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Lift Pump Pressure Switch Relay:

The current lift pump configuration uses an oil pressure activated switch to turn the lift pump on when oil pressure comes up in the engine. I believe the lift pump is also activated when starting the engine as well, but I need to look at schematics to verify this. Anyway this is about the pressure switch.

The current that drives the lift pump must pass through the contacts in the pressure switch. Lots of us with older trucks have replaced lift pumps and pressure switches. Recently I was helping a friend with a fuel delivery problem. His truck would start stumbling as the fuel delivery demands increased. After checking all the obvious things, fuel filter, line obstructions etc., we installed a relay to drive the lift pump. In this case it turns out the switch was limiting current available to the lift pump causing it to run slower than it was designed to. We bench tested the pump for pressure and flow. The pump ran much faster on the bench than it did on the truck, even when the pump was plugged while running a pressure test. The relay solved the problem.

This is a very simple modification to make to the Hummer. All that is required is a relay holder, relay, weatherpack pin and fuse holder. I installed the relay holder on a nut-sert just below the CTIS compressor and hooked the wires up as follows:

  • Remove the + wire to the fuel pump from the weatherpack connector at the lift pump. This wire provides the signal to the relay.
  • Connect the other side of the relay signal to ground. I put a ring lug on this wire and attached it to the relay holder screw.
  • +supply to fuse holder to junction block below CTIS compressor.
  • +switched supply to weatherpack connector.

The switch now provides the signal to the relay that activates the pump. This will supply the pump with all the current it needs to run at full speed, and the pressure switch should last a lot longer as well.


I did something very similar...

I used matching weatherpac connectors to splice in between the lift pump and the wiring harness. This provides both signal and ground, which leaves only the need for an external power lead.

I also added a manual switch that allows me to run the pump without the oil pressure switch working. I use this in case of a failure of the oil pressure switch, and when purging the fuel system when changing the fuel filter or running out of fuel. (The NA Diesel only runs the pump when cranking or with oil pressure - the TD runs it for several seconds when you turn the key to "on").

Dave Breggin
'95 Diesel Wagon

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