1994 gmc hd wiring trouble

[doych64]

Does anyone have a wiring diagram for a1994 3500hd ? I am having a problem with power to the lift pump . If i jump the relay it will continue to run .I need to know what activates the relay ? Where do i find oil pressure switch or cranking power ect.. thanks

[Kenneth]

The oil pressure switch sounds like it could be the culprit. The switch is on the back of the motor on the driver's side behind the head. My switch went out and it wasn't powering the lift pump. If it is the oil pressure switch make sure you replace it with an AC Delco or a Standard oil pressure switch. I got one from autozone and it only lasted 2 months. I then replaced it with a Standard one from carquest and it has lasted for over 6 months now.

Hope this helps.

[a5150nut]

You need to remove the fuel filter housing and the glow plug relay to find the OPS. Not much room , but doable. Some have added a relay and have the OPS just run the relay and use another source for voltage to the lift pump. It takes the load off the OPS.

[JohnC]

The relay feeds the pump through the NO contacts when the starter is engaged. When the engine is running power flows through the NC contacts of the relay and the oil pressure switch to the pump.

(NO = Normally Open...)