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Truck not shutting off

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Old 01-25-2022 | 04:04 PM
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Truck not shutting off

I have a 91 d250 12v, I have replaced the ignition and fss, truck will not turn off by key everything else turns off but motor stays running. Rack is not stuck either, any thoughts?
Old 01-25-2022 | 07:00 PM
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From: Hamilton,MO
Pull the plunger on your FSS. Probably have missing rubber tip.
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Old 01-26-2022 | 10:25 AM
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Originally Posted by dodgenstein
Pull the plunger on your FSS. Probably have missing rubber tip.
If he replaced it, not likely. How ever, if the tip of the old FSS is lodged in the chamber, it could hold the plunger open. Other possibility: Power is still going to the FSS even with the switch off. Pull the wire to the FSS and see if that shuts it down.
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Old 02-04-2022 | 09:22 PM
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Originally Posted by cougar
If he replaced it, not likely. How ever, if the tip of the old FSS is lodged in the chamber, it could hold the plunger open. Other possibility: Power is still going to the FSS even with the switch off. Pull the wire to the FSS and see if that shuts it down.
When the wire is pulled off it still stays running
Old 02-05-2022 | 08:03 AM
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From: Winterset, IA
Did u use an aftermarket FSS or a cummins one? There is a big difference. Pull the positive battery cable off the battery after you turn the key off so you can rule out a short to ground.
Old 02-05-2022 | 09:57 AM
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There are 2 wires connected to the FSS, one is power the other goes to a thermo switch then to the KSB solenoid. Make sure you are pulling the correct wire. Otherwise, I suspect debris holding the FSS plunger off the seat. As far as I know, that is the only way fuel can bypass the cutoff circuit. Unless it's not fuel but oil entering the intake air stream from say a turbo seal, head gasket, rings, or valve stim.
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Old 02-06-2022 | 12:19 PM
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Originally Posted by cougar
There are 2 wires connected to the FSS, one is power the other goes to a thermo switch then to the KSB solenoid. Make sure you are pulling the correct wire. Otherwise, I suspect debris holding the FSS plunger off the seat. As far as I know, that is the only way fuel can bypass the cutoff circuit. Unless it's not fuel but oil entering the intake air stream from say a turbo seal, head gasket, rings, or valve stim.
is it possible that I’m replacing the wrong one? I’ve replaced the one on the pump with a single wire, is there another close to the block with the 2 wires?
Old 02-07-2022 | 12:12 AM
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No there is only one wire connected to the FSS on the 89-93 12V motors.
Old 02-07-2022 | 05:59 AM
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Originally Posted by thrashingcows
No there is only one wire connected to the FSS on the 89-93 12V motors.
What I’ve been replacing is circled, what is the other solenoid with the red x I’ve been told that that’s the shutoff and the one circled also so which one?


Old 02-07-2022 | 06:32 AM
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Circled one you have been replacing is the KSB, which is the cold start unit.

The arrow is pointing close to the FSS you need to be checking out.
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Old 02-07-2022 | 09:18 AM
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Originally Posted by oliver foster
Circled one you have been replacing is the KSB, which is the cold start unit.

The arrow is pointing close to the FSS you need to be checking out.
is there a wrench or tool I can get to take that off easier
Old 02-07-2022 | 09:27 AM
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From: vermont
Yes I have seen several people cut and modify tools to make the job easier.

Try searching "FSS removal tool", or the like on this site and read up.
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Old 02-07-2022 | 10:06 AM
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This is a tool I fabricated to remove the FSS. A chopped and ground 15/16 offset box wrench. It works fine on my truck, but others said it won't work on theirs.


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Old 02-21-2022 | 08:31 AM
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Originally Posted by oliver foster
Circled one you have been replacing is the KSB, which is the cold start unit.

The arrow is pointing close to the FSS you need to be checking out.
Do they use the same solenoid for the ksb and fss?
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