Many lab scope signals may appear to be good at a glance. Close examination and experience will help to evaluate the features of a trace that will lead to the cause of the problem. The top traces are Ford PIP signals. Figure 1 has noise on the tops of the digital signals. The noise is caused by AC voltage riding on the DC signal. Figure 2 is an example of too much AC voltage that can interrupt the signal and cause a driveability problem.
Figure 1
Figure 2
When AC noise is detected on a DC signal, test the alternator ripple. Select the Diode Pattern test in the Engine Test Menu and connect the current probe to either battery cable and record the ripple pattern.
Figure 3 GOOD
Figure 4 BAD
