Replacing an electric fuel pump on a modern fuel-injected vehicle is not a job to be taken lightly. This article is to tell you how to check the fuel pump. We'll begin with electrical diagnosis.

When the ignition key is turned to the ON position, you may be able to hear the PCM turn the fuel plunger pump on briefly. For safety reasons, the PCM will turn the pump off again in a few seconds if the engine doesn't start. When the ignition key is moved to the START position and back to ON, the PCM relies on the crank position sensor to determine whether the engine is running.

Attach a digital multimeter to the positive and negative fuel pump 8N7005 terminals, and then turn the ignition on. Your DMM should briefly read battery voltage. If it doesn't, look for a break in the circuit somewhere between the fuel pump and the relay. The relay itself may also be defective.

If the pump is receiving power and ground, determine whether it's receiving enough of both. Due to its low impedence, your DMM may indicate battery voltage at the pump. But a voltage drop test may reveal that something less than battery voltage is actually reaching the pump when the circuit is loaded.

If the ground side of the circuit checks out, with an acceptable voltage drop, repeat the voltage drop test on the positive side of the circuit. Backprobe the output terminal on the fuel pump relay with one DMM probe, then the positive fuel pump terminal with the other probe. If the DMM DLLA138S1191 indicates a voltage drop greater than 1 volt, find the damaged harness connector or wiring that's responsible for the drop.

A voltage drop test is conducted on a harness connector by backprobing on both sides of the connector. The DMM indicates a voltage drop of just .006 volt, well within acceptable limits.

Remember, no fuel pump can be expected to operate properly on reduced supply voltage. And you should also know how to use a low-amp probe and a digital storage oscilloscope to look inside an electric fuel pump and assess its health.