Lift pump

[teedo]

Hello all new to the group been looking through tons of your topics and finding tons of help and wanted to thank you all.
I do have one question to ask about lift pumps I need some clarification on how they work I recently purchased a 1994 GMC K2500 with a 6.5TD my first diesel it was non running in my testing I found out that the lift pump was rewired to a hot wire while the key was in the on only position but lost power in the start position if I'm not mistaken there should be a prime in the key on for a few sec then be on in the on position while cranking and after the ops picks up pressure then that system takes over in the on position if this is not correct can you point me in the right direction on how this works?

Thank You

[happyauer]

Thats it.

[FloridaHorseman]

On your '94 the OEM system has the fuel pump (LP) relay on the firewall energize in the start position, then the OPS takes over after oil pressure has been established. Since the LP was rewired to be hot with the key in the on position it probably was tapped into a 12v source that gets cut off while all energy is diverted to the starter during the crank cycle. No telling which circuit that might be. Your best bet would be to start from scratch and do the LP relay mod in the Stickies section and also install a fresh AC Delco OPS. Yours will be located under the intake manifold forward of the fuel filter canister. ~FH

[JMJNet]

Welcome!!!

In a 94-95, there is never a priming function despite what people say.
It is suppose to but in my case, the LP never turns on when the key is on until the engine start running and OPS takes over. Which means if OPS is bad, then LP is dead also.

Priming function can be added by adding 2 relays into the LP/OPS circuit or buy a Kennedy Diesel LP/OPS harness.

[vstech]

Originally Posted by JMJNet

Welcome!!!

In a 94-95, there is never a priming function despite what people say.
It is suppose to but in my case, the LP never turns on when the key is on until the engine start running and OPS takes over. Which means if OPS is bad, then LP is dead also.

Priming function can be added by adding 2 relays into the LP/OPS circuit or buy a Kennedy Diesel LP/OPS harness.

this is different from the OPS relay mod? it's only mentioning one relay... what is the two relay mod you are talking about?
my truck only has one added relay, and I'd like to make sure the LP comes on with the key.
got a diagram for the 2 relay you are discussing?

[Graplr]

Originally Posted by FloridaHorseman

Yours will be located under the intake manifold forward of the fuel filter canister. ~FH

My OPS on my 94 is at the back of the block to the driver's side of the fuel filter.

[teedo]

Thanks for the advice i will look into the the mod i did some testing on the stock relay plug and i get voltage to the it from the hot wire in and voltage to the to the starter wire while cranking and voltage from the ops wire after about 2 revs of the engine assuming thats when it picks up oil pressure and that sensor completes its circuit which tells me that the ops is working it looks new so I think im good there. So if there is no prime mode the lp should be on while cranking at least and on after the ops kicks in for the on/run position. I trace the wire from the lp and it goes through the firewall where the hood release cable runs and was connected to a orange wire and orange plug that is mounted right there on the firewall to the left and above the brake pedal?

[57diesel]

I thought someone posted a diagram of the 94-95 system with that relay and it did run during cranking but dropped out and went back to just the oil switch for run.

[JMJNet]

Originally Posted by vstech

this is different from the OPS relay mod? it's only mentioning one relay... what is the two relay mod you are talking about?
my truck only has one added relay, and I'd like to make sure the LP comes on with the key.
got a diagram for the 2 relay you are discussing?

This comes from a member's post 94c1500, so I am not taking credit for this one.

Hopefully, it is clear. I converted it to PDF.

[teedo]

Originally Posted by 57diesel

I thought someone posted a diagram of the 94-95 system with that relay and it did run during cranking but dropped out and went back to just the oil switch for run.

Are you talking about the mod or the oem diagram