Primary ignition dwell was once adjustable via the air gap in the ignition points. We have no control over the dwell angle in pointless ignition systems. Since we cannot adjust this function, why test it?
The PDA dwell test, located in the Ignition Test Menu, monitors dwell in a bar graph for each cylinder and records the minimum, maximum, and live readings.
Figure 1
Figure 2
This function can be a quick check of crank reference, timing control, and ignition module functions. We expect to see a uniform change in the dwell as engine RPM changes. Figure 1 shows two cylinders that are very low and one that is high. This engine had an incorrect replacement distributor that had the wrong Hall effect shutter. The PCM could not correctly process the wrong input signals. The two cylinders with the low dwell angle had weak spark and misfired.
Figure 2 appears to be OK but this engine had an intermittent misfire. The most helpful clue in this figure is the minimum value. The dwell, on a running engine, should never be zero. Notice that cylinders #8 and #6 have a zero value in the minimum column. This quick check indicates that there is a problem and that more testing might be needed. Install the PDA Lab Scope module and test the crank sensor, timing control signal and the primary to determine the root cause of the problem.
The ignition timing signal, on most cars, is controlled by the PCM.
Figure 3
In Figure 3 the three signals follow the flow of events from the beginning to the end. The crank sensor signal inputs the PCM. The timing control signal is an output from the PCM to the ignition control module. The primary signal is the output from the ICM and shows dwell, primary discharge and ignition spark.
