- Nominal value
- Types of fuses
- Check fuses
- Practical situation with a blown fuse
There is a tangle of wires running through the car. These wires run close to metal in many places. It can happen that a wire is damaged and chafed, causing the conductive material to come into contact with the metal of the body. A short circuit can then occur. There can also be numerous other reasons that can cause a short circuit, for example due to incorrect wiring, internal shorting in components and moisture ingress into plugs and control devices. A fuse of too low a value or connecting too many consumers to one positive wire can also lead to a defective fuse.
Fuses serve to protect against overload and short circuit. We find fuses in different places in the car. Usually they are all centrally located in the dashboard on the driver's and / or co-driver's side behind the glove compartment, but sometimes we also find fuses in a plastic holder on the battery or in a fuse box in the trunk.
On this page, we take a closer look at the types of fuses in passenger cars and the ways to identify defective fuses. Making a diagnosis by measuring the voltage drop across a fuse to determine the amount of current is described on the page: measure voltage drop across the fuses.
Fuses all have a nominal value, in other words: a maximum current rating; this is marked on the top of the fuse, (eg 10 Amps). That means that a current can flow through it up to 10A. When a current greater than 10A flows through it due to an overload, electrical fault or short circuit, the conductive contact in the fuse is heated to such an extent that it eventually melts. The circuit is now interrupted.
This prevents current from flowing through that circuit, preventing damage to wires and components from the short circuit. Before replacing the fuse, the cause must be known. A fuse doesn't just blow. Here are some causes of a blown fuse:
- The wrong fuse may have been fitted: instead of a 20 A, there is a 10 A;
- Too many consumers are connected to one fuse, eg with retrofitted accessories. The fuse, but also the wiring is not designed for this. So don't just replace the fuse with one of a higher value, because there is a good chance that the wiring will be overloaded;
- The electrical component behind the fuse has a problem: think of worn / heavily moving bearings in the interior fan, or a high frictional resistance in the window rubbers, which puts a heavier load on the window motor. In both cases this is accompanied by a higher current, which may be close to the nominal value;
- There is an "occasional" short circuit, such as two worn wires in a door or trunk rubber. During the opening and closing of this, the conductive parts in two wires touch each other, resulting in a short circuit.
If the fuse continues to blow after it has been installed, there may be a short circuit. A test lamp can be used to search for the location of the short circuit. On the page find a short circuit with the test lamp describes how this works.
Types of fuses:
We usually find plug-in fuses in passenger cars. The blade fuses are available in six different sizes. The figure below shows the different fuses. The actual size may differ from the size shown below. The legend states the dimensions in millimeters and the nominal currents.
- Micro2: 9.1 × 3.8 × 15.3mm
- Micro3: 14.4 × 4.2 × 18.1mm
- Low mini: 10,9 x 3,81 x 8,73mm
- Mini: 10,9 x 3,6 x 16,3mm,
- Normal: 19,1 x 5,1 x 18,5mm
- Maxi: 29,2 x 8,5 x 34,3mm
- Micro2: 5, 7.5, 10, 15, 20, 25, 30
- Micro3: 5, 7.5, 10, 15
- Low mini: 2, 3, 4, 5, 7,5, 10, 15, 20, 25, 30
- Mini: 2, 3, 4, 5, 7.5, 10, 15, 20, 25, 30
- Normal: 1, 2, 3, 4, 5, 7.5, 10, 15, 20, 25, 30, 35, 40
- Max: 15, 20, 30, 40, 50, 60, 70, 80, 100, 120
Below are a few more types of fuses that we encounter in passenger cars. Cardridge fuses are mainly found in Japanese and Korean cars, glass fuses are often seen in retrofitted plus cables for amplifiers, high power fuses can be found both in the fuse boxes in the interior and in a box on top of the battery. The last variant is used for, among other things, the cooling fan because of the high electrical power.
The appearance of the fuses may differ slightly from the image above. For example, each manufacturer uses a slightly different shade of green and some use a colored plastic housing over a high power resistance, while others choose to expose the fuse wire and punch the nominal value into the metal.
In most cases, when consumers in the car no longer function, we first check the condition of the relevant fuse.
- On the way, you can find in the fuse card (usually located in the fuse box as in the images below, or on the sticker in the cover, or in the instruction manual) which fuse belongs to which consumer;
- In the workshop, the statement can be found in the workshop documentation or electrical diagrams (or a combination of both).
Be careful when pulling out the fuses one by one. Some consumers must be connected to a voltage source. When disassembling, a (harmless) malfunction can occur, the digital or analog clock on the dashboard can reset, etc. When measuring equipment is available, it is better to use measurements to locate the defective fuse instead of the fuses after removing them. pull to be checked visually.
Measurement with the multimeter (1):
With the voltmeter we can measure the voltage on both conducting sides of the fuse with respect to ground. If the fuse is OK, we will measure almost the same voltage on both sides. In this case, this voltage is 13,2 volts.
