The PMD (Pump Mounted Driver) is the little computer box that was originally mounted to the side of the injection pump under the intake. The designers thought it would be a great idea to have the super-hot running circuitry inside it get cooled by the diesel fuel running through the pump when the engine was running...only they didn't think about what happens when you shut the engine off and the heatsink effect fries the PMD.
I relocated it to behind the front bumper about ten years ago and never had a problem since. The truck went through three injection pumps when new. I bought it new in Florida and the first PMD fried five miles from the dealership the day I bought the truck! Back then GM refused to acknowledge the design defect and would replace the entire thousand dollar injection pump (with PMD attached) instead of just swapping out the PMD. GM eventually admitted the problem and extended the warranty on the pumps to 10 years or 100,000 miles. But they still would replace the entire pump instead of the PMD.
Here's a photo of the original PMD still on my injection pump. I bypassed it with an aftermarket four-foot long harness extension that reaches to the back of the front bumper behind the license plate brackets. It is mounted on a large heat sink in the protected airflow of the oil cooler duct holes in the bumper. I finally removed this old PMD from the pump last week and am keeping it as a spare - you always carry a couple spares in the tool box when you have a 6.5 diesel.
Kind of like the Mopar guys always carry a couple extra ballast resistors in the glove box for the old Mopar ignition systems.
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