Circuit Test When the ignition key is turned to the ON position, you can hear the PCM (Pulse Control Modulation) briefly turn on the fuel pump. If the engine is not started, the PCM will shut down the fuel pump again for safety reasons for a few seconds. When the ignition key is turned to the START position and back to the ON position, the PCM relies on the crank position sensor to determine if the engine is running. Assuming that the fuel pump is not being heard when you turn the ignition key on, the fuel pump will not operate for at least three of the following three reasons. • For some reason, the PCM decided not to switch on the fuel pump. • There was a malfunction between the PCM and the fuel pump. • The fuel pump was actually switched on but not running. We will exclude two obvious options. Assuming that the PCM is commanding the fuel pump relay to be switched on and energized, and assuming that the fuel pump itself is intact, the fuel pump is not operating properly for some reason between the PCM and the fuel pump. Connect the DMM (digital multimeter) to the positive and negative terminals of the fuel pump, and then turn the ignition key to the ON position. The DMM should immediately read out the battery voltage. If this is not the case, look for an open circuit somewhere between the fuel pump and the relay. In addition, the relay itself may also be faulty. If the fuel pump is energized and grounded, determine if the fuel pump is receiving enough power. Due to the low impedance of the fuel pump, the DMM can indicate the battery voltage at the fuel pump position. However, the voltage drop test reveals how much less the voltage to the fuel pump is actually than the battery voltage when the circuit is loaded. To test the entire fuel pump circuit for voltage drop, the circuit should be split into two halves: first the ground side of the circuit and then the positive side of the circuit. All tests must be done with the circuit "powered on". Use jumper fuses to energize the fuel pump relay and fuel pump circuit. When the circuit is powered on, connect a test lead from the DMM to the negative terminal of the battery and the other test lead from the DMM to the negative terminal of the fuel pump. If the ground circuit is normal, the DMM should indicate a voltage drop of about 0.1V. Any pressure greater than this will indicate the voltage was lost before the voltage reached the fuel pump. Damaged or corroded wires or harness connectors are likely to cause this phenomenon. If the voltage drop on the grounded side of the circuit is detected and there is no problem with the voltage drop, the voltage drop test is repeated on the positive side of the circuit. Place one probe of the DMM on the output terminal of the fuel pump relay and the other probe of the DMM on the positive terminal of the fuel pump. If the DMM shows a voltage drop greater than 0.1V, look for damaged or corroded wires or harness connectors. Use the "half points" method to determine the location of the faulty line. First, divide the circuit in half and then repeat the voltage drop test between the middle of the circuit and the relay, and between the middle of the circuit and the fuel pump. This allows you to determine if the failure occurred in the first half or the second half of the circuit. If the latter half of the pressure drop, then the second half of the circuit is divided into two halves, respectively, for pressure drop test. Each time the circuit is divided in half, the distance to find fault closer. Remember, do not expect the fuel pump to function properly with a reduced supply voltage. Below, we will show how to use the low current probe and DSO (Digital Storage Oscilloscope) to detect and diagnose the internal "health" of the fuel pump. Current Tests Inductive current probes vary in shape and size and can be used to test alternating current and direct current of various sizes. Inductive low current probe can be used to measure lower current in the fuel pump circuit (typically below 10A). The inductive probe transforms the measured signal into a signal recognized by the DSO (Digital Storage Oscilloscope) that can be read out of voltage. Although DSOs (digital storage oscilloscopes) show voltage signals, we know that they actually represent the current. Most inductive low current probes have two gears - 10mV / A and 100mV / A - that affect the waveform display on the digital storage oscilloscope's screen. If the inductive low current probe is tuned to 10mV / A, corresponding to 10mV / A shown by the digital storage oscilloscope, the probe is measuring 1A. If the inductive low current probe is tuned to 100mV / A, the probe measures 1A current for each 100mV / A shown on the digital storage oscilloscope. While viewing the fuel pump current waveforms on the digital storage oscilloscope, start with the time coordinate of 1ms or 2ms per indexing and the voltage coordinate of 100mV per index. The waveform can be analyzed in more detail at 1ms per division, while the fuel pump speed can be calculated more easily at 2ms per division. Adjust the oscilloscope and probe to the above settings before pinching the probe to the fuel pump harness, then use the inductive probe's ZERO adjustment dial to adjust the waveform to 0 (on the digital storage oscilloscope Ground) position. The clamp of the current probe can be placed anywhere within the fuel pump circuit as long as it is in series between the supply and ground. Clip the probe to a fuel pump line, if it is available, or to a power line where the fuel pump is in the fuse box, relay switch, or inertia switch, or to a test underneath the bonnet Connector. Although the voltage may be different at different points in the circuit, the current at all points is the same in the series circuit. Therefore, make sure that no other components (such as a transfer pump) are connected in parallel to the main pump in the circuit, because such components will affect the test result. In testing the fuel pump, the engine and other electronic components must not be in operation, their operation will affect the voltage on the fuel pump, will also affect the current reading. Some manufacturers provide a separate wire-test connector under the bonnet to power the fuel pump. To operate the fuel pump with this test connector while the engine is OFF, connect a power supply jumper to this test connector. Before using this method, first look at the road map. You can bypass fuses, relays, or other switching components when the fuel pump power is supplied through external wiring. If this is the case, the display of the pump current may be within the specification during lead detection. However, during normal operation of the vehicle, a voltage drop in the by-pass circuit may cause a low pump current. Ideally, when testing a fuel pump, the fuel pump's power wiring should be the same as the vehicle's power wiring during normal operation. A good fuel pump, will produce a characteristic curve. The oscilloscope is set at 100mV per index and the probe is set at 100mA / V. A vertical index equals 1A, so the pump current is between 5 and 6A. With the fuel pump in good condition, the peak waveforms should be consistent. Poor contact between the brush and one or more commutators will cause jagged peak or low amplitude peaks. Mechanical resistance in the fuel pump causes the pump current to exceed the normal average pump current. A blocked fuel filter or line can also cause an increase in pump current. The resistance in the fuel pump or elsewhere in the circuit reduces the voltage applied to the fuel pump, thereby reducing the pump current. Fuel pumps in high pressure fuel pumps, such as the PFI inlet fuel injection system (5.1 to 6.5 kPa), require more current than the fuel pumps required in the TBI throttle zone fuel injection system (1.3 to 1.9 kPa) Of the current. The normal current of the TBI system fuel pump can be as low as 3 to 5 A, while the PFI system fuel pump requires an average current of 4 to 6 A. The General Motors CPI center point injection system fuel pump (8.0 ~ 9.3kPa), need 8 ~ 10A current. Pressure and Volume Testing In the previous section, we focused on circuit diagnostics and current testing of electric fuel pumps. However, the fuel pump ultimately converts the energy and energy from the vehicle's electrical system into fuel, and in the following we will describe how to check the pressure and volume of the fuel pump. Obviously, a completely dead pump does not start the engine. However, the fuel pump on the engine performance in the end what kind of impact can produce it? Any kind of fuel injection engine, if difficult to start, idling does not work
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