Because the voltage is the same on both sides of the fuse, we know that it conducts well. The voltage from the plus of the battery is therefore properly passed on to the consumer.
We can also measure the voltage difference across a fuse. When the consumer to which the fuse is connected is switched off, there is of course no current flowing. The voltage difference is therefore 0 volts.
When the consumer is switched on, current flows from the fuse to the consumer. Due to the (very low) internal resistance of the fuse, some voltage is also absorbed. We lose this tension, but fortunately it is minimal. In the picture we measure a voltage difference of 6,4 milliVolts, or: 0,0064 volts.
In the table on the page “voltage drop across fuses” we can find that a current of about 2 Ampere flows through the fuse to the consumer.
This measurement can be useful when looking for a clandestine consumer.
When dealing with a blown fuse, we will measure the board voltage on one side (in the example 13,2 volts) and 0 volts on the other side. The voltage is therefore not passed on to the consumer by the fuse. The figure below shows the readings on the faulty fuse.
Measurement with the test lamp (2):
A quick way of checking the fuses is to check with a test lamp. The test lamp consists of a pointed end (used to measure the contacts on the fuse), a housing containing a lamp (tube lamp or LED) and a ground wire with an alligator clip at the end. We mount the alligator clip to a good earth point and with the plus side we go through the fuses one by one. The lamp in the housing lights up on all contact surfaces of the fuses that have voltage on them. The level of the voltage is not important in this case: it shows either the on-board voltage (between 11 and 13,8 volts) or 0 volts. In the latter case, the test lamp remains off.
In the following image we see that the test lamp is not lit. On the top contact yes. That means this fuse is defective.
Is the test lamp on both sides of the fuse not lit? Then there is probably no voltage at the fuse. This may be because the ignition of the car is not switched on or the consumer is not supplied with voltage. In the latter case, we can pull out the fuse without causing any interference and check it optically or with an ohmmeter.
The resistance measurement on a fuse is shown in the two figures below. The resistance of a good fuse is about 0,1 Ohm (very low). In the event of a defective fuse, there is no connection between the two measuring wires and the resistance is infinitely high. The ohmmeter will indicate this as OL or as 1. .
Practical situation with a defective fuse:
Anyone who drives, owns or tinkers with a car can be involved: a fuse has blown. As mentioned before, a fuse does not blow out of the blue. Usually there is something wrong: there is a short circuit in an electronic consumer, in the wiring or the plugs, or an electrical overload has occurred due to a mechanical problem. In this section we take a closer look at a practical situation.
The next problem has arisen: the horn no longer works. Pressing the horn switch (usually the airbag module in the center of the steering wheel or on the turn signal) does nothing. First we look up the fuse card and position description in the service booklet. The image below shows the following:
- top left: positions of the fuses, indicated with a numbering from 1 to 90;
- top right: the parts for which the fuses are. An explanation of these icons is usually also described on a page;
- bottom right: the nominal fuse values;
- bottom left: a photo of the fuse box.
Because we have a problem with the horn, we look it up in the overview and in the fuse box. The correct fuse is framed in red. After replacing the correct fuse (with a 15 A fuse, of course) it blows immediately when the horn is operated.
In the workshop one can consult the electrical diagrams to see how the horns are switched. The following diagram shows the circuit of the horn:
- from terminal 30 the power supply (terminal 183) of the horn relay is energized by the ignition lock A30;
- the horn relay (top right) switches on as soon as switch S15 closes (this is the horn switch that the driver operates);
- When the horn switch is operated, a current flows through the coil of the relay and the main current is switched on. The current flows through fuse F57 to both horns (B2-I and B2-II).
Since the fuse blows immediately as soon as the horns are switched on, there is probably a short circuit. By connecting a test lamp over the fuse, we can determine this:
- if the lamp glows faintly when the horns are switched on, there is a series connection and not a short circuit;
- a brightly lit test lamp indicates that there is a short circuit: the test lamp receives a direct power supply and ground and burns at 12 volts, i.e. at full power.
The two diagrams below show two situations: the current flow with a properly functioning system and with a short circuit.
- when the ignition is switched on (by ignition lock A183), the horn relay on pin 30 is supplied with voltage. The horn switch is pressed and the control current portion of the horn relay is energized (green). The main current (red) now finds its way through the relay (output terminal 87) and fuse F57 to the two horns (B2-I and B2-II). The horns are turned on and sound;
- now there is a short circuit. The positive wire of the right horn (B2-II) is connected to ground. There is now a direct connection between the plus (output relay) and ground. To ensure that the current does not rise to hundreds of Amperes, which will damage the wiring and components, the fuse interrupts the positive circuit when exceeding 15 A.
The short circuit could actually be a worn positive wire coming into contact with the body of the car. This can happen after the wiring harness has been incorrectly reassembled in its clamps / holders after removing the bumper and front panel. Or after a collision, where the wiring got pinched.