There's a PMD for that!
From its debut in 1994, the GMC/Chevrolet electronicly control 6.5L diesel engine has been plagued with a glaring reliability issue. Symptoms include stalling, no start, uncontrolled engine speed, engine surge, and hesitation. Anyone that has owned a 6.5L for more than a few years will most likely have run into one of these issues. The Pump Mounted Driver (PMD), also known as the Fuel Solenoid Driver (FSD), is usually the culprit but why?
Many speculate that the location of the PMD on the side of the injection pump is the reason for the repeated frailure. This is true to some extent but owners have still been left stranded even with the PMD mounted on the front bumper or in the cab of the vehicle. Why is this? The simple answer is "heat". Not from the engine as may be believed but from the PMD itself.
The reason the PMD was originally mounted on the side of the injection pump was for cooling. The diesel fuel running through the injection pump and returning to the tank acts like a coolant, removing heat from the system. This is evident when the engine coolant and fuel temperatures are monitored. While the engine will run at 90°C the injection pump is generally half that temperature or less. In theory this sounds like a good idea but in real world situations this is not the case.
Many factors can result in premature failure of the PMD in its original location. These include fuel tank level, transfer pump operation, injection pump solenoid condition, loss of torque on hold down screws, engine load and temperature. The manufacturer of the FSD Cooler, a remote mount heat sync, has performed extensive tests on PMD temperatures under "real world" conditions.
The Above are average PMD temperatures after driving at 100km/h for a distance of
As you can see from the table above the original black PMD was destine to fail as its electronic components were only rated for 85°C/185°F. Even the new grey "bullet proof" PMD has come up short on its promise to be more reliable. Initially offered with a two year warranty which has recently been reduced to just six months. A thicker black PMD has been developed in the aftermarket to withstand temperatures up to 125°C/257°F. This new PMD is much more reliable than the original. So much so that GM has started selling this unit as an OE replacement.
No matter how robust a PMD is, improper mounting will result in an imminent failure. The unit must be mounted on a heat sync with the capacity to dissipate a large amount of heat. The mounting surface has to be perfectly flat to allow full surface contact with the PMD. Under torqued hold down screws or omitting the thermal pad will decrease the heat transfer to any form of heat sync. The example below shows a PMD that over heated to the point that resin actually started to leak out of the housing.
This is one of the worst examples we have ever seen of PMD abuse.
***Note: It is highly recommended to have your vehicle diagnosed properly before replacing the PMD.***