Introduction:
Modern vehicles are equipped with a lot of electronics. They often contain dozens of ECU’s, each responsible for specific functions.
- Engine compartment: ECU for engine electronics, automatic transmission, ABS/ESP;
- Interior: ECU for airbags, in doors, under the seats, in the roof for the sunroof or lighting, in the trunk for the trailer electronics, etc.
These ECU’s and actuators receive their power directly from the fuse box. Because there are multiple power wires and fuses, we can often find multiple fuse boxes, such as in the engine compartment, the dashboard, and even in the trunk of passenger cars.
From the fuse box, power wires (positive) run to various components, such as ECU’s and actuators. The ECU’s receive information from sensors via signal wires.
An example in the interior is the door switch, which indicates 12 or 0 volts when open or closed, respectively. In the engine compartment, the coolant temperature sensor can send a 2.5-volt signal to the ECU at a temperature of 20 degrees Celsius and a 0.5-volt signal at 90 degrees Celsius.
The ECU then controls the actuator, supplying current to a passive actuator (e.g., an injector), sending a voltage signal to an active actuator (COP coil), or sending a digital message to an intelligent actuator (wiper motor). Each ECU and actuator is connected to a ground point on the body or chassis of the vehicle via one or more ground wires.
All positive, ground, signal, and communication wires between the fuse boxes, ECU’s, sensors, actuators, and ground points result in a huge amount of wiring. Manufacturers try to route the wiring as much as possible as one bundle through the vehicle. This is called a wiring harness.
In the following image, we see a part of the wiring harness where dozens of wires run through. The wiring harness is wrapped with tape to keep the wiring together. The colors are still visible between the windings of the tape, allowing a mechanic to easily trace the wire color when troubleshooting.
A wiring harness has many branches: it runs from the engine compartment to the trunk, but also from the left to the right doors, under the dashboard from left to right, and under the seats. The wiring harness is precisely tailored to the vehicle.
In a wiring harness, a wire can become damaged. If the insulation is frequently damaged by repeated bending (for example, at the hinge of a door or tailgate) or if the wire has been rubbing against something, the wire can often be repaired. The damaged section is removed, and a new piece of wire is soldered in between and then sealed with heat shrink tubing. However, when there is a short circuit and burned wires, it becomes more complicated. In such cases, especially with a car of high residual value, it may be decided to install a new wiring harness.
Wire Thicknesses:
In cars, we find many different wire thicknesses. In the engine compartment, we find thin wires from sensors and relatively thick wires to actuators. In the following schematic, we see a black (ground) wire on the battery (A) of 25.0 mm8 this is the thickest wire we find in the engine compartment. On the alternator (C) we see a black wire on the B+ of 16.0 mm8 on the control unit J367 we find significantly thinner wires ranging from 0.35 to 0.5 mm8.
The choice for the wire thickness is related to the maximum current and the length of the wire in relation to the specific resistance of the wire:
- A thick wire is suitable for higher current strengths;
- The longer the wire, the higher the resistance of the wire. Therefore, long wires are often made thicker.
A negative and B+ cable from the alternator must conduct high current. A thin wire would have too high an internal resistance, causing not only a voltage drop but also an increase in temperature. The wires to the ECU carry a small current.
The resistance in the wire has a significant effect on voltage drop. The current plays a major role in this. To make this insightful, two calculations follow below. In both examples, the resistance of the wire is 0.1 4.
We take a positive wire from a 21 Watt lamp and calculate the current by dividing the power by the source voltage of 12 volts (the power law). The current, depending on temperature, is approximately 1.75 A. Using Ohm’s Law, we calculate the voltage drop across a wire.
The voltage drop of 0.18 volts is acceptable, as the lamp burns at a voltage of (12 – 0.18) 11.82 volts. For clarity: the 0.18 is the V3 in the V4 measurement. The resistance in this wire is low enough not to have a negative impact on the operation of the consumer.
In the next example, we take the positive wire of the starter motor. Again, the resistance of the positive wire is 0.1 4. The measured start current is 90 Amperes.8
The resistance in the wire causes a voltage drop of 9 volts. With a voltage of 12 volts when the starter motor is on, only 3 volts will remain to operate the starter motor. This is, of course, too little; the starter motor will not or hardly move.
Conclusion: a resistance of 0.1 4 in a positive wire has hardly any effects on a lamp, but is so high for a starter motor that it no longer functions.
Specific Resistance of Wire:
Every wire has an ohmic resistance. The resistance value depends on:
- the material;
- the dimensions (length and diameter);
- temperature.
The following image shows four wires of the same material, with wire A having the highest and wire D having the lowest resistance.
- In proportion, 2L is twice as long as l;
- In proportion, 2d is twice as long as d.
A thick, short wire has less resistance than a thin, long wire.
