Fuel Pump Relay Test

If you are looking for guidelines on executing a test on the fuel pump relay, this article will be a helpful read. Here, I provide clear testing instructions.
Modern automobiles are some of the most complex machines ever manufactured. Each system is supported by several subsystems. Every component has a large role to play in the overall functioning, as all systems depend on each other. In case of fuel injection systems, one of the most important parts is a fuel pump relay. A failure of this part can cripple your car's functioning. In case of any fuel injection problems, it is customary to run a test on the relay, to determine if the source of the problem is that component.


The fuel pump relay switches the pump on and off. All the vital systems of a car are controlled by the 'Engine Control Unit (ECU)', which monitors and executes all of its functions. The ECU controls the fuel pump, via the relay. It is usually packed in a plastic casing and attached to the ignition system. It is usually located near the steering column, on the firewall. It is basically like a small transformer system, which works on the principle of magnetic induction.

For the engine cycle to begin, it's important that it is supplied with the right amount of fuel, at the right pressure. For that, the pump needs to be in a working condition. When there are problems with the relay, the pump cannot function, leading to major car starting problems. The relay comes into the picture twice, during the operation of a fuel pump. At the very start of the engine cycle, it engages with the pump, to switch it on and later, it disengages after the pump begins operation.

Later, the power required for functioning of the pump is usually supplied by the oil pressure sending unit. The relay is again switched on when the pump needs to be powered off. So, it is vital that the relay stays functional, if you want the car ignition to be smooth.

Bad Relay Symptoms

When a relay stops functioning, the engine cycle cannot begin. So, one of the prime symptoms is a car starting problem. Other than that, a delayed triggering of the engine may also indicate a bad relay. Your fuel pump problems may actually be relay problems. However, to isolate a badly functioning relay as the cause of car starting problems, you have to run a test.

Test Procedure

You will need a ohmmeter for testing the relay. Also needed, will be a circuit diagram of the relay, if you get one. The diagram is mostly printed in the relay itself. Firstly, locate the relay, using the car reference manual. Detach the relay and remove it from its casing. All you have to do throughout the whole test, is check for continuity and resistance between terminals.

There are four main terminals to check. These include two power terminals and two control terminals. Set the ohmmeter on its lowest resistance value. Connect one of the ohmmeter leads with the first power terminal and second with the other one. If you find the resistance to be anything less than infinite, then the relay has undoubtedly shorted and it needs a replacement.

Next, get hold of jumper wires and connect one from the positive terminal of the car battery to the positive control terminal. Also connect another jumper cable from the negative of car battery terminal to ground. If you hear the relay clicking, as soon as you make the ground connection, it is in working condition. Lastly, check the continuity and resistance between the power and control terminals. The resistance between them should be a few ohms. If it is infinite, then you need to go for a relay replacement.

All you need is a ohmmeter to check voltage values and continuity. In case you are not sure about how to go about it, just visit your nearest car repair center for help.