94 lift pump oem wiring questions

[78ta]

1994 k2500 6.5td

I was doing some research on lift pump relay mods when I got to wondering about the oem fuel pump relay and fuse on the fire wall. Where do these come into play? Correct me if I'm wrong, the oil pressure switch is what directly feeds the lift pump with 12v. So the oem relay and fuse are before the oil pressure switch?
When is that relay powered up and from what? And is that where the ops gets it's 12v from?


Does anyone have wiring diagrams for this for a 1994?

 

[JMJNet]

Welcome!!!

Not sure of the wiring diagram.

Here is a good explanation which is valid for 94-95 and may be 93 also differences with 96+ (OBD-II).

http://www.thetruckstop.us/forum/showthread.php?2647-Reference-Material-Lift-Pump-OPS

 

[78ta]

Thanks for the link. After reading up some more it appears that there is a fuse in fuse box that feeds power during engine crank to the fuse and oem fuel pump relay on firewall. And that powers the lift pump on my 94 only during cranking, after start up then the ops feeds power to lift pump. Now I'm thinking would there be a problem with re-wiring that relay so it powers up and runs also in the key on position and do away with the ops powering lift pump? I know and have read up on the norm "ops relay mod" not really what I want.

 

[JMJNet]

GM idea for going through the OPS was for safety reason.

The OPS relay mod diagram shown by Buddy is to make sure that the OPS is still in the circuit so it preserve that safety feature while rendering the OPS to not passing on any electricity to the LP. So the OPS is just a switch on the relay but let the relay pass on the electric.

You can also wire the relay in a way that it will prime the LP during WTS or once the key turn on.
IIRC, this is a double relay OPS relay mod.