Ignition Diagnostic Monitor (IDM)

PURPOSE
The Ignition Diagnostic Monitor (IDM) signal is a function of the Powertrain Control Module (PCM). Information on system failures is stored within the PCM and used for diagnostic self-tests.


OPERATION
The IDM signal is transmitted on each spark firing. The PCM acts as an electronic switch to ground in the coil primary circuit. When the switch is closed, battery power applied to the coil primary circuit builds a magnetic field around the primary coil. When the switch opens, the power is interrupted and the primary field collapses inducing the high voltage in the secondary coil windings and the spark plug is fired. A kickback voltage spike occurs when the primary field collapses. The PCM uses this voltage spike to generate the IDM signal. IDM communicates information by pulsewidth modulation in the PCM.