Engine/Transmission (1998.5 - 2002) Lift pump control voltage & fuel solenoid

[Gordon Maney]

To prepare for some diagnosis; a question. What diagnostic process do you use if the lift pump is not getting a voltage applied through its connected lead?

Is that voltage controlled by switching, or is it controlled by a computer? Are people ever seeing a lift pump not working because it is not getting its control voltage?

Do these trucks have a solenoid controlled fuel valve that cuts off fuel when the truck is shut off? I was visiting with a guy who had a 12 valve; he said his truck had such a valve.

 

[jelag]

On most vehicles, the fuel pump is controlled by a relay in the fuse panel... the function of the relay is to switch on and off the pump... .

But there are several circuits that effect this... . when you turn the key on, but do not start the engine (no rpm) the circuit allows for about 20 seconds of fuel pump operation to charge the system if there was bleed down, or a leak... to restart the pump you need to go to key off and key on... . or start the engine and supply the computer with RPM...

In addition, most vehicles have an inertia switch in the dash... . if the vehicle is bumped hard (rear ends another vehicle) the inertia switch trips and shuts down the pump, so that in event of severe damage and rupture of the fuel system it won't dump fuel all over... and burn the vehicle down. .

I've upgraded several trucks from from lift pumps to in tank pumps and have never seen a problem with this circuit... in addition I've jumped off this lead, to run a relay to allow the operation of other electric fuel pumps for a back up... during the time we had the lift pumps...

Hope this is what your looking for... . so yes, there is computer control... but I've never seen problems with this power..... I'm guessing that if you overloaded it... . it could fail. .

 

[Gordon Maney]

On most vehicles, the fuel pump is controlled by a relay in the fuse panel... the function of the relay is to switch on and off the pump... .

But there are several circuits that effect this... . when you turn the key on, but do not start the engine (no rpm) the circuit allows for about 20 seconds of fuel pump operation to charge the system if there was bleed down, or a leak... to restart the pump you need to go to key off and key on... . or start the engine and supply the computer with RPM...

In addition, most vehicles have an inertia switch in the dash... . if the vehicle is bumped hard (rear ends another vehicle) the inertia switch trips and shuts down the pump, so that in event of severe damage and rupture of the fuel system it won't dump fuel all over... and burn the vehicle down. .

I've upgraded several trucks from from lift pumps to in tank pumps and have never seen a problem with this circuit... in addition I've jumped off this lead, to run a relay to allow the operation of other electric fuel pumps for a back up... during the time we had the lift pumps...

Hope this is what your looking for... . so yes, there is computer control... but I've never seen problems with this power..... I'm guessing that if you overloaded it... . it could fail. .

Good stuff, thank you.

Out of curiosity, where in the dash is this inertia switch?

 

[Gordon Maney]

I ordered the test pressure gauge from Geno's Garage; the Vulcan unit. I am also going to pour five gallons of diesel into the tank and try it again, to verify it is not the gauge issue.

 

[mhenon]

... ... . in addition I've jumped off this lead, to run a relay to allow the operation of other electric fuel pumps for a back up... .

That's just what I did with my main LP. Got a little suspicous of the ECM voltage powering the pump so now it just has to power a relay. The battery runs the pump. I keep a spare relay all wired up and ready to go under the hood. I've already had one relay go unexpectedly.

Now lets hope the ECM doesn't go belly up.

Mike