. On some bulbs this may be on the base on others it may be printed or etched on the glass. It is worth making a note of this number before you contact your supplier. If they ask for the number at least you will have it to hand.
Okay, so you have checked the fuse and checked the bulb, what if it is neither of these. Well there is a possibility that the problem lies in the actual wiring itself. So the next step is to make sure we have a voltage reading at the lamp’s socket. Again we can make use of our multi-meter. Set the meter to DC volts, probably best on the 20V scale, fig 15. This has been highlighted in our example. Now with the socket off the lamp or bulb turn on the headlamp switch and probe the socket with your muti-meter, fig 16. If your vehicle has a 12V battery you should see approximately a 12V reading on the meter. It is always worth trying to take this reading with the engine both off and running. The reasoning behind this is quite simple, if the voltage regulator on the alternator is failing or has failed this could be sending a high voltage through the system causing the bulbs to blow. So if you have a reading in excess of 14.5V it may be worth checking the vehicles alternator or voltage regulator for a fault. However, if the reading significantly lower than 12V then the problem probably lies with the battery or the wiring. Let us first of all check the wiring. Initially check the earthing of the system. Make sure all earth straps have a good clean connection and that they are secure


Fig. 15



Fig. 16

Check your battery terminals, making sure that these are also clean and secure, fig 17. It is amazing how many electrical faults can be traced back to a poor earth or loose battery terminals. With these checks made, recheck the voltage at the lamp socket. If you still have nothing reading on the meter, the chances are either the wiring has a break in it somewhere or the headlamp switch is at fault. In tracing a wire fault this can be a time consuming job. This can be done by checking the continuity of various sections of the wire using the multi-meter. However it is also worth checking the wiring visually.
If all appears to be well with the wiring then there are only two real possibilities where the fault may now lie. Firstly the relay or secondly the headlamp switch. The easiest of these items to check is the relay. Once located you will need to turn on the headlamp switch and listen. You will be hoping to hear a click sound, this noise will be the relay activating. If you want to be completely thorough you may want to use your multi-meter to check the voltage in and out of the relay. If the relay is not activating the headlamp circuit this is obviously where the fault lies. Again the relay will have a number on it, copy it down in case your parts supplier asks for it.
Should the relay check out as being okay, through a process of elimination and pure deductional genius, the fault must lie with the headlamp switch. Well at least we hope it does. Depending on the type of headlamp switch you have, it is either going to be very easy or very difficult to test. In all honesty we find it is rarely worth trying to remove and test headlamp switches. It is usually easiest to just replace the switch, fig 18, and be done with it. This is especially true if you have an older vehicle where, as mentioned earlier, the component may be suffering from old age.

Fig 17

Fig. 18

Hopefully we have helped guide you through some basic electrical fault finding. In this case on the headlamp system. However, it should be noted that the basic principles of the fault finding remain the same whether you are trying to find a fault with the headlamps, tail lamps or the interior lamps. Some circuits may contain other components that may be at fault, so be aware of this. For example, if you suspect a fault on the turn signal or hazard circuits, you may have a flasher unit in the circuit, which could be at fault. If you are having problems with your headlamps not being bright enough, then there are plenty of upgrade options. We shall endeavour to cover these in a later article.

Just as a final note, if you do not feel confident in carrying out any of the procedures mentioned in the article please make sure you get a qualified auto-electrician to carry out the work. Whilst we are talking about relatively low voltages that are not particularly life threatening, changing any components incorrectly may lead to an electrical fire. This of course maybe life threatening and will almost certainly mean the demise of part or all of your vehicle........always like to end on a high note.