Figure 1
When we connect our ignition scope to an engine, we expect to see one firing signal per cylinder on the screen. I was testing an eight cylinder engine and the primary ignition parade pattern would intermittently display from four to fifteen firing signals. The engine would misfire and stall at idle. Why were there too few and too many firing cycles on this engine? The next step was to test the computer input and output signals that control the ignition primary.
The first three signals to test on this GM system, Figure 1, are the distributor reference (DREF), spark timing (EST), and by-pass (timing advance control). The graphic below shows these three signals. The top signal (DREF) is an input to the computer, and it was good. The middle and bottom signals are output signals from the computer and they are bad. The EST signals should mirror the DREF signals, with a ratio of one to one at approximately 5V. The by-pass signal should be a constant 5V while the engine is running. You can see that the timing signal drops to 0.5V when the by-pass signal is 0 volts. We had good information going into the computer and garbage signals coming out. A replacement computer corrected this probl
