GM-GMC: 94 6.5l diesel power loss, oil pressure switch, chevy silverado

Question
Hello, I hope you can help me with the problem im having whith my 94 chevy silverado 6.5l turbo diesel dually . First, the list of some of the parts that have been replaced include the following: Harmonic balancer, Fuel pump, Fuel injection pump, fuel filter, turbo waistgate solinoid, Engine replaced about 1 1/2 years ago. The problem i had first was stalling, after replacing the fuel injector pump, that problem was fixed. Then, another problem accured, here are the symptoms: When under load for a little while, the check engine light comes on and the truck suddenly loses a lot of power, even after changing the waist-gate solinoid the problem was still there. PPPPPPLLLLLLLEEEEEEAAAAAAASSSSSSSEEEEEE help me the truck needs to get fixed because it is towing a trailer and it will be under load most of the time. Also if you have ANY tips, oppinions, please tell me. thanks

Answer
I had a similar problem yesterday when toaing mor ethan 5,000 pounds behind my 84 K-2500 6.2 diesel.  It turned out to be the secondary fuel filter and I replaced it after stopping off the highway.

Your 94 is more complicated becasue GM runs the power to the transfer (lift) pump through a set on contacts in the oil pressure switch (OPS).  The OPS contacts are not rated for the power that the pump requires, so the contacts burn out.  Your engine may run without the transfer pump, but it will starve for fuel under heavy loads.

The pump is under the vehicle and inline with the fuel line from the tank to the engine.  You should be able to hear and/or feel it operate while the engine is running.  If not, try running a source of +12V to the gray wire on the pump.  If the pump won't run with power applied directly, the pump is bad (and they are known to fail).  If it does run with power directly applied, the OPS is bad.

Also, recheck the fuel filter.  There seems to be a lot of dirty fuel out there and filters can plug with just one tank of bad fuel.

You should also consider a kit to remote mount the pump mounted driver (PMD) from the passenger side of the injection pump to a cooler location.  The PMD (also known as the fuel solenoid driver or FSD) does not like heat and it will fail unless it is placed where it can stay cool.