About the PMD / FSD
The Pump Mounted Driver or PMD (also known as Fuel Solenoid Driver or FSD) is the brains behind the electronic fuel injection pump.
The driver module is used to control the Stanadyne DS4 injection pump used on all GM and GEP 6.5L Turbo-Diesel engines. This includes Chevrolet, GMC, and Hummer / H1 vehicles and General Engine Products Optomizer and P400 engines.
When this module fails, the truck will not operate and no code or "check engine" light is displayed. Failure is often intermittent to begin with, and gets progressively worse until finally it will not work at all.
The module is located on the driver's side of the injection pump, on top of the engine. The driver is cooled by fuel flowing through the pump.
Failure is usually heat-related, so it will happen mostly after the engine is fully warmed up. Symptoms include rough running, stalling, surging, and difficulty in restarting. Pouring cool (not cold) water over the driver may cool it enough to temporarily reduce or eliminate symptoms.
Be sure to check for stored codes before you replace the driver since these symptoms have other causes as well.