fuel filter pressure switch

[F350camper]

Hey guys, how does the pressure switch that controls the fuel filter light work? Or maybe a better question is how do you test it? I assumed that the terminal which the wire connects to would be shorted to ground if the switch didn’t have like 5psi or so. Meaning that if you had your switch on the bench, you should get a short from the terminal to the metal body, and if you apply 5psi, it should open up.

Well, I have two of these switches, and both are open with no pressure applied. So either i have two bad switches, or I’m not testing them right.

 

[JPR]

I just checked mine and open with no pressure is correct.

I can't check it with pressure at the moment.

 

[F350camper]

ok, thanks John. I didnt think that I could have 2 bad ones, but I guess I still dont understand how it works then.

 

[towcat]

For what its worth, the switch is on the intake side of the filter. If there is too much blockage in the element, the switch trips from the pressure created by the blockage. usually, your throttle foot would already be telling you that there is a problem due to the lack of power.

 

[F350camper]

For what its worth, the switch is on the intake side of the filter. If there is too much blockage in the element, the switch trips from the pressure created by the blockage. usually, your throttle foot would already be telling you that there is a problem due to the lack of power.

on the intake side?! ohhhhh, I thought is was on the clean side.

 

[Guntherx]

your right

on the intake side?! ohhhhh, I thought is was on the clean side.

The clean side is the intake side. In (to the filter) take.

 

[Guntherx]

For what its worth, the switch is on the intake side of the filter. If there is too much blockage in the element, the switch trips from the pressure created by the blockage. usually, your throttle foot would already be telling you that there is a problem due to the lack of power.

Ok so if its on the intake side of the filter, I have a ? While on the big trip and on grades with the toes wiggling out the front, I would see the filter likght slowly come on only when I'm on the new 45 gal. back tank. If I would switch the light would go out. I replaced the filter with no change.

When I put the tank in at first I did get some kinks in the lines, but took it down and fixed it. What else could be doing it then?

That is why I don't understand that it comes on with psi. I assumed it was a fule level thing in the filter housing. I was going to do the red pump to see if it fixes it.

 

[F350camper]

Ahhh ha!

I figured it out. This thing is a vacuum switch, not a pressure switch. I never would have guessed. I mean sure, that’s how the air filter ones are (vacuum triggered) But on the fuel system I would have expected a 0psi condition to cause the light to come on. But no, when your fuel filter light comes on, it’s because you’re filter is so clogged that your IP is actually making vacuum on the clean side of the filter. Now that is one clogged filter.

 

[F350camper]

Another thought just crossed my mind on this. Since that fuel filter light is activated by vacuum, and not just low fuel pressure. It paints a much different picture for guys that have posted about installing too little of an electric lift pump, and see the light come on during hard acceleration.

 

[Guntherx]

That makes sense. It is what is happening to me on my back tank. It is a long way back there and a very deep lift. Time to put in the red pump.