The ignition relay controls the power coming from the battery for the fuel and the ignition system.

The moment you turn on the ignition the relay will come alive. The ignition relay is located under the hood between the fuse and the relay panel. It is designed to last for the life of the vehicle, but there are moments when it fails. If the ignition relay is damaged, you will notice that the car stalls while driving.

Before we begin testing a relay, you may first need to diagnose the symptoms of a faulty or damaged ignition relay.

Testing with a multimeter


Before you can start testing your relay with a multimeter, you must first have a basic understanding of how relays work. Inside the relay, there is an electromagnet which opens and closes depending on the current flowing through it. The relay helps to extend the life of switches by preventing the flow of high current.

The terms open and closed circuit are used in the relay to indicate whether a current flow is present or not. When contact points are open, the circuit is open, and when they are connected, the circuit is closed and current flows.

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1. Disconnect power sources

Before you can start working on the relay, you must first switch off all power sources. Most relays are supplied with a datasheet that shows the pin configurations. You will also be given information about the voltage values; this will help you to avoid errors when repairing the relay. For the automotive relay, this information is displayed on the PDC.

If overheating has occurred, you will notice the melting of the black plastic housing that houses the connectors. Some relays have an LED light that indicates when they are ON/OFF.

2. Check Resistance

A digital multimeter is used to check the resistance of each pole of the relay and its normally closed or normally open contacts. You should read an infinite resistance with the appropriate pole for the normally open contacts, while the normally closed contact should read 0 ohms.

When testing solid state relays, use the multimeter to perform a diode test. The multimeter works by applying a small voltage to the semiconductors in the relay. It tests the NPN transistor; if it is faulty, it shows 0, but if the silicon transistor is working, it shows 0.7 and 0.5 for a germanium transistor.

3. Test for voltage drop

There are two types of tests you can perform on an ignition relay – electrical resistance and voltage drop. When testing for voltage drop, set the multimeter to 20V DC. The black multimeter probe of the multimeter should be connected to the circuit of the ignition switch, while the red probe should connect to the red battery cable.

You need someone to help you switch on the ignition. A bad relay is a relay that indicates a voltage of more than 0.2 V. The resistance should be less than 5 ohms.

Cleaning your relay

If dirt and particles get into the fuse box, current surges occur which eventually destroy the relay. It is important that you clean your relay occasionally. This can be done with a vacuum cleaner and a damp cloth. The relays are usually located in the fuse box. Use a blower to remove dirt particles inside the fuse box.

Do not use water to clean a relay because it does not work well with electrical components. Ethanol is an alternative. Remove a relay and immerse the clothing in the ethanol. Use it to remove dirt particles on the relay.

Symptoms of a Bad Relay

1. Car stalls while driving

The internal combustion engine works by igniting a fuel-air mixture. The spark plugs provide the spark necessary for ignition. The ignition relay sends current to the fuel system. In case of a defect, the power supply to the fuel system and the spark plugs is interrupted. This brings the vehicle to an immediate standstill. If the relay is short-circuited, you may be able to drive again, only for the car to come to a standstill again.

2. Unresponsive ignition

In the fuse box you can see the relay and other fuses. Dirt and impurities find their way inside the fuse box and destroy the fuses. If this happens, you may have a damaged relay. The ignition relay helps your car start, and if it is damaged, you will find that the engine does not come to life when you turn on the ignition.

You will also find that some of the other car accessories receive power, but not the engine. This is because the ignition relay supplies power to the fuel and ignition system.

3. Dead battery

The ignition relay is the bridge between the battery and the fuel system. If the ignition relay is defective, the battery is still supplied with power from the ignition system even after the ignition is switched off. You will notice that even if the battery is new and working properly, it will always be drained of power, especially in the morning.

4. Faulty relay

The relay works with circuits and will overheat at certain moments. If the relay burns out, other fuses in the box will melt. If this happens, you will be forced to replace the entire fuse box.

If you want to test a relay, you must first determine where the circuit goes in and out. There are standard numbers that are used to identify the terminal pins.

The relay box will have a list of numbers. For the control side, you will have pins 85 and 86. Pin 85 is used to take current in, and it flows out through pin 86. This complete circuit is what activates the electromagnet. Pin 30 is used to regulate the current flow from the battery and closes the circuit when energized.

When using a multimeter, make sure that there is voltage on pin 85. Then check pin 86, if no current is flowing, note that you may have a blown fuse.


An important relay in your car is the ignition relay. It controls the fuel and ignition system. The relay can be destroyed over time, which can cause your car to stall during operation. If this happens, you must identify the ignition relay under the car and test the relay. A multimeter is used to test the relay for voltage.