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EDIT: I will keep posting on this thread with more info as I get time if you have things you would like me to add or explain please PM me. I will be trying to make this thread have as much info regarding the FSS as possible, Thank you
The FSS (Fuel Shut off Solenoid) is the way our trucks can turn on and off the engine. First off it is one of the most needlessly replaced parts on these engines. The most common way to kill them is the starter to stick on giving the "Pull" wire power for a long period of time, which produces a lot of heat inside the FSS, and it burns up, you most likely don't need to replace it unless it looks burnt. If your FSS is not working correctly then it's most likely your wiring or a dirty shaft.
When the FSS lever is down, the Injector lines will not receive ANY fuel.
Shut off solenoid location:
And this is the arm that need to be up to let fuel in the injector lines, this is under the shut off solenoid. The rubber boot and the arm coming out of the boot are part of the shut off solenoid. the lever the the shut off solenoid pulls up is part of the Injector Pump
Here's a wiring diagram: (I would recommend clicking the bar on top the image to view it full size)
There are 3 wires going to the FSS, Ground, Hold and Pull.
"Ground" wire, truck side is black with a orange stripe, FSS side is all black, it is connected to ground all the time.
"Hold" wire, truck side is small light green with a black stripe, FSS side is normal sized Red wire, the Hold wire gets power when the key is in the run position. Since the soleniod is spring loaded to push the lever down (turn off engine). The Hold wire only has enough power to hold the lever up but does not have enough power to pull it up (compress the spring).
"Pull" wire, truck side is Red, FSS side is White, the Pull wire gets power only when the starter is engaged. it has plenty of power to pull up the lever (compress the spring), but if it's kept on to long it will create a lot of heat and will burn up the FSS. the Power wire gets power from the FSS relay(Part# SS70A), the relay gets it signal power from the small wire coming off the starter.
If you're ever on the road and it shuts off or won't start then you should be able to tie that lever up with a bungee cord, some wire, rope or something. that lever needs to be up for the truck to start.
You can test the FSS by trying to start the truck and then leaving the key in the "Run" position, feel under the FSS to make sure it's all the way up, if it's all the way up then it shouldn't be your problem, unless you have a lack of power problem then the lever might not be going all the way up then you can test it by going HERE and check that the arm is really going all the way up, if it isn't it could be that your shaft is dirty or rusted or simply it might not be adjusted correctly.
And for people that would like a little more info:
here's a picture of of the rack when the FSS is fully down and the throtal is at idle, notice that the arm compresses the spring to bring the rack back to the shutoff position, yet the fueling arm is still the same spot (refer to the next 2 pics):
And here's a picture of the rack at idle with the FSS lever all the way up:
notice that the fueling arm (one that touches the AFC foot and fuel plate) does not move because it is up against the low idle screw. And see how if the FSS lever is not all the way up it will restricted rack travel. That is why you need to make sure that the arm is adjusted correctly to allow full fueling.
Here's a picture of the FSS lever all the way up at WOT with out a Mack plug:
With the Mack plug installed there is 0.2127" of gap between the lever and where it hits the rack.
With the Mack plug not installed there is a 0.11" gap.
The blue fusible link is a common problem (Diagram HERE), it fuses the "Pull" wire going to the FSS. On the 94-97 it is located on the drivers side positive battery terminal. it often gets corroded at the connector or inside the insulation, go to your auto parts store and ask for a "14 ga fusible link" (it might not be blue but that does not matter there both the same) then cut the old one off and use a crimp connector to install the new one.
(not sure about 98, if you have info please PM me, I will try to get the info ASAP)
Pull Relay is located on the fire wall in picture below (it's the bigger one), Part# SS70A, this relay is what gives power to the "Pull" wire to lift the lever when the starter is engaged. Here's a good place to get the Relay LarryBs.com Dodge
Rotted Boot, the Boot will often rot off and let dirt on the shaft which will cause it to get stuck, To clean it, (I will try to get a full right up later with pics) take it off the truck and clean the shaft with a rag, stick the shaft in and pull it back out, each time cleaning the shaft off and spraying it with a spray can of some kind (oil or cleaner, just make sure it will not eat away at rubber). and put it back on truck. If your boot is rotted then it would be best to replace it, Here's a good place to get a new boot LarryBs.com Dodge
Truck Starts and shuts off
Connections are the most common cause of that I think, check the FSS connection and make sure it's not corroded. and check to see what all things you loose when the truck shuts off, gages/lights/radio/etc.
Hold Power: if the truck starts and then shuts down immediately then it sounds like you hold wire isn't getting power, sorry to say but that means checking a bunch of connections to see where it's corroded, the hold wire is fed by the 'green w/black stripe' wire. that's the same wire that powers a bunch of other things. so that's probably your problem if you loose gauges, radio, ect.
Low power/will not rev as high
Fuel shut off lever not coming up all the way
There's two things that can happen, A) the FSS can get dirty so the shaft doesn't come up all the way (refer to "rotted boot" just a little ways up). B) The FSS is not adjusted correctly, Go HERE to see how to adjust it, if it's not adjusted correctly, adjust the FSS shaft as necessary
Notice I didn't say anything about replacing the FSS its self, there's a reason for that. it almost never goes bad and if it does you can tell by looking at it that it is burnt. in which case you can get one from here LarryBs.com Dodge
First here are the tools I used, your truck may be different so use what works best for you:
3/8" ratchet w/5/16" socket and 9" extension
1/4" ratchet w/5/16" socket
10mm open-end box-end wrench
Needle nose pliers
Now loosen up these two bolts that hold on the FSS, take them out about 1/4" or until the end of the bolt is flush with the plate (we want to loosen them but don't want them to fall out, you will know why in a bit).
Working on it...
OK we loosened the bolts up, and now have some gap, the arm is spring-loaded so it will want to push the FSS body up.
Now turn on the key to the "run" position, Go out to the FSS and and push down on the main part to close the gap and lift up on the lever until it clicks up.
The FSS clicks up when the ARM is all the way up, so what you want to do is make sure the Lever is all the way up when the arm clicks up. So sense the bolts holding down the FSS body are loose try lifting on the FSS body to see how high you can lift it, if you can lift it more then 1/8" then the lever is not coming up enough and is limiting fueling on the top end(go here know how it works).
Loosen up the lock/jam nut on the shaft,
First take out your cotter pin on the shaft,
Slide the end of the FSS arm off the shaft, before you do, either turn off the key or unplug the FSS so the arm falls down (every time you turn on the key the grid heater will cycle if it's under 60* out),
And spin it to adjust the height of the lever when the FSS arm is fully up (clicked up),
How much you adjust it is based on the results you had on step 6, then repeat steps 7-10 until you like how the test performed in step 6 goes.
Put back together
Slip the arm Back on the shaft, before you try, either turn off the key or unplug the FSS (every time you turn on the key the grid heater will cycle if it's under 60* out).
Put the cotter pin in,
Tighten the lock nut over the shaft,
Tighten the bolts that hold on the FSS body to the bracket,
If there's something I didn't explain very well or you would like me to add please PM me. Thank you!
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