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Discussion Starter #1
Hey,

I went home this Sunday to move the truck and trailer to their "winter homes" i.e. inside the truck shed as it had been sitting outside for a few weeks waiting to move some equipment around. Been pretty cold out last week or so and the truck wouldn't start. Didn't have winter blend or #1 in it when left it so I figured it may have gelled up. Towed it into the heated shop and left it for a while so I could go move the trailer into the shed and had a battery charger on it. Long story short, did some quick TS when it still wouldn't start foundthe fuel filter to be pretty dirty. Now the last few trips with it I had noticed power seemed to be down and so was fuel economy, but the fuel filter restriction light has never come on. I thought it may be the fuel filter, but was getting ready to install my new fuel system with filters and didn't want to "waste' money on another stock replacement filter.

So my question is, does anyone know what pressure or vacuum the Fuel restriction switch trips? I know it must be a normally closed switch, but is it held open with pressure (i.e. when the pressure drops below "X" it closes) or is it pulled closed with vacuum? Seeing it's position on the bowl, I am inclined to think it is a minimum pressure holding it open. I am intergrating it into my new fuel system and would like to know if anyone knows this info. I am not ready to remove it or I would test it myself.

Switch could be bad I guess.....

Cory
 

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I don't know if I've ever heard of one being bad, other than leaking fuel like a sieve. I don't recall ever seeing a spec on the switch, but it is a vacuum switch. Cheers!
 

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Hate to start a debate, but its a fuel operated pressure switch not a vacuum switch. It is on the low pressure side of the bowl assembly which means its only reading at max 7-8 psi. You can also interchange the switch off of an IDI as its the same readings. I believe it comes on below 5psi, but I won't swear to it. I have an old switch I will see if I can reverse my vacuum diaphragm set up to reg pressure and get you an accurate reading. Good luck.
 

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That doesn't really make sense to me. I know the switch closes with a plugged filter. If it closed simply on low pressure (say below 5 psi), why wouldn't it come on with a bad fuel pump? Cheers!
 

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The switch is a N/C (normally closed)ground switch. With the truck running the fuel pressure opens the contacts of the switch removing the ground and turning off the light on your dash. Also why you can unplug it and the light doesn't come on. The main reason that the light doesn't normally come on when a fuel pump goes out is that its a 2 stage pump. The restriction sensor is on the low pressure side or lift side of the pump. Once the fuel goes through the filter on the low pressure side it goes back into the high pressure side of the pump (remember 2 stages) and from their through the heads and injectors and back to the regulator and return on the side of the fuel bowl. The switch is not in the high pressure side of the pump which is normally the side that will fail on these trucks. I hope that makes sense.
 

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Hmmm, the EVTM states that the switch closes with plugged fuel filter. Cheers!
 

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Discussion Starter #7
The point is that it is held open by a minimum pressure thus no light. If the filter becomes restricted, the pressure drops due to the pump not being able to pull fuel through the restriction, thus allowing the switch to return to its normally closed position and grounding the negative side of the circuit causing the light to come on. Since the low stage of the pump is only about 6 PS I, I would imagine it is around <5psi. Remember negative pressure = vacuum. Thanks for the replys.

Cory
 

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Thanks Cory. But just to clarify the switch is on the pressure side not a vacuum side of the pump. Looking from the front of the engine the top right port of the pump is vacuum side directly from the tank. The top left is low pressure through the filter, it returns from the filter to the bottom side of the pump through the second stage and high pressure out the back to the back of the heads, then out of the front of the heads to the braided lines to the regulator on the side of the fuel bowl and back to the tank. So it truly is a pressure switch not a vacuum switch.
 

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Discussion Starter #9
I know I agree it would be located on the pressure side of the first stage (diapraghm lift section) where fuel pressure is approx 6psi. If the filter becomes plugged, the fuel flow is reduced, which I turn reduces the pressure across this stage and the second stage. I would imagine if the first stage of the pump became weak enough or failed, you could get this light for that as well since it is simply a pressure switch.

On a side note, I realize I miss typed on my earlier post today. I believe this switch grounds an input to the ECM which commands it to power the restriction light. This is why those indicator lights go through a test flash at startup. I will have to confirm with a wiring diagram, but I doubt this switch directly applies power or ground to the indicator light.

Cory
 

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You are correct it does go through the main harness.
 

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Fuel Filter Restriction does not have any connection to the PCM. Power will come from UD fuse #17 and the ground will come from either the switch or the ignition switch (when turned to the Start position). Cheers!
 

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Discussion Starter #12
Update - Fuel bowl starting leaking like a sieve. So I pulled it and completely rebuilt it. The restriction switch was clean and dry, but there is no continuity across the terminal to the body (ground). The switch is bad, which is why it isn't setting off the light. Didn't have time to get another one, so I threaded it back in for the time being. The filter assembly was so nasty on the inside. Regulator screen was plugged, and the catch screen had debris in it as well. All clean and leak free now. Truck runs very well. Just thought I would update. Still trying to put an E-Fuel system together, but now thinking about a hybred system using a Carter 6 psi pump pre-mechancial and my new fuel filtration system.

Cory
 
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