Hello,

Lift pump runs when you jump the orange and grey wires at the OPS. The lift pump runs when cranking over the motor ....But doesn't run when the truck is running. Cranks over..pump runs fine but the minute the truck starts the pump stops pumping.

Replaced the fuel pump relay and the problem is still there. Checked relay wires and if you jump the orange and grey wires at that location the pump also will run. OPS is new (3rd) new lift pump (2nd)

My question is what transfers the control gate of the relay. Is there a wire that when energized pulls the control gate over so the orange and grey become hot? I know those two wires need to connect so the pump will run while the truck does.

Can I just make these two wires hot whenever the truck is running?

I know it might not be wreck proof and could catch fire....but at this rate that is what I would really like to see.

My 6.2 has 315,000 and I would take off anywhere in it..the 6.5 has 175,000 and I can't even make it to the parts store....6.5's and GM are starting to really suck!

P.S. Just finished dropping the tank, draining and cleaning the tank, installing a new sending unit and new factory line to the lift pump... AND NO..the problem was there before I did all that work :)

Any help and ideas are greatly appreciated.:help:

Bill

There is an oil pressure sensor on the back side of the block that makes the lift pump run when proper oil pressure is present. As most guys will tell you this is a bad design because the power for the pump flows through the switch. I have never had to replace one but hear a lot of guys have. Search other threads for the oil pressure switch.

As far as lift pump power goes, you can by-pass the OPS all together by jumping fuel pump relay pins in the under-hood fuse box. But don't do this if you're worried about diesel fuel continuing to pump if you were in an accident in which the engine shut down but electrical power was still flowing.

Obviously, I wasn't worried about it. And I'm not worried about OPS failure anymore either. ;)