This explores some ways to use a milliamp current probe with your lab scope. Most vehicle PCM input signals do not produce enough current to test with a current probe, it is best to view the voltage trace of those signals. The engine management signals that we want to test with the milliamp probe, are the output signals that control solenoids, motors, and relays.
The milliamp probe can be used for testing fuel pumps and other low current electric motors. Ignition coil current can also be tested with the lab scope. Your PDA will allow you to view ignition coil current and secondary voltage at the same time, using the Scope Channel in the Engine Test menu.
The milliamp current probe allows quick and accurate non-intrusive testing. Figure 1 is a new fuel pump voltage and amperage trace on a GM CMFI system. This system requires a minimum of 8.5 amps. Figure 2 voltage signal is good and the amperage signal is faulty. The fuel pump motor has an open circuit and the vehicle had a loss of power. Figure 3 displays secondary ignition and coil current. This screen captured the crossfiring in the secondary and verified that the coil triggering was not at fault.
Test good current signals and note their shapes and amplitude so that you may detect a faulty signal. A great tutorial on current testing fuel pumps and ignition circuits is available on the net. Visit Linder Technical Services, www.lindertech.com and download Current Ramping I and II.
