Fuel pump relay testing is one way to diagnose problems in fuel delivery in vehicles. A malfunctioning relay could disrupt the power flow to the fuel pump and lead to conditions such as an engine stall or failure of the engine to start. The fuel pump relay usually works on a 12-volt system, and once it fails, the engine will not get the required pressure to keep running. To start testing, visually inspect the relay for any physical damage. A burned-out or damaged relay usually has visible scorch marks, broken pins, or a distinct smell of burnt plastic. Most Ford F-150s experience relay failure at approximately 100,000 miles and cause the engine to stall or not start.
The next step is to check for power at the terminals of the fuel pump relay. Using a digital multimeter, measure the voltage on both the input and output terminals of the relay. At the input and output of a typical fuel pump relay, there should be a constant 12 volts when the ignition is turned on. Anything less or fluctuating voltage indicates that the relay might need replacement. Sometimes, you may notice that the failure of the relay is linked to a worn-out ignition switch. This has happened in some Chevrolet Silverado trucks where an ignition failure prevented proper relay activation. A General Motors service bulletin shows this problem usually affects models with over 150,000 miles.
If the voltage is proper and the relay still does not work, then the coil resistance of the relay should be checked using a multimeter. A relay coil should have resistance in the range of 50-200 ohms depending on the model. If the coil resistance is out of this range, then the relay is probably faulty. In some Honda Civics, a common fault is too high coil resistance that is out of the recommended range, which causes intermittent fuel pump operation.
Substitution of the relay with a relay that is known to work in another system is another well-tested method of testing for malfunctioning relays-a common example being the use of a horn relay, where often the specification is compatible. If the fuel pump functions with the substitute relay then it will indicate the original faulty relay. A P0446 DTC on the Toyota Corolla could be a fault in the relay, which provides the wrong fuel pressure needed to run the engine, while performance will suffer. Check whether the relay is the problem-a relay swap will quickly confirm yes or no.
In cars with electronic control units, checking the control signal to the relay must also be performed. This is because the ECU will send a signal to the fuel pump relay to turn it on. If this signal is missing, it may indicate wiring problems, a faulty ECU, or a bad relay. In most Mercedes-Benz models, for instance, failures in the ECU often cause the fuel pump relay to not get the required signal for correct fuel delivery.
Lastly, check the wiring from the relay to the fuel pump. A loose, corroded, or broken wire will not allow the relay to feed the pump power and thus can create fuel pressure problems. Corrosion especially reduces the effectiveness of the electrical connection tremendously and is very common for vehicles driven through harsh climates.
The diagnosis of the Fuel Pump relay should be done in a structured manner to identify the problem correctly. If the relay does not pass these tests, it needs to be replaced with a high-quality Fuel Pump relay to reinstate the correct operation of the fuel system and avoid problems with the performance of the engine.