Delphi Technologies outlines the process of performing a voltage drop test.
Insufficient pressure and flow are indications that a fuel pump is underperforming, but do not necessarily mean that it is failing. Low voltage, or a bad connection or ground, will cause a pump to underperform, producing the same test results as if the pump was faulty. Before replacing any fuel pump based on pressure and flow test results, always follow the vehicle manufacturer’s recommended procedure for testing all electrical connections and the electrical system charge. A minor issue such as a loose ground can cause many problems that resemble more serious malfunctions.
When testing from the power side, it’s important to ensure safety is a priority. Always wear safety glasses, and never probe the vehicle electrical harness connector with digital multimeter (DMM) leads. Doing this can damage the terminals in the harness, creating the potential for an overheated fuel pump connector due to excessive electrical resistance. Always use the proper test probe to perform electrical tests.
To do the test, firstly take the DMM and set it to 20V D-C scale, or D-C if the multimeter has auto-ranging capabilities. Then, connect the DMM positive probe to the battery positive terminal. With the fuel pump wire harness connected to the fuel pump, connect the DMM negative probe to the power feed wire at the fuel pump connector.
Turn the ignition on, but keep in mind that the pump will only run for about two seconds whilst the relay is in prime position, or until it gets an RPM signal. The reading should be less than 0.2V. If the reading is greater, check for resistance in the power supply circuit for the fuel pump.
Alternatively, to do the test from the ground side, the DMM needs to again be set to 20V DC, or D-C if the multimeter has auto-ranging capabilities. It’s a very similar process.
Connect the multimeter positive probe to the ground terminal of the fuel pump connector. Make sure the fuel pump wire harness is connected to the fuel pump. Then, connect the DMM’s negative probe to the battery’s negative terminal.
Turn the ignition on, and again keep in mind that it will only run for about two seconds whilst the relay is in prime position, or until it gets an RPM signal. The reading should be less than 0.2V D-C. If it’s greater than that, check for resistance in the ground supply circuit for the fuel pump.