What exactly does the PCM/PDM do???

[cheyenne1500]

Tha black box on the injector pump...what exactly does it do/control thanx :)

[Hubert]

PMD = pump mounted driver. It takes the signal from the pcm = powertrain control module and boosts the power and relay's it to the fuel solenoid to meter the fuel at the back of the injection pump going out to the injectors. The pcm monitors all sensors and determines the appropriate fuel delivery.

You have to appreciate the quickness of the signal, different magnitudes of injection event, and precise / accurate fuel metering required for smooth economical power.