lift pump/ops?

[TCole2008]

I have '95 6.5TD P30 chassis Trek motorhome. Used pressure gauge on T valve while engine running and got no pressure. Checked lift pump and definitely not running. Turned on my manual override switch, installed years ago to aid filter changing, and pump fires up 7lbs of pressure.
So I get a new ops and install it and still no pressure? What else should I be checking?
Tom

[simon]

check Fuse
Power to OPS
OPS itself,could be bad,even when new.
continuity in wire from OPS to LP
LP ground

[TCole2008]

All the fuses in box under dash are ok. But don't know if fuse you are referring to is in under dash box? Is there another location where it might be located?

[simon]

no way of telling where all fuses are in your vehicle,i'm only familiar with trucks. test OP pluggin,1 of the 2 parallel terminals should have power with key on,the offset terminal is for oil pressure.there could be a fuse on pass side firewall,is in that location on 89/94 trucks,maybe you still have a LP relay[could also be the culprit] in that location too?
95 and up trucks uses the computer to initially power the LP at startup.

[JohnC]

Put the trans in drive and turn the key to start (engine off). If the pump runs then most of the wiring including the fuse is OK. (Note, this only works for MY '95) If this checks out, try jumping the oil pressure switch. Go from there... Current flows through the NO contact on the relay when the starter is engaged and the NC contact via the OPS all other times, even key off.

[TCole2008]

Simon, I followed your lead today, I'll try John's ideas tomorrow. First I've got over 12.5 volts at the orange wire in the ops plug, with the key on. Second I attached a probe to the gray wire across from the orange in the ops plug and ran the other probe to the plug going into th LP, White wire with yellow stripe (yellow wire on LP side) NO tone! I'm thinking that if I try John's idea and jump the ops by connecting the orange and gray wires at the ops plug with a jumper I'm going to end up at the same place with no current at the LP.
So what is between the gray wire at the ops plug and the white with yellow stripe wire at the LP plug?

[TCole2008]

Fixed it. Tracked it down to the connection between the chassis wire harness plug to the engine wire harness plug connection. One side of the plug had pulled loose just enough to cause the LP connection to fail. I reinstalled the orig OPS and it works perfect now. So I've got a spare OPS and learned another lesson. Thanks for your help.

Tom '95 6.5LTD P30 chassis 28' Trek motorhome