Is the fuel system relay in the engine compartment electrical box what powers the lift pump? I have my old pump producing pressure only at idle and the new pump will not even turn. The new pump runs when plugged into a truck at the Cummins shop. They're telling me that I have a lack of proper voltage and was wondering if that relay can slowly go bad or do they just burp and are out for the count. Or is there another power supply, I am just about ready to cut off one of my ten thumbs in frustration. Yep, I'm a dingbat but I'm getting pretty good at changing out that bleeping pump.

 

The power supply for the lift pump comes from the ECM mounted on the side of the engine. The ECM gets its power from the PDC on the driver’s side fender well. Inside the PCM look for fuse #3 (20 amp). This fuse powers the ECM.

The ECM has a circuit inside that controls power to the pump by turning it on and off with the engine cranking and solid on with the engine running. If you try to measure lift pump voltage while cranking the engine there will be a low average voltage. You don’t say when you are measuring the voltage but this may look low when cranking.

I would start by checking the fuse in the PDC. The problem may also be caused by a poor or corroded connection in the ECM plug or lift pump plug. It might be a good idea to disconnect these and check for a good shiny metal connection or a bent connector pin.

There is a small chance that the ECM lift pump circuit has failed due to a short causing the driver inside to burn up. This would show up as no voltage to the lift pump. You say one of the pumps was running so it is not likely.

The fuel system relay is used to power the VP44 and also can cause the engine not to start.

 

Thanks for the info ! Turns out the sending unit for the fuel pump is going bad. The lift pump is weak but is being replaced anyway.