The resistance of a wire can be calculated using the following formula:
In this formula:
- R is the resistance of the wire in ohms [4];
- l is the length of the wire in meters [m]
- 2 (rho) is the resistivity of the wire in ohm meters [4m]
- A is the cross-sectional area of the wire in square meters [m8]
The formula shows that the resistance of the wire increases with a larger length (l) and decreases with a larger cross-section (A).8 the resistivity of a wire is expressed in ohm-meter (4m). Since we are dealing with small values, we use a unit 10^6 times smaller, specifically micro-ohm-meter (4m).
Example:
We calculate the resistance of a copper wire with a length of 2 meters, a cross-section of 1.25 mm8, and a resistivity of 0.0175 * 10^-6 4m.
Plug Connections:
In cars, wires are connected to a sensor, actuator, or control unit via a plug connection. It is also possible that a plug is located somewhere in a wiring harness, allowing two wiring harnesses to be connected to each other.
The following image shows part of a schematic of a Ford Fiesta. Here we see the component code B31 (mass air flow sensor) and Y34 (charcoal filter solenoid valve). The mass air flow sensor is a sensor, and the solenoid valve is an actuator. They are both connected to the engine control unit (at the top).
On the mass air flow sensor, we see a 5-pin plug (5p) with four occupied positions: 2 to 5.
The solenoid valve is equipped with a two-pin plug (2P).
The numbers on the plug in the schematic are actually displayed on the plug itself. This way, one can compare wire colors, or when the same wire color is used in multiple positions, distinguish wire functions from each other (positive, ground, signal, etc).
Wire Repair:
During a wire repair, it is possible that a new plug needs to be crimped onto the wire. This is done with a crimping pliers, also known as a crimping tool. In this example, uninsulated metal plugs are crimped onto the wire and clicked into plastic connector blocks.
The crimping pliers have a mechanism that applies a large force to the cable terminal or metal connector plug with minimal force on the handle. Often, a holding mechanism is also included, so that the pliers “click” during crimping and hold the cable terminal when the handle is released. Only when the pliers are fully squeezed in the extreme position, or when the unlocking mechanism is operated, do the pliers release the cable terminal again.
Determine the length of the wire and cut off a section. Keep in mind that part of the insulation will be removed from the ends with the wire stripper.8 the two images below show the wire stripper and the end of the green wire:
- left: first, determine the length you want to strip the wire by setting the red part to another position. Far left, as in the image, the length is 2 mm. Squeeze the pliers. The jaws close and the metal mechanism grips the insulation. Squeeze the pliers completely. The insulation is slid off the wire at the adjusted distance;
- right: release the pliers. The copper wire is now visible.
After the wire is stripped (the copper wire is 2 mm long), cable terminals (insulated/uninsulated) or metal connectors can be crimped onto it. In the following three images, the following is visible:
- Left: a crimping pliers with two metal connectors (male and female);
- Middle: the metal connector is clicked into the crimping pliers, and the stripped wire is inserted into the back of the metal connector;
- Right: the other side of the crimping pliers with the metal connector.
Correct (1)
Mistakes are sometimes made when crimping cable terminals. It is important to know how far the electrical cable should be stripped and how far the wire should be inserted into the cable terminal. Here are five examples with the three most common mistakes.
The following image shows a properly mounted wire.
Correct (2)
This is the same wire, drawn from a different angle.
Error (1)
The insulation is stripped too far. The copper wire protrudes and can, after bending the ends, cause a short circuit in some connector housings.
Error (2)
Not all of the copper wire is crimped into the cable terminal. The protruding wire can, when bent, cause a short circuit with another wire in the connector, or against the vehicle body.
Error 3:
The insulation is stripped too short and is crimped into the inner part of the cable terminal. Because this part is thicker than the copper wire, the cable terminal is not completely crimped closed. The possible consequence of this is poor contact between the copper wire and the cable terminal.
After crimping the two metal connectors onto the wire, they can be clicked into the plastic connector blocks.
It is possible to accidentally click the wire into the wrong position. With a lamp fitting screwdriver or a connector remover, one can carefully bend the metal tab at the connector and pull the wire out of the plug. Of course, the tab must be bent back up again, as otherwise, the connector will not click securely.
Unlocking Connectors:
It may be necessary to remove a wire from a plug. The metal connector crimped onto the end of the wire must therefore be removed from the plastic connector housing. This requires a tool; a so-called connector remover. This tool can bend the metal tabs on the connector inside the plug so that the wire can be pulled out. To do this, however, the lock in the plug must first be removed; in the image, the lock can be recognized by the purple plastic part, halfway along the plug. The lock prevents the wire from being pulled out of the plug, even if the connector is unlocked with the tool. The animation shows unlocking and removing the wire from a four-pin plug used in an Audi.
