Faulty fuse:
When a fuse blows immediately after being installed in the fuse holder, there are two possibilities;
- The fuse has a rating that is too low. If a 5 Amp fuse is installed while the consumer requires 10 Amps to operate, the fuse will immediately fail when the consumer is turned on. In this case, there is no short circuit; a higher-rated fuse needs to be installed.
- There is a short circuit present. The fuse blows to protect the system.
This page explains how to locate a short circuit.
Short circuit in lighting system:
In a fault-free lighting system, the lamp lights up as soon as the ECU (the electronic switch) passes the voltage. A closed circuit is formed, allowing current to flow. In the image below (Fault-free), the positive is indicated in red and the ground in brown.
In the image (titled Short circuit), the positive connection of the lamp is directly connected to ground. This extra wire is shown in brown in the image. In reality, the wires are close to each other, which means damage to the insulation of two cables can cause them to make contact.
Between the positive and negative of the battery, there must always be a consumer present. This is not the case with the brown wire in the image below. As soon as the battery’s positive is connected to ground, the fuse will blow.
It can be very difficult to find a short circuit. The short circuit may be located in a wiring harness, but also in the ECU or in a consumer. As soon as a new fuse is installed, it will blow immediately. If a short circuit is suspected, this can be verified by connecting a test lamp into the circuit.
Any 12-volt lamp used in the vehicle’s lighting system can serve as a test lamp. Special test lamps are also available at auto parts stores. This test lamp should be connected to the fuse’s connection points (see the diagram). The consumer should be turned on.a0When the test lamp lights up, it confirms a short circuit is present in the circuit.
When the ECU is activated, the test lamp’s ground wire is connected to the battery’s ground, causing the test lamp to light up. In a fault-free situation, the ECU switches the positive through, but in this case, the ground is passed through. Testing with the test lamp is not harmful to the system. The test lamp acts as a consumer, so the short circuit no longer causes an overload.
Now that it is known there is a short circuit in the system, the location of the short circuit can be found by disconnecting plug connections. In the above diagram, the lamp connections can be disconnected, but the test lamp remains on. Therefore, the lamp connections are not causing the short circuit.
When the connector from the ECU’s output is disconnected, the ground connection is interrupted. The test lamp extinguishes. Using the diagram, it can be determined that the short circuit is located in the wiring between the ECU and the lamp. Knowing the location on the diagram makes it easier to find the relevant wire in the car. Repairing the damage or replacing a section of the wires is often enough.